Gas Furnaces

This chapter is concerned primarily with introducing the basic proce­dures recommended for installing, operating, servicing, and repairing gas-fired forced-warm-air furnaces. Additional information about furnaces is contained in Chapter 10, “Furnace Fundamentals.”

Only gas-fired furnaces approved by the American Gas As­sociation (AGA) should be used in a heating installation. Not only does AGA certification ensure a certain standard of quality, it is often required by most local codes and ordinances.

Types of Gas Furnaces

There are a variety of different ways to classify residential gas fur­naces. One method is to classify them by their shape, which is com­monly determined by the direction the warm air flows out of the furnace into the spaces being heated. Furnace manufacturers manufacture upflow, downflow (counterflow), lowboy, and hori­zontal flow models for installation in attics, basements, closet spaces, or at floor level (Figures 11-1 through 11-3).

Furnaces are also available in different capacities. The furnace capacity (Btu output) is determined by the heating requirements of the structure—in other words, the amount of heat the furnace must provide to replace heat loss and maintain the desired comfort level.

Note

It is very important to match the furnace capacity as closely as possible to the sizing requirements of the heating system. An oversized or undersized furnace will not heat efficiently.

Furnaces can also be distinguished by their heating efficiency. In recent years, the government and furnace manufacturers have made enormous strides in improving the heating efficiency of residential gas furnaces. Two widely used methods, the AFUE measurement and the Energy Star program of the United States Environmental Protection Agency (EPA), are described below. A furnace that meets the minimum efficiency requirements of these ratings is referred to as a high-efficiency gas furnace.

BLOWER ACCESS PANEL

Gas Furnaces

AIR FILTERS

FLUE

AUTOMATIC GAS VALVE

PILOT TUBE

BLOWER

HEAT EXCHANGER

GAS MANIFOLD "v’ ‘GAS BURNERS

Figure 11-1 Downflow standing-pilot furnace. (Courtesy Coleman Co.)

Gas Furnace Energy Efficiency Ratings AFUE Rating

The energy efficiency of a natural gas furnace is measured by its annual fuel utilization capacity (AFUE). The AFUE ratings for furnaces manufactured today are listed in the furnace manufacturer’s literature. Look for the EnerGuide emblem for

FLUE CONNECTION

Gas Furnaces

Figure 11-2 Horizontal standing-pilot gas furnace.

(Courtesy Lennox Industries Inc.)

The efficiency rating of that particular model. The higher the rating, the more efficient the furnace. The government has established a minimum rating for furnaces of 78 percent. Mid-efficiency furnaces have AFUE ratings ranging from 78 percent to 82 percent. High-efficiency furnaces have AFUE ratings ranging from 88 percent to 97 percent. Traditional standing-pilot gas furnaces have AFUE ratings of approxi­mately 60 to 65 percent.

Energy Star Certification

Energy Star is an energy performance rating system created in 1992 by the U. S. Environmental Protection Agency (EPA) to identify and certify certain energy-efficient appliances. The goal is to give special recognition to companies that manufacture products that help reduce greenhouse gas emissions. This vol­untary labeling program was expanded by 1995 to include fur­naces, boilers, heat pumps, and other HVAC equipment. Both the Energy Star label and an AFUE rating above 80 percent will identify the gas furnace as an energy-efficient appliance.

Finally, gas furnaces can be classified according to the method

Used to light the gas burner. The traditional method was to use a

FLUE CONNECTION

Gas Furnaces

Figure 11-3 Upflow standing-pilot gas furnace. (Courtesy Lennox Industries Inc.)

Pilot light, which was always burning. These types of furnace are called standing-pilot furnaces. They are gradually being replaced by the more energy efficient electronic ignition furnaces (see High­Efficiency Furnaces).

Standing-Pilot Gas Furnaces

The majority of residential gas furnaces in use today are still the conventional standing-pilot types, although they are gradually being replaced by the more efficient mid-efficiency and high­efficiency furnaces (Figures 11-2 and 11-3).

A conventional standing-pilot gas furnace consists of a naturally aspirating gas burner, draft hood, a solenoid-operated main gas valve, a continuously operating pilot light, a thermocouple safety device, a 24-volt AC transformer, a heat exchanger, a blower and motor assembly, and one or more air filters. The furnace is vented through a flue connection to a chimney.

The main gas valve used on a conventional standing-pilot gas fur­nace has a valve knob with a pilot position for lighting the pilot, and both a pilot tube and a thermocouple connection. A transformer provides 24-volt AC power to operate the main gas valve. When the pilot light is lit, the thermocouple sends an electrical current to hold the pilot valve open. If the pilot goes out, the thermocouple closes the valve.

Some standing-pilot gas furnaces use a millivolt generator instead of a 24-volt transformer to open the main gas valve. The main gas valve in these furnaces is specifically designed for use with the millivolt generator.

The energy efficiency of conventional gas furnaces has been improved by equipping them with a direct-drive blower and motor. Adding the direct-drive blower and motor creates a dual-capacity, variable-speed furnace that can match the heat output and circulat­ing fan speed of the furnace to the actual heating requirements of the structure. The furnace is able to maintain the desired tempera­ture comfort levels while, at the same time, saving energy and reducing energy costs.

Mid-Efficiency Gas Furnaces

Mid-efficiency gas furnaces are equipped with a naturally aspirat­ing gas burner and a pilot light, but the pilot does not run continu­ously (Figure 11-4). The pilot is shut off when the furnace is idle (that is, when the thermostat is not calling for heat).

These furnaces have a more efficiently designed heat exchanger than the one found in conventional furnaces. The heat exchanger provides greater resistance to the flow of the combustion gases up through the flue and chimney. This allows the slowed gases to cool and condense. The heat normally trapped in these gases can be extracted from the condensate instead of being lost up the chim­ney. There is no draft hood as in a conventional standing-pilot gas furnace. It is replaced by a small fan in the flue pipe. The induced draft produced by the fan overcomes the cooler (heavier) exhaust gases and the resistance created by the more efficient heat exchanger. Furnaces equipped with these fans are sometimes called induced-draft furnaces. Other components (automatic controls, blower and blower motor assembly, venting, and so on) are the same as those found in a conventional standing-pilot furnace. Mid­efficiency gas furnaces are approximately 20 percent more energy efficient than a conventional gas furnace. The AFUE ratings are in the 78 to 82 percent range.

HEAT

EXCHANGER

ASSEMBLY

 

FRESH AIR INTAKE CONNECTION

 

FLUE COLLAR

 

BURNERS

 

TOP CAP

FLAME SIGHT
GLASS

LOW HEAT

PRESSURE SWITCH
HIGH HEAT

PRESSURE SWITCH
(-75 models only)
UPPER ACCESS
PANEL

TWO-SPEED
COMBUSTION AIR
BLOWER

DOOR
INTERLOCK
SWITCH

SURELIGHT™
INTEGRATED
CONTROL

 

CABINET

 

GAS MANIFOLD

 

BURNER

BOX

 

GAS

VALVE

 

CONDENSATE TRAP

 

BLOWER

MOTOR

CAPACITOR

 

TRANSFORMER

 

CONTROL BLOWER ACCESS BOX

DOOR

 

TWO-STAGE

CONTROL

 

Figure 11-4 Mid-efficiency gas furnace. (Courtesy Lennox Industries Inc.)

 

Gas Furnaces

Note

Mid-efficiency furnaces with AFUE ratings higher than 85 percent often experience condensation problems in the furnace and the exhaust venting.

Some manufacturers of mid-efficiency furnaces install a motor­ized damper in the exhaust flue pipe. The damper automatically closes when the furnace is idle to prevent heat from escaping up the chimney.

High-Efficiency Gas Furnaces

High-efficiency gas furnaces (also sometimes called condensing furnaces or electronic ignition furnaces) have AFUE ratings of 90 percent or more (Figure 11-5). The ignition process in these furnaces is controlled by a solid-state circuit board. There is no continuously burning pilot flame.

High-efficiency furnaces are equipped with two and sometimes three heat exchangers to extract the maximum amount of heat from the flue gases. The condensate resulting from the heat extrac­tion process is then passed out of the bottom of the furnace to a nearby drain. The flue gas temperatures are low enough for the remaining gases to vent outdoors through a narrow PVC pipe.

HEAT

Gas Furnaces

There is no need to vent the gases up a chimney. There are two types of high-efficiency furnace: the intermittent-pilot furnace and the hot-surface ignition (HSI) furnace.

Intermittent-Pilot Furnace

An intermittent-pilot furnace (also sometimes called a spark igni­tion furnace) has many of the same components as a conventional standing-pilot type, including a pilot light assembly, except for a more efficient heat exchanger and an electronic ignition system. When the thermostat setting calls for heat, there begins a short igni­tion trial period during which a high-voltage spark is generated. The spark ignites the pilot. If the attempt to light the pilot was successful, the pilot flame must be proven through the flame rectifi­cation process. If the flame is proven, the solid-state circuit board in the furnace control box sends a signal to the main gas valve, open­ing it to permit gas to flow to the burner (Figure 11-6). The pilot flame then lights the gas burner, which continues to burn until the heat has reached the desired level inside the structure, whereupon the thermostat signals the electronic module or circuit board to stop the ignition process and shut off the pilot and burner.

Some intermittent-pilot furnaces use a pilot relight module and a mercury flame sensor instead of a control module to operate the pilot and gas valve. The gas valve is identical to the one used in fur-

Gas Furnaces

Naces with solid-state control modules. These furnaces are some­times called cycle pilot furnaces.

Hot-Surface Ignition (HSI) Furnace

Figure 11-7 Hot-surface

Ignition igniter. (Courtesy Lennox Industries Inc.)

подпись: 
figure 11-7 hot-surface
ignition igniter. (courtesy lennox industries inc.)
The hot-surface ignition (HSI) furnace is an electronically controlled furnace with two or more heat exchangers. An electronic ignition device called a hot surface igniter (also sometimes called a glow stick or glow plug) is used instead of a pilot light to start the gas burner (Figure 11-7). The tip of the igniter is positioned directly above the gas burner opening. When the thermostat setting calls for heat, the furnace begins a sequence of pre-ignition steps including a purge cycle. After the purge cycle, an electric current is sent to the igniter and heats the surface of the element until it reaches a high enough temperature to ignite the burner. At that point, the main gas valve is opened and gas flows to the burner and is ignited. A flame-sensing rod connected to a solid-state control module is used to detect (prove) the gas flame at the burner nozzle through flame rectification. If the flame can be proven, the electrical current to the igniter is shut off. In these furnaces, the solid-state ignition unit provides direct igni­tion and burner control.

Note

Some of these furnaces ignite the gas with a high-voltage direct spark instead of a red-hot igniter element. These fur­naces are sometimes called direct-spark furnaces. All the other components are the same as those used in a conventional HSI furnace.

Gas Furnace Components

This section contains a brief description of the principal components of a gas-fired, forced-air furnace. Additional and more detailed informa­tion about these components is found in Chapter 2, “Gas Burners”; Chapter 4, “Thermostats and Humidistats”; Chapter 5, “Oil and Gas Controls”; and Chapter 6, “Other Automatic Controls” in Volume 2. The basic furnace components are as follows:

1. Furnace controls

2. Heat exchanger

3. Gas burners

4. Gas pilot assembly

5. Blower and motor

6. Air filter(s)

Furnace Controls

Each gas furnace is equipped with a variety of different controls used to ensure its safe and efficient operation. Not every gas fur­nace will have all of the controls listed here. For example, a ther­mocouple is a safety device used in standing-pilot gas furnaces, but not in those equipped with electronic ignition systems. Check the furnace specifications and manuals for the controls used with a specific model. Detailed descriptions of these devices are cov­ered in Chapter 2, “Gas Burners”; Chapter 4, “Thermostats and Humidistats”; Chapter 5, “Gas and Oil Controls”; and Chapter 6, “Other Automatic Controls.”

Thermostat

A room thermostat controls the operation of the furnace (Figure 11-8). The thermostat senses air-temperature changes in the space or spaces being heated and sends an electrical signal to open or close the automatic main gas valve. The thermostat is generally wired in series with the pilot safety valve, the automatic main gas valve, and

Gas Furnaces

The limit control. See Chapter 4 of Volume 2, “Thermostats and Humidistats” for a detailed description.

Main Gas Valve

The principal function of the main gas valve is to control the flow of gas from the outside supply line (natural gas) or storage tank (propane) to the burner manifold (in the case of a burner assembly) or to the gas burner. These valves are manufactured in a variety of different shapes and sizes. They are commonly located behind the front panel of the furnace. Several different types of gas valves are illustrated in Figure 11-9. These valves are often combined with

Gas Furnaces

Safety shutoff devices that turn off both the main gas and the pilot gas, and/or pressure regulators that regulate the flow of gas. Such valves are commonly called combination gas valves, combination main gas valves, and intermittent-pilot dual-valve combination gas valves. The main gas valves in modern furnaces where there is no standing-pilot are sometimes called electronic ignition gas valves. The various types of gas valves are covered in greater detail in Chapter 6, “Gas and Oil Valves” of Volume 2.

Thermocouple

A thermocouple is a heat-sensing safety device used in a standing­pilot gas furnace to determine whether the pilot is lit before the main gas valve is opened to supply gas to the burners (Figure 11-10). The heat of the pilot flame is converted by the thermocouple into an elec­tric current. The current is strong enough to open the pilot portion of the main gas valve, which then supplies gas to the pilot light. If the thermocouple does not detect a pilot flame, it will turn off the gas supply to the pilot. The main gas valve is operated by a current from a 24-volt AC transformer. The thermocouple is described in greater detail in Chapter 5, “Gas and Oil Controls” in Volume 2.

Gas Furnaces

Thermopile

A thermopile (sometimes called a thermopile generator or a milli­volt generator) is a pilot flame, heat-sensing safety device used in some standing-pilot gas furnaces instead of a thermocouple (Figure 11-11). It is larger than a thermocouple, delivers more electricity, and operates both the main gas valve and the pilot light. No transformer is required in gas furnaces equipped with a thermopile. The thermopile is described in greater detail in Chapter 5, “Gas and Oil Controls” in Volume 2.

Mercury Flame Sensor

A mercury flame sensor used in an electronic ignition system is illustrated in Figure 11-12. It consists of a mercury-filled sensor end, a capillary tube, and an SPDT switch assembly on the main gas valve. The sensor end, which is filled with mercury, is directly heated by the burner flame. The mercury vaporizes and expands, forcing the remaining nonvaporized mercury in its liquid state down the capillary tube to the switch on the gas valve. The weight of the liquid mercury triggers the switch and opens the gas valve.

Gas FurnacesFigure 11-11 Thermopile.

(Courtesy Lennox Industries Inc.)

Gas Furnaces

Figure 11-12 Electronic ignition system flame sensor.

(Courtesy Lennox Industries Inc.)

The mercury flame sensor is described in greater detail in Chapter

5, “Gas and Oil Controls” in Volume 2.

Gas Pressure Regulator

The gas in the supply main is frequently subjected to pressure vari­ations. A gas pressure regulator ensures a constant gas pressure in the burner manifold. In a natural gas heating system, the pressure regulator is located in the furnace on the main gas valve. The gas pressure regulator in an LP (propane) gas heating system is located between the supply tank and the main gas valve.

Fan and Limit Control

The fan and limit control is a safety device that operates on a ther­mostatic principle (Figure 11-13). This combined control is designed to govern the operation of the furnace within a specified temperature range (usually 80 to 150oF), and is located in the furnace plenum, where it responds to outlet air temperatures.

One of the functions of this control is to prevent the furnace from overheating by shutting off the gas supply when the plenum air temperature exceeds the upper temperature setting (180OF) on the control. The limit-control switch (and pilot safety relay) re­sponds to excessive plenum temperatures by closing the automatic main gas valve and thereby shutting off the flow of gas to the burner or burner assembly.

Move fan control lever to its lowest setting to put blower into continuous operation.

To return blower to intermittent or automatic operation, move fan control lever to 90°.

 

Gas Furnaces

TYPE III

подпись: type iii

Figure 11-13 Typical fan and limit controls. (Courtesy Lennox Industries Inc.)

подпись: figure 11-13 typical fan and limit controls. (courtesy lennox industries inc.) Gas FurnacesTYPE I

Move fan control levers to their lowest settings to put blower into continuous operation.

TYPE II

Move fan control levers to their lowest settings to put blower into continuous operation.

To return blower to intermittent or automatic operation, move fan control levers to approximately 115° "on" and 90° "off".

подпись: type ii
move fan control levers to their lowest settings to put blower into continuous operation.
to return blower to intermittent or automatic operation, move fan control levers to approximately 115° "on" and 90° "off".
To return blower to intermittent or automatic operation, move fan control levers to approximately 115° "on " and 90° "off".

Another function of the fan and limit control is to stop the fan when the gas burner or burner assembly has been turned off. When the plenum air drops below the lower setting (80oF) on the control, the fan is automatically shut off. For additional information about fan and limit controls, read the appropriate sections in Chapter 6 of Volume 2, “Other Automatic Controls.”

Heat Exchanger

A traditional standing-pilot gas furnace has a burner compartment containing the gas burner and pilot. The gas and air necessary for combustion are mixed in the venturi or mixing tube prior to ignition.

This mixture is then ignited by the pilot flame as it leaves the burner. The burner flame is encased in a combustion chamber or firebox located directly below the furnace heat exchanger. The heat of the combustion process is transferred to the metal walls of the heat exchanger and then to the air rising through it (Figure 11-14).

The air flowing through a heat exchanger is subject to tempera­tures that frequently approach the 700 to 800°F range. Because of these high temperatures, the airflow must be uniform across all sec­tions of the heat exchanger (as shown in Figure 11-15) and must move at a sufficient rate of speed to keep these temperatures from building any higher. A low volume of air flowing across one section of the heat exchanger will result in overheating and the failure of the entire heat exchanger assembly (Figure 11-16).

Gas FurnacesHEAT EXCHANGER ASSEMBLY Figure 11-14 Heat exchangers. (Courtesy

Lennox Industries Inc.)

Figure 11-15 Uniform air flow across all sections of the heat

Gas FurnacesExchanger. (Courtesy Trane Co.)

Overheating can also be caused by a portion of the gas flame impinging on the inner surface of the heat exchanger (Figure

11- 17). This results in the generation of extremely high tempera­tures. Under normal operating conditions, the gas flame should be directed upward through the center of each section.

Another cause of heat exchanger failure is overfiring. This con­dition is usually traced to an excessively high manifold pressure or

Gas FurnacesFigure 11-16 Low air flow across one heat exchanger section may result in premature failure due to

Overheating. (Courtesy Trane Co.)

Figure 11-17 Flame impingement in one section of the heat exchanger.

Gas FurnacesAn orifice that is too large. The result is a firing rate that exceeds the design limits.

If the heat exchanger becomes so covered with soot, carbon deposits, and other contaminants that it reduces the operating effi­ciency of the furnace, it should be thoroughly cleaned. Gaining access to the heat exchanger on some furnaces is not easy because it may require removing the burners and manifold assembly, vent connector, draft hood (diverter), and flue baffles. For this type of furnace, you are advised to call a local service repairman if you want to clean the heat exchanger. Many furnace manufacturers now provide convenient and accessible cleanout openings to make this maintenance chore easier.

Gas Burners

Gas burners and gas burner assemblies are designed to provide the proper mixture of gas and air for combustion purposes. The gas burners used in residential heating installations operate primarily on the same principle as the Bunsen burner. See Chapter 2, “Gas Burners” in Volume 2.

Gas furnaces require both primary and secondary air for the combustion process. The gas passes through the small orifice in the mixer head, which is shaped to produce a straight-flowing jet mov­ing at high velocity (Figure 11-18). As the gas stream enters the throat of the mixing tube, it tends to spread and induce air in through the opening of the adjustable shutter. This is the primary air, which mixes with the gas before the air-gas mixture is forced

Gas Furnaces

GAS-AIR MIXTURE

GAS FLOW

PRIM

PRIMARY AIR

Gas Furnaces

PRIMARY AIR

Figure 11-18 Schematic of air movements in a typical gas burner.

Through the burner ports (Figure 11-18). The pressure in the gas stream forces the mixture through the mixing tube into the burner manifold casting, from which it issues through ports where addi­tional air must be added to the flame to complete combustion. This secondary air supply flows into the heat exchanger and around the burner. Its function is to mix with unburned gas in the heat exchanger in order to complete the combustion process.

The primary air is admitted at a ration of about five parts pri­mary air to one part gas for manufactured gas, and at a 10:1 ratio for natural gas. These ratios are generally used as theoretical values of air for purposes of complete combustion. Most burners used in gas-fired furnaces operate efficiently on 40 to 60 percent of the the­oretical value. Primary air is regulated by means of an adjustable shutter. When burning natural gas, the air adjustment is generally made to secure as blue a flame as possible. Burner manufacturers provide a number of different ways to field adjust primary air. For example, shutters for primary air control are part of the main burn­ers. Factory adjusted, angled orifices are also used on some burners.

The secondary air is drawn into the burner by natural draft. No provision is made by manufacturers to field adjust secondary air. Secondary air is controlled primarily by the design of the flue restric — tors and draft diverter. The general configurations of the heat exchanger and burners are also important factors in secondary air

Gas Furnaces

Control. Excess secondary air constitutes a loss and should be reduced to a proper minimum (usually not less than 25 to 35 percent).

As shown in Figure 11-19, a typical gas flame consists of a well — defined inner cone surrounded by an outer envelope. Combustion of the primary air and gas mixture occurs along the outer surface of the inner cone. Any unburned gases combine with the secondary air and complete the combustion process in the outer envelope of the flame.

As Figure 11-19 illustrates, the major portion of the inner cone consists of an unburned air-gas mixture. It is along the outer sur­face of the inner cone that combustion takes place. The height of the inner cone can be reduced by increasing the amount of primary air contained in the air-gas mixture. The extra oxygen increases the burning rate, which causes the inner cone to decrease in size.

A burner is generally considered to be properly adjusted when the height of the inner cone is approximately 70 percent of the maximum visible cone height. The gas should flow out of the burner ports fast enough so that the flame cannot travel or flash back into the burner head. The velocity must not be so high that it blows the flame away from the port(s).

Gas Pilot Assembly

The gas pilot assembly in a standing-pilot gas furnace consists of a pilot burner and a pilot safety device (Figure 11-20). The most commonly used pilot safety is the thermocouple lead operating in conjunction with a companion valve or switch. The thermocouple itself is actually a miniature generator that converts heat (from the pilot flame) into a small electrical current. The upper half of the thermocouple is located in the pilot flame. As long as the pilot flame is operating, an electrical circuit is maintained between the pilot burner and the gas valve. If the pilot flame goes out, the circuit between the thermocouple and the gas valve is broken. As a result,

Gas Furnaces

The gas valve closes and shuts off the gas flow to the pilot burner. Other pilot safety devices used with gas-fired furnaces are described in Chapter 5 of Volume 2, “Gas and Oil Controls.”

Smaller gas furnaces, such as those used in RV furnaces, mobile homes, and swimming pool heaters, may use a millivolt system instead of a thermocouple. The pilot heats the millivolt device until it generates enough electricity to operate the main gas valve. There is no need to incorporate a 24-volt transformer, because with a millivolt system the burner will not operate if the pilot is not lit.

Many gas furnaces contain several individual heat exchanger sec­tions with a gas burner inserted in each section. In this arrangement, all burners must fire almost simultaneously, or there will be danger of unburned gas flowing into the combustion chamber. Simultaneous ignition can be provided by installing a crossover burner across the top of the main burners near the port area. Figures 11-21 and 11-22 show the types of crossovers in Trane gas-fired furnaces.

CROSSOVER

Gas Furnaces

CROSSOVER Crossover is fed from the burner manifold.

ORIFICE Air for combustion is supplied through the

Primary air opening (not shown) near the orifice.

In this design there are two orifices: one on either end of the crossover.

BURNER

MANIFOLD

Figure 11-21 Crossover located perpendicular to and across the top of the main burners near the port area. (Courtesy Trane Co.)

MUST FIT SNUGLY GAS-AIR MIXTURE FED TO THIS SECTION

AGAINST THE BURNER CROSSOVER OF CROSSOVER BY BURNER # 3

Gas Furnaces

Burner #2 feeds this section of crossver.

Scoop in burner directs flow up into crossover.

Figure 11-22 Crossover arrangement in which each burner feeds a portion of the gas-air mixture into the crossover, where it is ignited by the pilot flame as it leaves the crossover part. (Courtesy Trane Co.)

High-efficiency gas furnaces do not have a continuously burning pilot light. In these furnaces, an electronic pilot assembly takes the place of the standing-pilot and thermocouple. These pilot assemblies contain a pilot, a spark ignition, and a flame sensor (Figure 11-23).

Blower and Motor

Forced-warm-air furnaces are equipped with variable-speed blowers designed for continuous duty. Blower motors are available as either direct-drive or belt-driven units (Figures 11-24 and 11-25). Most modern furnaces are equipped with direct-drive motors. See the sec­tion Blowers and Motors in this chapter for a detailed description.

Air Filter

A forced-warm-air, gas-fired furnace is supplied with either a dis­posable air filter or a permanent (washable) one. Typical gas fur­nace air filters are illustrated in Figure 11-26. See the section Air Filters in this chapter for a detailed description.

Gas Furnaces

NOTE : PILOT LOCATED ON SAME SIDE OF BURNER AS GAS VALVE

Figure 11-23 Electronic ignition pilot assembly. (Courtesy Lennox Industries Inc.)

Gas Furnaces

WHEEL (B3)

Gas Furnaces

1/4" TO 1/2" Deflection

Figure 11-25 Belt-driven blower.

(Courtesy Lennox Industries, Inc.)

Gas FurnacesThe first step in planning a heating system is to calculate the maximum heat loss for the structure. This should be done in accordance with procedures described in the manuals published by the Air Conditioning Contractors of America (ACCA), espe­cially their Manual J, Residential Load Calculations, or by a com­parable method. Correctly calculating the maximum heat loss for

Gas Furnaces

CDB1 SERIES CDB1 SERIES

(COUNTERFLOW APPLICATION) (RIGHT SIDE HORIZONTAL APPLICATION)

The structure is very important because the data will be used to determine the size (capacity) of the furnace selected for the heat­ing system. The ACCA’s Manual J contains the most accurate cal­culation methods for sizing heating and air conditioning equipment. Computer programs and worksheets designed to sim­plify the calculation procedures found in Manual J are also avail­able for purchase.

Note

High-efficiency (condensing) gas furnaces must be correctly sized. Oversizing will not lead to increased energy costs, as is the case with the traditional standing-pilot gas furnaces, but it will cause short firing cycles. If the furnace operates with short firing cycles, it will never get hot enough to dry out the condensate extracted from the combustion process. The condensate accumulates, becomes gradually more and more acidic, and begins to corrode parts of the furnace. Oversizing high-efficiency furnaces may also result in uncomfortable tem­perature swings in the living areas and excessive air flow from the room registers. Oversized furnaces require larger blowers than correctly sized ones.

Caution

A gas furnace must be properly set up and installed by a certified HVAC gas heat technician, a representative of the gas furnace manufacturer, or someone with equivalent experience.

Warning

If the heating or heating/cooling installation is to be approved by either the FHA or VA, heat loss and heat gain calculations must be made in accordance with the procedures described in the ACCA’s Manual J.

Some Installation Recommendations

New furnaces for residential installations are shipped preassembled from the factory with all internal wiring completed. In order to install the new furnace, the gas piping from the supply main, the electrical service from the line voltage main, and the low-voltage thermostat must be connected. Directions for making these connec­tions are to be found in the furnace manufacturer’s installation instructions.

A gas furnace must be properly set up and installed by a certified gas heat technician, a representative of the gas furnace manufacturer, or an HVAC professional with equiva­lent experience.

Make certain you have familiarized yourself with all local codes and regulations that govern the installation of a gas-fired furnace. Local codes and regulations take precedence over national stan­dards. Your furnace installation must comply with the local codes and regulations.

Always check a new furnace for damages as soon as it arrives from the factory. If shipping damages are found, the carrier (not the factory) should be notified and a claim filed immediately. Heating equipment is shipped FOB from the factory and it is the responsi­bility of the carrier to see that it arrives undamaged.

Always place the furnace on a solid and level base. Doing so will reduce vibrations from the equipment and keep operating noise to a minimum. In a crawl-space installation, the furnace is either sup­ported on a slab or on concrete blocks, or it is suspended from floor joists with 3/8-in hanger rods.

A furnace installed in an attic should be placed on a fiberboard sheeting base to absorb vibrations. Many furnaces designed for attic installations are certified for installation directly on combustible material such as ceiling joists or attic floors. In an attic installation, the furnace should be placed over a load-bearing partition for additional support.

Locations and Clearances

The proper location of a gas furnace depends on a careful consider­ation of the following three factors:

1. Length of heat runs

2. Chimney location

3. Clearances

A gas forced-air furnace should be located as near as possible to the center of the heat-distribution system and chimney. Centralizing the furnace reduces the need for one or more long supply ducts, which tend to lose a certain amount of heat. The number of elbows should also be kept to a minimum for the same reason. Installing the furnace near the chimney will reduce the length of the horizontal run of flue pipe required for the traditional, standing­pilot type gas furnace. The flue pipe should always be kept as short as possible.

Sufficient clearance should be provided for access to the draft hood and flue pipe. It is particularly important to locate a gas-fired furnace so that the draft hood is at least 6 in from any combustible material.

Allowing access for lighting the furnace and servicing it is also an important consideration. Most furnace manufacturers recom­mend a clearance of 24 to 30 inches in front of the unit for access to the burner and controls.

Clearances between the furnace and any combustible materials are usually provided by the manufacturer in the furnace specifications.

Wiring

All of the electrical wiring inside the furnace is completed and inspected at the factory before the unit is shipped. Wiring instruc­tions will accompany each furnace and the internal (factory installed) wiring will be clearly marked. The wiring to controls and to the electrical power supply must be completed by the installer, and the wiring should be done in accordance with the diagram sup­plied by the manufacturer. This wiring is usually indicated on wiring diagrams by broken lines.

All wiring connected to the furnace must comply with the lat­est edition of the National Electric Code and any local codes and ordinances. Local codes and ordinances will always take prece­dence over national ones when there is any conflict. In order to validate the furnace warranty, many manufacturers require that a local electrical authority approve all electrical service and connections.

The two types of electrical connections required to field wire a furnace are (1) line-voltage field wiring, and (2) control-voltage field wiring. Both are illustrated in the wiring diagram for the gas furnace shown in Figure 11-27.

The line-voltage wiring connects the furnace to the building power supply. It runs directly from the building power panel to a fused disconnect switch. From there, the wiring runs to terminals L1 and L2 on the power-supply terminal block (that is, the furnace junction box).

The unit must be properly grounded either by attaching a ground conduit for the supply conductors or by connecting a separate wire from the furnace ground lug to a suitable ground. The furnace must be electrically grounded in accordance with the National Electrical Code (ANSI-CI-1971).

Gas Furnaces

Furnaces equipped with motors in excess of 3/4-hp or 12-amp ratings must be wired to a separate 240-volt service in accordance with the National Electrical Code and local codes and regulations. A 240-volt transformer should be installed whenever 240-volt service is required and/or a motor larger than % hp is used.

The external control-voltage circuitry consists of the wiring between the thermostat and the low-voltage terminal block located in the control-voltage section of the furnace. Instructions for the control-voltage wiring are generally shipped with the thermostat.

Duct Connections

The warm-air plenum, warm-air duct, and return-air duct should be installed in accordance with the furnace manufacturer’s specifi­cations and the local building code requirements. Detailed informa­tion about the installation of an air-duct system for a gas furnace is also contained in the following publications:

1. Residence Type Warm Air Heating and Air Conditioning System (National Fire Protection Association, No. 90B)

2. Installation of Air Conditioning and Ventilating Systems of Other than Residence Type (National Fire Protection Association, No. 90A)

3. Manual D, Residential Duct Systems (Air Conditioning Contractors of America)

Additional information about duct connections can be found in Chapter 7 of Volume 2, “Ducts and Duct Systems.”

It is important for you to remember the following facts about furnace duct connections and the air distribution ducts:

1. Duct lengths should be kept to a minimum by centralizing the furnace location as much as possible.

2. The duct system must be properly sized. Undersizing will cause a higher external static pressure than the one for which the furnace was designed. This may result in a noisy blower or insufficient air distribution.

3. Duct sizing should include an allowance for the future instal­lation of air conditioning equipment.

4. The furnace should be leveled before any duct connections are made.

5. Seal around the base of the furnace with a caulking com­pound to prevent air leakage if a bottom air return is used.

6. Ducts should be connected to the furnace plenums with tight fittings.

7. Warm-air-plenum and return-air-plenum connections should be the same size as the openings on the furnace.

8. Use tapered fittings or starter collars between the ducts and the furnace plenum.

9. Ductwork located in unconditioned spaces (for example, unheated attics, basements, and crawl spaces) should be insu­lated if air conditioning is planned for some future date.

10. Install canvas connectors between air plenums and the casing of the furnace to ensure quiet operation. Check local codes and regulations to make certain canvas connectors are in compliance.

11. Line the first 10 ft or so of the supply and return ducts with acoustical material when extremely quiet operation is necessary.

12. Install locking-type dampers in each warm-air run to facilitate balancing the system.

Ventilation and Combustion Air

A gas furnace requires an unobstructed supply of air for combus­tion. In the older standing-pilot furnaces, this combustion air sup­ply was commonly supplied through natural air infiltration (cracks in the walls and around windows and doors) or through ventilating ducts. Newer furnaces include a separate air combustion fan next to the burners to provide a sufficient supply of combustion air to the furnace. The houses are now too tightly constructed and well insulated for natural ventilation to provide enough air. These small air combustion fans or blowers are in addition to the large indoor blower on the furnace.

If a gas-fired furnace is located in an open area (basement or utility room) and the ventilation is relatively unrestricted, there should be a sufficient supply of air for combustion and draft hood dilution. On the other hand, if the heating unit is located in an enclosed furnace room or if normal air infiltration is effectively reduced by storm windows or doors, then certain provisions must be made to correct this situation. Figure 11-28 illustrates one type of modification that can be made to provide an adequate supply of combustion and ventilation air to a furnace room. As shown, two permanent grilles are installed in the walls of the furnace room, each of a size equal to 1 in2 of free area per 1000 Btu per hour of burner output. One grille should be located approximately 6 in

VENTILATING AIR OPENING 1 SQ. IN. FOR EACH 1000 Btu PER HR. INPUT

Gas Furnaces

1000 Btu PER HR. INPUT

Figure 11-28 Recommended air openings in furnace room wall.

From the ceiling and the other one near the floor. A possible alter­native is to provide openings in the door, ceiling, or floor as shown in Figure 11-29.

If the furnace is located in a tightly constructed building, it should be directly connected to an outside source of air. A perma­nently open grille sized for at least 1 in2 of free area per 5000 Btu per hour of burner output should also be provided. Connection to an outside source of air is also recommended if the building con­tains a large exhaust fan.

Provisions for ventilation and combustion air are described in greater detail in Installation of Gas Appliances and Gas Piping (ASA Z21.30 and NFPA No. 54). These standards were adopted and approved by the National Fire Protection Association and the National Board of Fire Underwriters.

OPENING FOR VENTILATION AIR IN DOOR OR CEILING

Gas Furnaces

Figure 11-29 Alternative method of providing air supply openings.

Venting

Provisions must be made for venting the products of combustion to the outside in order to avoid contamination of the air in the living or working spaces of the structure. The four basic methods of vent­ing the products of combustion to the outside are:

1. Masonry chimneys

2. Low-heat Type A prefabricated chimneys

3. Type B gas vents

4. Type C vents

5. Wall venting

6. PVC pipe venting

Masonry and prefabricated chimneys are described in later sec­tions (see Chimney and Chimney Troubleshooting in this chapter).

Warning

Any furnace that produces heat from the combustion process (that is, burning gas, oil, coal, or wood) can potentially pro­duce carbon monoxide gas as well. Carbon monoxide gas is odorless, invisible, and deadly toxic. The only safeguard against the accumulation of this gas before it reaches unsafe levels is to install a carbon monoxide detector.

Low-heat Type A flues are metal, prefabricated chimneys that have been tested and approved by Underwriters Laboratories, Inc. (UL). These chimneys are easier to install than masonry chimneys and are generally safer under abnormal firing conditions.

Type B gas vents (Figure 11-30) are listed by UL and are recom­mended for venting all standard gas-fired furnaces. They are not recommended for incinerators, combination gas-oil appliances, or appliances convertible to solid fuel. Check the furnace specifica­tions for the type of vent to use. It will usually be AGA certified for use with a Type B gas vent. A Type C vent is used to vent gas-fired furnaces in attic installations (Figure 11-31).

Wall venting involves gas-fired appliances, which have their combustion process, combustion air supply, and combustion prod­ucts isolated from the space that is being heated. These appliances are generally vented through the wall.

High-efficiency gas furnaces vent the gases and other byproducts of the combustion process through PVC (plastic) pipe, not the metal pipe used with standing-pilot gas furnaces. The PVC exhaust pipe extends vertically through the roof or horizontally through a sidewall to the outdoors. The latter is the typical connection to a replacement furnace.

The PVC exhaust pipe must be firmly and securely supported. Weak or inadequate support can cause the piping to sag. A sagging horizontal exhaust vent pipe will cause condensate (water) to collect in low spots. The condensate cannot drain back down into the fur­nace and eventually builds up to a point where it blocks the exhaust vent. A blocked exhaust vent will cause the furnace to shut down.

Note

Provide at least a V4-inch per foot upward pitch for horizontal exhaust vent piping to allow the condensate to drain back down into the furnace. Never pitch the exhaust pipe down­ward toward the outdoors.

Gas Furnaces

PLACEMENT OF REDUCING TEE AT MINIMUM HEIGHT Lm ABOVE SMALLER OF TWO OR MORE APPLIANCES PERMITS MULTIPLE SYSTEMS.

STORM COLLAR

ADJUSTABLE ROOF FLASHING ALSO FLAT AND TALL CONE

ROUND VENT SIZES FROM 3" TO 12"

45° ADJUSTABLE ELBOW ALSO IN 90°

INCREASER OR REDUCER DRAFT HOOD CONNECTOR

ALUMINUM INNER PIPE

AIR SPACE

GALVANIZED STEEL OUTER PIPE

FIRE STOP SPACER

BELMONT TOP

SECTION LENGTHS: 6", 12", 18", 3′, 5′, AND 12" ADJUSTABLE

Figure 11-30 Double-wall gas-vent pipe and fittings for single or multiple Type B gas-vent system.

Gas Furnaces

High-efficiency (condensing) furnaces should not be vented into a chimney. The exhaust gases from these furnaces are too cool to create a chimney draft. Unable to rise, they condense inside the chimney. The trapped condensate will eventually damage the chimney materials.

Flue Pipe

The flue is the passage through which the flue gases pass from the combustion chamber of the furnace (or boiler) to the outside. A flue is also referred to as a flue pipe, vent pipe, or vent connector.

The term appliance flue refers to the flue passages inside a fur­nace or boiler. A chimney flue is the vertical flue passage running up through the chimney. A vent connector is specifically the flue pas­sage between the heating unit and the chimney. It is also variously referred to as a chimney connector or smoke pipe. A flue outlet, or vent, is the opening in a furnace or boiler through which the flue gases pass.

Conventional Flue Construction Details

The flue pipe (vent pipe) connects the smoke outlet of the furnace with the chimney. It should never extend beyond the inner liner of

The chimney and should never be connected to the flue of an open fireplace. Furthermore, flue connections from two or more sources should never enter the chimney at the same level from opposing sides.

The horizontal run of flue pipe should be pitched toward the chimney with a rise of at least Vi in per running foot. Some furnace manufacturers recommend as much as a 1-in minimum rise per running foot. Do not allow the horizontal run to exceed, in length, 75 percent of the vertical run. Vent pipe crossovers in an attic must not extend at an angle of less than 60° from the vertical.

At intervals, fasten the horizontal run of flue pipe securely with sheet-metal screws and support the pipe with straps or hangers to prevent sagging. Sagging can cause cracks to develop at the joints, which may result in releasing toxic flue gases into the living and working spaces of the structure.

The point at which the flue pipe enters the chimney should be at least 2 ft above the cleanout opening of the chimney (Figure 11-32). The flue pipe must be the same size as the outlet of the flue collector

Gas Furnaces

FLUE PIPE

2 FT. MINIMUM

CLEANOUT DOOR-

подпись: cleanout door-T

(furnace flue collar). Never install a damper in a flue pipe or reduce the size of the flue pipe. Run the flue pipe from the draft hood to the chimney in as short a distance as possible. All joints in the flue pipe should be made with the length nearest the furnace overlapping the other.

If excessive condensation is encountered, install a drip tee in place of an elbow. Never use dampers or other types of restrictors in the vent pipe. A minimum distance should be maintained between the vent pipe and the nearest combustible material.

When more than one appliance is vented into a common flue, the area of the common flue should be equal to the area of the largest flue plus 50 percent of the area of the additional flue.

PVC Vent Construction Details

High-efficiency furnaces vent the byproducts of the combustion process through horizontal PVC piping extending from the fur­nace to an opening in an exterior sidewall. PVC piping is also used to bring combustion air from the outdoors to the furnace. Vent outlet and combustion air inlet details are illustrated in Figure 11-33. Figure 11-34 illustrates typical terminations for PVC piping on the outside of the sidewall. Note that the maxi­mum wall thickness through which both the vent and combus­tion air pipes can pass is 18 inches. The minimum thickness is 2 inches.

Carefully follow the local codes and ordinances plus the furnace manufacturer’s requirements when installing PVC vent and air intake piping. Note the following requirements:

• Install the bottom edge of the vent outlet termination elbow a minimum of 12 inches above the outlet of the combustion air termination elbow, as shown in Figure 11-32.

• Maintain a minimum horizontal distance of 4 ft between the outlet and inlet vents and any electric meters, gas meters, reg­ulators, and relief devices.

• Insulate PVC vent pipe passing through an unheated space with 1.0-in-thick foil faced with fiberglass insulation.

• Cut PVC pipe at right angles with a hacksaw. Remove all burrs before installing the pipe.

• Seal PVC joints with a silicone caulk. Do not butt and glue cut pipe ends together. Do not use caulk to seal the PVC sleeve or coupling to the metal air intake collar on the furnace burner box.

Gas Furnaces

COUPLING

TERMINATION

COMBUSTION AIR INLET TERMINATION BLOW

Gas Furnaces

329

подпись: 329Figure I 1-33 Sidewall vent outlet and combustion air inlet details. (CourtesyThermo Pride)

Gas Furnaces

Figure 11-34 Location of vent outlet and air inlet in sidewalls.

(Courtesy Thermo Pride)

Chimney

Most chimneys are constructed of brick or metal. If a brick chim­ney is used, it should be lined with a protective material to prevent damage from water vapor. The most commonly used liner is smooth faced tile. Prefabricated factory-built chimneys are also used, but only those listed by Underwriters Laboratories are suit­able for use with fuel-burning equipment.

The standard chimney must be at least 3 ft higher than the roof or 2 ft higher than any portion of the structure within 10 ft of the chimney flashing in order to avoid downdrafts (Figure 11-35).

An existing chimney should always be checked to make sure it is smoketight and clean. Any dirt or debris must be cleaned out before the furnace or boiler is used. See Chapter 3, “Fireplaces, Stoves, and Chimneys” in Volume 3 for additional information about chimney construction, maintenance, and repair.

Caution

If an original furnace has been replaced by a mid-efficiency or high-efficiency furnace, the chimney cannot be used to vent the combustion gases unless it has been rebuilt or modified. The cooler gases of these more efficient furnaces lack the buoyancy of those produced in the conventional standing-pilot furnaces. Consequently, they do not rise quickly up the chimney. The condensate of these cool, slower moving gases will collect on the chimney walls and eventually destroy the masonry.

DOWN DRAFT BECAUSE OF PRESSURE DIFFERENCE

Gas Furnaces

Figure 11-35 Examples of correct and incorrect chimney designs.

Chimney Troubleshooting

The chimney must give sufficient draft for combustion or the fur­nace will not operate efficiently. It must also provide a means for venting the products of combustion to the outside. Although a chimney-produced draft is not as important for the combustion process in gas-fired appliances as it is in other types of fuel-burning equipment, the venting capacity of the chimney is extremely impor­tant. The chimney must be of a suitable area and height to vent all the products of the combustion process.

Figure 11-36 illustrates some of the common chimney problems that can cause insufficient draft and improper venting. Some of these problems are detectable by observation; others require the use of a draft gauge.

Sometimes a chimney will be either too small or too large for the installation. When this is the case, the chimney should be rebuilt with the necessary corrections made in its design.

Draft Hood

A standing-pilot (continuously burning pilot) gas furnace is equipped with a draft hood attached to the flue outlet of the appliance. The draft hood used on the appliance should be certified by the American Gas Association. Only gas conversion furnaces equipped with power-type burners and conversion burner installations in large steel boilers with inputs in excess of 400,000 Btu/h are not required to have draft hoods.

A draft hood is a device used to ensure the maintenance of con­stant low draft conditions in the combustion chamber. By this action, it contributes to the stability of the air supply for the combustion process. A draft hood will also prevent excessive chimney draft and
downdrafts that tend to extinguish the gas burner flame. Because of this last function, a draft hood is often referred to as a draft diverter.

Note

Mid-efficiency furnaces (the so-called induced-draft furnaces) do not have a draft hood. They use a small fan located in the flue to induce a draft. High-efficiency gas furnaces also do not have draft hoods.

Problem

Top of chimney is lower than surrounding objects. Chimney cap or ventilator obstruction.

Coping restricts opening. Obstruction in chimney.

Joist projecting into chimney.

Break in chimney lining.

Collection of soot at narrow space in flue opening.

Offset.

Two or more openings in same chimney.

Smoke pipe extends into chimney.

Loose-fitted cleanout door.

Failure to extend the length of flue partition down to the floor.

Loosely fitted cleanout door.

Remedy

Extend chimney above all objects within 20 ft.

Remove.

Make opening as large as inside of chimney.

Use rod or weight on string or wire to break and dislodge. Change support for joist so that chimney will be clear. Should be handled by a competent brick contractor. Rebuild chimney with a course of brick between flue tiles. Clean out with weighted brush or bag of loose gravel on end of line. May be necessary to open chimney.

Change to straight or long offset.

Least important opening must be closed using some other chimney flue.

Shorten pipe so that end is flush with inside of tile.

Leaks should be eliminated by cementing all pipe openings. Extend partition to floor level.

Close all leaks with cement.

Common chimney problems and their corrections. (continued)

TROUBLES

Gas Furnaces

Draft hoods may be either internally or externally mounted, depending on the design of the furnace. Never use an external draft hood with a furnace already equipped with an internal draft hood. Either vertical or horizontal discharge from the draft hood is possible (Figure 11-37). Locations of draft hoods in conversion burner instal­lations are illustrated in Figure 11-38.

In some installations, a neutral pressure-point adjuster is installed in the flue pipe between the furnace and the draft hood. The procedure for making a neutral pressure-point adjuster is illus­trated in Figure 11-39. Always leave the neutral pressure-point

BUILT-IN DRAFT HOOD WITH VERTICAL DISCHARGE

подпись: 
built-in draft hood with vertical discharge
5′-0"

BULLET-IN DRAFT HOOD WITH HORIZONTAL DISCHARGE

подпись: 
bullet-in draft hood with horizontal discharge
2′-0"

-f _ ZD

‘-0"

J

1— 1 1——- 1

1 i

I

‘ :

S

>

S

S

EXTERNAL DRAFT HOOD

WITH HORIZONTAL DISCHARGE

EXTERNAL DRAFT HOOD WITH VERTICAL DISCHARGE

подпись: 
external draft hood with vertical discharge
Figure 11-37 External and internal (built-in) draft hoods with vertical and horizontal discharge. (Courtesy American Gas Association)

Gas Furnaces

335

Gas Furnaces

Figure 11-38 Location of draft hoods in conversion burner installations.

(Courtesy Magic Servant Products Co.)

 

подпись: 335

SAW SLOT

 

Suggested form of neutral pressure point adjuster. Insert in slot cut in flue pipe.

 

When adjustment is completed, scribe a line on adjuster so it can be relocated

 

SCRIBE LINE AROUND FLUE PIPE

 

Gas Furnaces

TRIM OFF EXCESS STOCK

подпись: trim off excess stock
 
Remove adjuster, trim off excess, slit remainder vertically, and bend segments in alternate directions.

Replace in flue pipe, recheck adjustments to ensure unchanged conditions, and fasten adjuster in place with sheet metal screws.

Figure 11-39 Suggested construction of a neutral pressure-point

Adjuster. (Courtesy Magic Servant Products Co.)
adjuster in wide open position until after the burner rating has been established.

Pilot Burner Adjustment

It is sometimes necessary to adjust the pilot flame. On some combi­nation gas controls, a pilot adjustment screw is provided for this purpose. On units using natural gas, the screw should be adjusted until the pilot flame completely envelops the thermocouple. If propane gas is used, the screw should be wide open.

Sometimes a pilot gas regulator is used on a furnace to control pressures in areas where gas pressure variation is great. The pilot valve is placed in wide open position, and pilot flame adjustments are made by adjusting the gas pressure regulator.

Gas Input Adjustment

The gas input of a furnace must not be greater than that specified on the rating plate. However, it is permissible to reduce the gas input to a minimum of 80 percent of the rated input in order to balance the load.

The gas input can be checked by timing the flow through the meter. The procedure for clocking the meter is as follows:

1. Turn off all gas units connected through the meter except the furnace. The main burners of the furnace must be on for the timing check.

2. Time one complete revolution of the hand (or disk) on the meter. The hand should be the one with the smallest cubic footage per revolution (usually V2 or 1 ft3 per revolution on small meters and 5 ft3 on large meters).

3. Using the size of the test meter dial (V2, 1, 2, or 5 ft3) and the number of seconds per revolution, determine the cubic feet per hour from the appropriate column in Table 11-1.

4. Determine the actual Btu/h input of the burner by multiplying the cubic feet per hour rate (obtained in Step 3) by the Btu per cubic feet of gas (obtained from the local gas company).

By the way of example, let’s assume that you have contacted the local gas company and they have given you a value of 1040 Btu per cubic foot. When you clock the meter, you find that it takes exactly 18 seconds for the hand to make one complete revolution on the V2 ft3 meter dial. With this information, turn to Table 11-1 and find 18 seconds in the extreme left-hand column. Opposite the value

Table I l-l Gas Rate in Cubic Feet per Hour

Size of Test Meter Dial Size of Test Meter Dial

‘/2 ft3 I ft3 2 ft3 5 ft3 ‘/2 ft3 I ft3 2 ft3

For One

Cubic Feet per Hour Revolution Cubic Feet per Hour

180

360

720

1800

50

36

72

144

164

327

655

1636

51

35

71

141

150

300

600

1500

52

35

69

138

138

277

555

1385

53

34

68

136

129

257

514

1286

54

33

67

133

120

240

480

1200

55

33

65

131

112

225

450

1125

56

32

64

129

106

212

424

1059

57

32

63

126

100

200

400

1000

58

31

62

124

95

189

379

947

59

30

61

122

90

180

360

900

60

30

60

120

86

171

343

857

62

29

58

116

82

164

327

818

64

29

56

112

78

157

313

783

66

29

54

109

75

150

300

750

68

28

53

106

72

144

288

720

70

26

51

103

69

138

277

692

72

25

50

100

67

133

267

667

74

24

48

97

28

64

129

257

643

29

62

124

248

621

30

60

120

240

600

31

58

116

232

581

32

56

113

225

562

33

55

109

218

545

34

53

106

212

529

35

51

103

206

514

36

50

100

200

500

37

49

97

105

486

38

47

95

189

474

39

46

92

185

462

40

45

90

180

450

41

44

88

176

440

42

43

86

172

430

43

42

84

167

420

44

41

82

164

410

45

40

80

160

400

46

39

78

157

391

47

38

77

153

383

48

37

75

150

375

49

37

73

147

367

Courtesy Dunham-Bush, Inc.

339

подпись: 339

24

47

23

46

22

45

22

44

21

43

21

42

20

41

20

40

19

38

18

37

18

36

17

35

17

33

16

32

15

31

15

30

14

28

13

26

12

24

11

22

11

21

10

20

95

237

92

231

90

225

88

220

86

214

84

209

82

205

80

200

76

192

74

184

72

180

69

173

67

167

64

161

62

155

60

150

55

138

51

129

48

120

45

ИЗ

42

106

40

100

18 seconds under the */2 ft3 column heading read 100 ft3. The rest is simple multiplication:

100 ft3 per hour X 1040 Btu per ft3 = 104,000 Btu/h

If the Btu/h rate is not within 5 percent of the desired value, change the burner orifices or adjust the pressure regulator. Pressure regulator setting changes are used only for minor adjustments in Btu input. Major adjustments are made by changing the burner orifices.

When making adjustments, remember the following: Btu input may never be less than the minimum or more than the maximum on the rating plate.

There is no meter to clock on furnaces fired with propane. The burner orifices on these units are sized to give the proper rate at 11 in W. C. (water column) manifold pressure.

After the correct Btu input rate has been determined, adjust the burner for the most efficient flame characteristics (see Combustion Air Adjustment).

High-Altitude Adjustment

For elevations above 2000 ft, the Btu input should be reduced (der­ated) 4 percent for each 1000 ft above sea level. This adjustment is required because of the increase in air volume at higher elevations. Air expands at approximately 4 percent per 1000 ft of elevation. At ele­vations above 2000 ft, the volume of air required to supply enough oxygen for complete combustion exceeds the maximum air-including ability of the burners. Because the primary air volume cannot be increased, the gas flow (input) must be decreased (that is, derated).

Changing Burner Orifices

A gas-fired furnace is supplied with standard burner orifices (spuds) for the gas shown on the rating plate. The orifice size will depend on the calorific value of the gas (Btu per cubic foot) and the manifold pressure (3.5 in W. C. for natural gas and 11 W. C. for LP gas). The burner orifice size partially determines the firing rate by allowing only a predetermined volume of gas to pass. An orifice that is too large will allow too much gas to pass through into the burner. This condition sometimes results in overheating the heat exchanger or restricting the air-inducing ability of the burner.

The gas burner orifices supplied with a furnace are suitable for average calorific values of a gas at the listed manifold pressures. Table 11-2 lists the various orifice capacities for different drill sizes. The procedure for determining the suitability of a particular drill size is as follows:

1. Divide the total Btu input of the furnace by the total number of burner orifices in the unit.

2. From Table 11-2, select the burner orifice size closest to the calculated value (plus or minus 5 percent) for the gas used.

Table 11-2 Orifice Capacity Table

Drill Size

Approximate Orifice Capacity (But/h)*

Natural Gas @ 3 ‘/i" W. C.

Propane Gas @ »" W. C.

60

4650

13,000

59

4900

13,750

58

5150

14,450

57

5400

15,150

56

6300

17,600

55

7900

22,000

54

8800

24,750

53

10,400

28,800

52

11,750

33,000

51

13,000

36,700

50

14,250

40,000

49

15,500

43,500

48

16,750

47,200

47

17,900

50,200

46

19,000

53,500

45

19,500

55,000

44

21,500

60,050

43

23,000

42

25,250

41

26,750

40

27,800

39

28,600

38

29,900

37

31,100

36

33,000

35

35,000

*Values determined using the following data: propane at 1.52 specific gravity/2500 Btu/ft3 and natural gas at 0.62 specific gravity/1000 Btu/ft3. Courtesy The Trane Company

This procedure can be illustrated by determining the most suitable drill size for a 150,000-Btu-input, gas-fired furnace. This furnace has four burners, with two orifices per burner, and uses natural gas.

• , , , 150,000

Capacity of each burner orifice = ^

_ 150,000 = 8 = 18,750

In Table 11-2, the figure closest to 18,750 in the left-hand column (natural gas) is 19,000. The drill size opposite this figure is No. 46.

Manifold Pressure Adjustment

A suitable manifold pressure is important for efficient furnace operation. If the gas pressure is too low, it will cause rough igni­tion, incomplete and inefficient combustion, and incorrect fan control response. An excessively high manifold pressure may cause the burners to overfire the heat exchanger. Overfiring the heat exchanger not only reduces the life of this component, but it also may result in repeated cycling of the burner on the high limit control.

Manifold pressure should be set at 3.5 in W. C. for natural gas. Control manufacturers normally preset the pressure regulator in natural-gas valves at the time of manufacture so that natural-gas units are fired at this 3.5 in W. C. rate. Only small variations in the gas flow should be made by adjusting the pressure regulator. This adjustment should never exceed plus or minus 0.3 in W. C. Major changes in the gas flow should be made by changing the size of the burner orifice (see below).

The manifold gas pressure can be tested by using a U-tube water manometer (Figure 11-40). The manometer is connected to the manifold through an opening covered by a 1/8-in plug cap. The pressure test must be run while the unit is operating. On Dunham- Bush gas-fired furnaces, the test is run through an opening in the gas valve (Figure 11-41). The furnace manufacturer’s installation literature should contain instructions for testing the manifold pressure.

Once you have determined the manifold pressure, you may find it necessary to adjust it for better operating characteristics. Manifold pressure can be increased by turning the adjusting screw clockwise, and decreased by turning it counterclockwise.

Gas Furnaces

(Courtesy Trane Co.)

LP gas units are fired at 11 in W. C. manifold pressure. LP gas (propane and butane) is heavier than natural gas and has a higher heating value. As a result, LP gas needs more primary air for combustion. Thus, a higher manifold pressure is required to induce the greater volume of primary air into the burner than is the case in natural gas units

An LP furnace does not have a pressure regulator in the gas valve. Pressure adjustments are made by means of the regulator at the supply tank. These adjustments must be made by the installer and regularly checked by the serviceman for possible malfunctions.

Combustion Air Adjustment

Primary air shutters are provided on all furnaces to enable adjustment of the primary air supply. The purpose of this adjust­ment is to obtain the most suitable flame characteristics.

Gas Furnaces

Figure 11-41 Applying a U-tube manometer

To gas valve. (Courtesy Dunham-Bush, Inc.)

Experience has shown that the most efficient burner flame for natural gas has a soft blue cone without a yellow tip. For pro­pane gas, there should be a little yellow showing in the tips of the flame.

The procedure for adjusting the primary air on a Coleman gas furnace may be summarized as follows:

1. Light the pilot burner.

2. Turn up the setting on the room thermostat until the main burners come on.

3. Allow the main burners 10 minutes to warm up.

4. Loosen locknut on adjusting screw (Figure 11-42).

5. Turn adjusting screw in (clockwise) until the yellow tip appears in the flame.

6. Turn adjusting screw out (counterclockwise) until the yellow tip just disappears.

7. Hold adjusting screw and tighten locknut.

8. Repeat Steps 4 through 7 on each of the other burners.

Gas Furnaces

Figure 11-42 Position of the locknut and adjusting screw on a

Coleman gas furnace. (Courtesy Coleman Co., Inc.)

On a Fedders gas furnace, a locking screw is located above the primary air opening on each gas burner (Figure 11-43). The pri­mary air adjustment procedure is as follows:

1. Loosen the locking screw at the base of the burner.

2. Adjust the air-shutter opening to a position that gives a slight yellow tip on the end of the flame.

3. Open the air shutter until the yellow tip just disappears.

4. Tighten the locking screw.

5. Repeat Steps 1 through 4 for each of the burners.

A primary air-shutter assembly is provided on all Carrier gas fur­naces in order to simplify the adjustment procedure. As shown in Figure 11-44, adjustment of the end burner results in a simultane­ous adjustment of all burners.

After the primary air adjustments have been made, check to see that the burners are level. The burner flames should be uniform and centered in the heat exchanger (see Heat Exchanger in this chapter).

Gas Furnaces

Figure 11-43 Primary air adjustment on a Fedders gas

Furnace. (Courtesy Fedders Corp.)

Before allowing the furnace to continue operating, you must check to see that it has the proper draft. This can be done by pass­ing a match along the draft-hood opening. If the vent is drawing properly, the match flame will be drawn inward (that is, into the draft hood). If the furnace is not receiving proper draft, the prod­ucts of combustion escaping the draft hood will extinguish the flame. The draft must be corrected before the furnace is operated.

Gas Supply Piping

The gas supply piping (also referred to as the gas service piping) must be sized and installed in accordance with the recommendations con­tained in local codes and ordinances, or, if unavailable, the provisions

Gas Furnaces

OUTBOARD BURNERADJUSTS ALL BURNERS SIMULTANEOUSLY Figure 11-44 Primary air-shutter assembly on Carrier

Furnace. (Courtesy Carrier Corp.)

Gas Furnaces

Figure 11-45 Location of manual valve on horizontal gas supply

Line. (Courtesy Carrier Corp.)

Of the latest edition of the National Gas Code (ANSI Z223.1) and the regulations of the National Fire Protection Association (ANSI/NFPA70). The location of the main shutoff valve will usually be specified by local codes or regulations. Consult the local codes or regulations for recommended sizes of pipe for the required gas vol­umes. Local codes or regulations always take precedence over national standards when there is a conflict between the two.

Generally, 1-in supply pipe is adequate for furnace inputs up to 125,000 Btu/h. A lVi-in pipe is recommended for higher inputs. The inlet gas supply pipe size should never be smaller than Vi2 in. The gas line from the supply should serve only a single unit.

A drip leg (also called a dirt leg or a dirt trap) should be installed at the bottom of the gas supply riser to collect moisture or impurities carried by the gas. A manual main shutoff valve should be installed either on the gas supply riser or on the horizontal pipe between the riser and ground union joint (Figures 11-45 and 11-46). The location of the main shutoff valve will usually be specified by local codes and regulations.

As shown in Figure 11-45, the ground union joint should be installed between the manual main shutoff valve and the gas control valve on the furnace or boiler. The ground union joint provides easy access to the gas controls on the unit for servicing or repair.

Sizing Gas Piping

Gas Furnaces

Figure 11-46 Location of union and drip leg for connecting furnace to main gas supply line.

подпись: 
figure 11-46 location of union and drip leg for connecting furnace to main gas supply line.
The gas supply pipe should be sized per volume of gas used and allowable pressure drop. The pipe must also be of ade­quate size to prevent undue pressure drop. The diameter of the supply pipe must at least equal that of the manual shutoff valve in the supply riser. Minimum acceptable pipe sizes are listed in Table

11-3.

The volume rate of gas in cubic feet per hour is deter­mined by dividing the Btu/h required by the furnace by the Btu per cubic foot of gas being used. The cubic feet per hour value can then be used to determine pipe size for gas of a specific gravity other than

0. 60. The data used for these

Table 11-3 Capacity of Pipe (ft3/h) (Pressure Drop 0.3—Specific Gravity 0.60)

Length of Pipe (feet)

Diameter of Pipe (inches)

3/4

1

V/4

I’/2

15

172

345

750

1220

30

120

241

535

850

45

99

199

435

700

60

86

173

380

610

75

77

155

345

545

90

70

141

310

490

105

65

131

285

450

120

120

270

420

Specific Gravity

Multiplier

Specific Gravity

Multiplier

0.40

0.813

1.30

1.47

0.50

0.910

1.40

1.53

0.60

1.00

1.50

1.58

0.70

1.08

1.60

1.63

0.80

1.15

1.70

1.68

0.90

1.22

1.80

1.73

1.00

1.29

1.90

1.77

1.10

1.35

2.00

1.83

1.20

1.42

Courtesy Janitrol

Calculations are listed in Tables 11-3 and 11-4. The procedure is as follows:

1. Multiply the cubic feet per hour required by the multiplier (Table 11-4).

2. Find recommended pipe size for length of run in Table 11-3.

Tables 11-3 and 11-4 apply to the average piping installation where four or five fittings are used in the gas piping to the furnace. Existing pipe should be converted to the proper size of pipe where necessary. In no case should pipe less than 334-in diameter be used. Any extensions to existing piping should conform to Tables 11-3 and 11-4.

Installing Gas Piping

The principal recommendations to be followed when installing gas piping are:

1. A single gas line must be provided from the main gas supply to the furnace.

2. The gas supply pipe must be sized according to the volume of gas used and the allowable pressure drop.

3. The supply riser and drop leg must be installed adjacent to the furnace on either side. Never install it in front of the furnace, where it might block access to the unit.

4. A manual main shutoff valve or plug cock should be installed on the gas supply piping (usually on the riser) outside the unit in accordance with local codes and regulations.

5. Install a tee for the drop leg in the pipe at the same level as the gas supply (inlet) connection on the furnace.

6. Extend the drip leg and cap it.

7. Install a ground union joint between the tee and the furnace controls.

8. Use a pipe compound resistant to LP gas on all threaded pipe connections in an LP gas installation.

9. Test for pressure and leaks. Use a solution of soap or deter­gent and water to detect any leaks. Never check for leaks with a flame.

Typical Startup Instructions for a Standing-Pilot Gas Furnace

The startup procedure for most standing-pilot gas furnaces is very similar, particularly insofar as use of the thermostat is concerned. The major difference is in the lighting of the pilot burner, and this will depend on the gas valve used.

Caution

Always carefully follow the furnace manufacturer’s lighting and operating instructions when attempting to start a furnace. Failure to do so may cause a fire or explosion resulting in pos­sible injury or even death.

The startup procedure for a standing-pilot gas furnace may be summarized as follows:

1. Open all warm air registers.

2. Shut off main gas valve and pilot gas cock, and wait at least 5 minutes to allow gas that may have accumulated in the burner compartment to escape.

Warning

If you smell gas after waiting 5 minutes, immediately call your local gas company (utility) from a telephone outside the house and follow their instructions.

3. Set the room thermostat at the lowest setting.

4. Open the water supply valve if a humidifier is installed in the system.

5. Turn off the line voltage switch if one is provided in the fur­nace circuit.

6. Light the pilot (see below).

7. Turn electrical power on.

8. Replace all access doors.

9. Light main burners by setting the room thermostat above the room temperature.

10.Set the room thermostat at the desired temperature after the main burners are lighted.

The pilot lighting instructions for a standing-pilot gas furnace should be included with the furnace and prominently displayed. Lighting instructions for the 24-volt Honeywell V8280, V8243, and Robertshaw (Unitrol) 7000 combination gas valves basically summarize the pilot-lighting procedure (Figure 11-47). The proce­dure is as follows:

1. Depress gas cock dial and turn it counterclockwise to “pilot” position.

2. Keep gas cock dial depressed and light pilot with a match.

3. Keep dial depressed until pilot remains lighted when released.

4. Depress dial and turn counterclockwise to ON position.

REGULATOR ADJUSTMENT

Gas Furnaces

Figure 11-47 Robertshaw Unitrol 7000 series combination gas

Valve. (Courtesy Robertshaw Controls Co.)

Typical Startup Instructions for an Electronic Ignition Furnace

The modern high-efficiency gas furnace uses an electronic ignition system instead of a standing pilot to light the gas burners. The igni­tion device automatically lights the burners. Do not try to light the gas burners manually in these furnaces.

On gas furnaces equipped with electronic ignition systems, there is a sequence of safety steps that must be followed to light the burner. This information is normally attached to the inside of the burner or blower access door. Follow the procedure according to the manufacturer’s guidelines to avoid the risk of fire or explosion. If you have any doubts about the startup procedure, call a qualified technician to start the furnace.

A typical procedure for starting a furnace equipped with an elec­tronic ignition system is offered here simply as a guideline. The steps are as follows:

1. Set the room thermostat to its lowest possible setting.

2. Turn off all electrical power to the furnace.

3. Remove cabinet panel to gain access to the main gas valve.

4. Turn gas valve knob to the OFF position.

Caution

Turn the knob manually. Do not force it, do not use a tool to turn it, and do not attempt to repair it. Doing so may result in a fire or explosion with possible injury or death. If it cannot easily be moved manually, call a qualified HVAC technician for service.

5. Wait 5 minutes for any gas to clear out of the burners and furnace.

Warning

If you smell gas after waiting 5 minutes, immediately call your local gas company (utility) from a telephone outside the house and follow their instructions.

6. If no gas odor has been detected, turn the gas valve knob to the ON position. Do not force the knob.

7. Replace the access panel and restore all electrical power to the furnace.

8. Adjust the thermostat for the desired room temperature setting.

9. Replace the cabinet access panel.

10.Repeat steps 1 through 9 if air needs to be purged from the pilot line.

If the furnace still will not start, shut it off and call a qualified HVAC technician for assistance. The furnace shutoff procedure is as follows:

1. Set the room thermostat to its lowest possible setting.

2. Turn off all electrical power to the furnace.

3. Remove the access panel to the main gas valve.

4. Turn the gas valve knob to the OFF position. Do not force the knob to turn.

5. Replace the access panel.

Blowers and Motors

A forced-warm-air furnace should be equipped with a variable­speed electric motor designed for continuous duty. The motor should be provided with overcurrent protection in accordance with the National Electrical Code (ANSI C1-1971).

Blower motors are available as either direct-drive or belt-driven units. On direct-drive units, the blower wheel is mounted directly on the motor shaft and runs at motor speed. The blower wheel of a belt-driven unit is operated by a V-belt connecting a fixed pulley mounted on the blower shaft with a variable-pitch drive pulley mounted on the motor shaft.

The blower motors used in residential furnaces are single-phase electric motors. Depending on the method used for starting, they may be classified as:

1. Shaded pole

2. Permanent split capacitor

3. Split-phase

4. Capacitor-start motors

A direct-drive blower generally uses a shaded pole (SHP) or per­manent capacitor (PSC) motor. Belt-driven blower motors are usu­ally split-phase (SPH) types up to the V3-hp size. Larger motors are capacitor-start, split-phase motors.

Blower motors are fractional horsepower motors commonly available in the following sizes: V12, Wo, V6, W, V3, W, and % hp. A

Fractional horsepower motor should be protected with tempera­ture — or current-sensitive devices to prevent motor winding temper­atures from exceeding those allowed in the specifications.

Provisions should be made for the periodic lubrication of the blower and motor. Use only the type and grade of lubricant speci­fied in the operating instructions. Do not overlubricate. Too much oil can be just as bad as too little. A direct-drive blower should be capable of operating within the range of air temperature at the static pressures for which the furnace is designed and at the voltage specified on the motor nameplate.

Motors using belt drives must be supplied with adjustable pul­leys. The only exception is a condenser motor used on a forced — warm-air furnace with a cooling unit. Always provide the means for adjusting a belt-driven motor in an easily accessible location.

Most forced-warm-air furnaces are shipped with the blower motors factory-adjusted to drive the blower wheel at a correct speed for heating. This is a slower speed than the one used for air conditioning, but it is relatively easy to alter the air delivery to meet the requirements of the installation (see Air Conditioning in this chapter).

Air Delivery and Blower Adjustment

The furnace blower is used to force air through the heat exchanger in order to remove the heat produced by the burners. This removal of heat by the blower serves a twofold purpose: (1) It distributes the heat through the supply ducts to the various spaces in the structure; and (2) it protects the surfaces of the heat exchanger from overheating.

Most blowers are factory-set to operate with a specific air tem­perature rise. For example, Janitrol Series 37 gas-fired furnaces are certified by the American Gas Association to operate within a range of 70 to 100°F air temperature heat rise. In other words, the tem­perature of the air within the supply ducts may be 70 to 100°F higher than the air temperature in the return duct. A temperature rise of 85 to 90°F with approximately 0.1-in external pressure in the area beyond the unit and filter is generally considered adequate for most residential applications.

The furnace air delivery rate should be adjusted at the time of installation to obtain a temperature rise within the range specified on the furnace rating plate. Many furnace manufacturers recom­mend that the temperature rise be set below the maximum to ensure that temperature rise will not exceed the maximum require­ment should the filter become excessively dirty.

Although the blower and motor are factory-set to operate within a specific temperature-rise range, the unit can be field adjusted to deliver more or less air as required. The air delivery rate is adjusted by changing the blower speed. On direct-drive motors, this is accomplished by moving the line lead to the desired terminal. Belt — driven motors are adjusted by changing the effective diameter of the driver pulley (see Direct-Drive Blower Adjustment and Belt­Drive Blower Adjustment).

Direct-Drive Blower Adjustment

The blower (fan) of a direct-drive unit is operated by a drive shaft connected to the motor. Three common direct-drive motors used with blowers are:

1. Two-speed, direct-drive shaded pole motors

2. Three-speed, direct-drive PSC motors

3. Four-speed, direct-drive PSC motors

These blower motors are shipped from the factory set for low — speed heating operation. If a higher operating speed is desired, the red blower motor wire must be moved from the low-speed red ter­minal to a higher speed terminal.

Belt-Drive Blower Adjustment

Air-temperature rise can be decreased by increasing the blower speed. A belt-drive unit can be adjusted for a higher speed as follows:

1. Loosen the set screw in the movable outer flange of the motor pulley.

2. Reduce the distance between the fixed flange and the movable flange of the pulley.

3. Tighten the set screw.

The procedure for decreasing the blower speed (and thereby increasing the air-temperature rise) is similar except that in Step 2 above, the distance between the fixed flange and the movable flange of the pulley are increased.

The motor and blower pulley should be aligned to minimize belt wear. The belt should be parallel to the blower scroll. Adjustments can be made by moving the motor or adjusting the pulley on the shaft.

Correct belt tension is also important. Belt slippage occurs when there is too little tension. Too much tension will cause motor over­load and bearing wear. Correct belt tension will usually be indicated

Gas Furnaces

Figure 11-48 Checking belt tension and alignment.

By the amount of depression in the belt midway between the blower and motor pulleys. Usually this will be a 1-in depression at the cen­ter of the belt, but it may vary among different furnace manufactur­ers. Check the furnace manufacturer’s specifications and service manual for the preferred amount of belt tension (Figure 11-48).

Air Filters

A forced-warm-air, gas-fired furnace is supplied with either a dis­posable air filter or a permanent (washable) one. The type of filter will be indicated on a label attached to the filter. Never use a filter with a gravity warm-air furnace, because it will obstruct the airflow.

Proper maintenance of the air filter is very important to the operating efficiency of the furnace. A dirty, clogged air filter reduces the airflow through the furnace and causes air temperatures inside the unit to rise. This increase in air temperature results in a reduced life for the heat exchanger and a lower operating efficiency for the furnace.

If a disposable air filter is used, it should be inspected on a monthly basis. If it is dirty, it should be replaced. This is usually done at the beginning and middle of the heating season. The same schedule should be followed during the summer months when air conditioning is used.

If a permanent filter is provided, it must be removed and cleaned from time to time. Cleaning the filter involves the following procedure:

1. Shake it to remove dust and dirt particles.

2. Vacuum it.

3. Wash it in a solution of soap or detergent and water.

4. Allow time for it to dry.

5. Replace it in the furnace.

Read the label on the filter for any special instructions that may apply to cleaning it. The location of filters in upflow and downflow furnaces is shown in Figures 11-49 and 11-50. Note that, in each case, the air filter is placed directly in the path of the air entering the furnace.

A filter should be installed in the return plenum or duct when air conditioning is added to the system. Because air conditioning nor­mally requires a greater volume of air than heating, the filter should be sized for air conditioning rather than heating.

Access to internally installed filters is generally provided through a service panel on the furnace. Instructions for cleaning or replacing the filters should be placed on the service panel. It would be a good idea to include the dimensions of the replacement filter(s) in the information.

An adequate filter should provide at least 50 in2 of area per cfm of air circulated. For an air velocity of 1000 cfm, this would mean a filler area of 400 in2. Filters sized on 50 in2 for each 100 cfm meet the requirements of air conditioning—a greater volume of air than heating.

Never use a smaller-size filter than the one required in the speci­fications. To do so would create excessive pressure drop at the filter, which would reduce the operating efficiency of the furnace.

Additional information about air filters can be found in Chapter

13, “Air Cleaners and Filters” in Volume 3.

Air Conditioning

If you intend to install air conditioning equipment at some future date, the duct sizing should include an allowance for it. Air condi­tioning involves a greater volume of air than heating. Quite often,

T

Gas Furnaces

Gas Furnaces

The blowers and motors of the furnace will be sized for the addition of cooling with matching evaporator coil and condensing unit. Check the specifications to be sure.

All ductwork located in unconditioned areas (for example, attics, crawl spaces) downstream from the furnace should be insulated. A furnace used in conjunction with a cooling unit should be installed in parallel or on the upstream side of the evaporator coil to avoid condensation on the heating element. In a parallel installation, dampers or comparable means should be provided to prevent chilled air from entering the furnace.

Installation Checklist

A properly installed and maintained furnace will operate efficiently and economically. Figure 11-51 and the following installation checklist is offered as a guide for the installer:

1. Adjust primary air shutter.

2. Provide sufficient space for service accessibility.

3. Check all field wiring.

4. Supply line fuse or circuit breaker must be of proper size and type for furnace protection.

5. Line voltage must meet specifications while furnace is operating.

6. Airflow must be sufficient.

7. Ventilation and combustion air must be sufficient.

8. Ductwork must be checked for proper balance, velocity, and quietness.

9. Measure the manifold pressure.

10. Check all gas piping connections for gas leaks (use soap and water, not a flame).

11. Cycle the burners.

12. Check limit switch.

13. Check fan switch.

14. Adjust blower motor for desired speed.

15. Make sure air filter is properly secured.

16. Make sure all access panels have been secured.

17. Pitch air conditioning equipment condensation lines toward a drain.

18. Check thermostat heat anticipatory setting.

19. Check thermostat for normal operation.

20. Clear and clean the area around the furnace.

Gas Furnaces

Figure 11-51 Installation and maintenance checklist.

(Courtesy Robertshaw Controls Co.)

Gas Furnace Inspections, Service, and Maintenance Tips

Gas furnaces should be inspected annually, preferably before the beginning of the heating season, by a qualified service technician to ensure continued safe operation. The inspection should include the following:

Caution

Always carefully read the service and maintenance instruction for the furnace first. As a rule, the electrical power is shut off

First at the disconnect switch if service or an inspection is to be

Performed. After switching off the power, turn off the gas valve.

• Inspect the vent pipe for water accumulation, sagging pip­ing, dirt, loose joints, and damage.

• Check the return air duct for a tight connection to the fur­nace. The duct connection must provide an airtight seal at the furnace and must terminate at its other end outside the room or space where the furnace is located.

• Inspect the furnace wiring for burnt or damaged wires and loose connections.

• Inspect interior and exterior furnace surfaces for dirt or water accumulation.

• Make sure the blower access door is tightly closed.

• Inspect the burners for dirt, rust, or signs of water.

• Make sure the fresh air grilles and louvers are open, clean, and unobstructed.

• Inspect and clean the condensate traps and drain to prevent water accumulation in the furnace.

• Inspect the blower wheel and remove any debris.

• Check the furnace support and base. The base of the fur­nace must form a tight seal with the support.

If the above items all pass inspection, restore power and start the furnace according to the furnace manufacturer’s instructions. It will involve first turning on the gas valve and then turning on elec­trical power to the furnace. Continue the inspection as follows:

Caution

If you smell gas, call the local gas company immediately from

A telephone outside the house and follow their instructions.

• Run the furnace and observe its operation. The furnace should operate smoothly and quietly. Check the vent pipe and return duct to make sure they are not leaking.

• Analyze the combustion gases to make sure they meet the furnace manufacturer’s specifications.

Note

Although the hot surface igniter used in a hot surface ignition

(HSI) gas furnace may be made of a material (such as silicon

Carbon) capable of resisting the high temperatures encoun­tered when firing the gas-air mixture, these igniters are fragile and easily damaged if not handled carefully. The damage most commonly occurs during shipment from the furnace manufacturer or supplier, or during installation. The damage is not always visible to the naked eye and a cracked igniter ini­tially can function according to specifications. However, even­tually, its operating efficiency will degrade and its service life will be much shorter. Igniter damage too small to see can be discovered by checking for inconsistencies in its glow pattern immediately after installation.

Troubleshooting a Gas Furnace

Table 11-5 contains the most common operating problems associ­ated with residential and small commercial gas furnaces. Each prob­lem is given in the form of a symptom, the possible cause, and a suggested remedy. The table is intended to provide the operator with a quick reference to the cause and correction of a specific problem.

Table 11-5 Troubleshooting Gas Furnaces

Possible Remedy

подпись: possible remedySymptom and Possible Cause

No heat (standing-pilot furnace vented to chimney)

(a) Thermostat not turned to HEAT.

(b) Thermostat temperature setting above room temperature.

(c) Blown fuse.

(d) Tripped circuit breaker.

(e) Dirty filter.

(f) Faulty gas valve.

(g) Pilot light out.

(a) Turn to HEAT.

(b) Lower thermostat heat setting until furnace turns on.

(c) Replace fuse. Call electrician if problem reoccurs.

(d) Reset circuit breaker. Call electrician if problem reoccurs.

(e) Clean or replace filter.

(f) Replace gas valve.

(g) Light pilot according to the furnace manufacturer’s instructions or request a service call from an HVAC technician.

Symptom and Possible Cause

(h) Emergency power switch off.

(i) Defective thermostat.

(j) Defective thermocouple.

(k) Open plenum switch.

(l) Defective plenum switch.

(m) Open limit switch (plenum or exhaust).

(n) Blocked flue, vent, or chimney.

No heat (high-efficiency furnace vented through PVC pipe)

(a) Thermostat not turned to HEAT.

(b) Thermostat temperature setting above room temperature.

(c) Blown fuse.

(d) Tripped circuit breaker.

(e) Dirty filter.

(f) Defective thermostat.

(g) Defective hot surface igniter.

(h) Faulty spark igniter.

(i) Faulty or tight blower motor.

(j) Defective fan relay.

(k) Defective plenum switch.

(l) Blocked or restricted PVC vent outlet.

(m) Blocked or restricted PVC intake opening.

(n) Defective gas valve.

(o) Open limit switch (plenum or exhaust).

(p) Defective pressure switch.

(q) Blower door off, open, or not properly closed.

Possible Remedy

(h) Turn switch on.

(i) Replace thermostat.

(j) Replace thermocouple. (k) Close.

(l) Replace switch.

(m) Close limit switch or replace.

(n) Locate and remove obstruction.

(a) Turn to HEAT.

(b) Lower thermostat heat setting until furnace turns on.

(c) Replace fuse. Call electrician if problem reoccurs.

(d) Reset circuit breaker. Call electrician if problem reoccurs.

(e) Clean or replace filter.

(f) Replace thermostat.

(g) Replace hot surface igniter.

(h) Replace spark igniter.

(i) Repair or replace blower motor.

(j) Replace fan relay.

(k) Replace plenum switch.

(l) Locate and clear blockage.

(m) Locate and clear blockage.

(n) Replace gas valve.

(o) Close limit switch or replace.

(p) Replace pressure switch.

(q) Close door firmly and secure.

Table 11-5 (continued) Symptom and Possible Cause Possible Remedy

Not enough heat

(a) Thermostat set too low.

(a)

Raise setting.

(b) Lamp or some other heat source

(b)

Move heat source away

Too close to thermostat.

From thermostat.

(c) Thermostat improperly located.

(c)

Relocate thermostat.

(d) Dirty air filter.

(d)

Clean or replace.

(e) Thermostat out of calibration.

(e)

Recalibrate or replace.

(f) Limit set too low.

(f)

Reset or replace.

(g) Fan speed too low.

(g)

Check motor and fan belt and tighten if too loose.

Too much heat

(a) Thermostat set too high.

(a)

Lower setting.

(b) Thermostat out of calibration.

(b)

Recalibrate or replace.

(c) Short in wiring.

(c)

Locate and correct.

(d) Valve sticks open.

(d)

Replace valve.

(e) Thermostat in draft or on

(e)

Relocate thermostat to

Cold wall.

Sense average temperature.

(f) Bypass open. Flame too large

(f)

Close bypass.

(a) Pressure regulator set too high.

(a)

Reset using manometer.

(b) Defective regulator.

(b)

Replace.

(c) Burner orifice too large.

(c)

Replace with correct size.

Noisy flame

(a) Too much primary air.

(a)

Adjust air shutters.

(b) Noisy pilot.

(b)

Reduce pilot gas.

(c) Burr in orifice.

(c)

Remove burr or replace orifice.

Yellow Tip flame

(a) Too little primary air.

(a)

Adjust air shutters.

(b) Clogged burner ports.

(b)

Clean ports.

(c) Misaligned orifices.

(c)

Realign.

(d) Clogged draft hood.

(d)

Clean.

Floating flame

(a) Blocked venting.

(b) Insufficient primary air.

(a) Clean.

(b) Increase primary air supply.

Table 11-5 (continued) Symptom and Possible Cause Possible Remedy

Delayed Ignition

(a) Improper pilot location.

(a)

Reposition pilot.

(b) Pilot flame too small.

(b)

Check orifice; clean,

Increase pilot gas.

(c) Burner ports clogged near.

(c)

Clean ports.

(d) Low pressure.

(d)

Adjust pressure regulator.

Failure to ignite

(a)

Pilot light off.

(a)

Light pilot according to the furnace manufacturer’s instructions or request a service call from an HVAC technician.

(b)

Main gas supply off.

(b)

Open manual valve.

(c)

Blown fuse.

(c)

Replace fuse. Call an electrician if the problem reoccurs.

(d)

Circuit breaker tripped.

(d)

Reset circuit breaker. Call an

Electrician if the problem reoccurs.

(e)

Limit switch defective.

(e)

Replace.

(f)

Poor electrical connections.

(f)

Check, clean, and tighten.

(g)

Defective gas valve.

(g)

Replace.

(h)

Defective thermostat.

(h)

Replace.

Burner will not turn on

(a)

Pilot flame too large or too small. (a)

Readjust.

(b)

Dirt in pilot orifice.

(b)

Clean.

(c)

Too much draft.

(c)

Shield pilot.

(d)

Defective automatic.

(d)

Replace.

(e)

Defective thermocouple.

(e)

Replace.

(f)

Improper thermocouple.

(f)

Properly position thermopile.

(g)

Defective wiring.

(g)

Check connections; tighten and repair shorts.

(h)

Defective thermostat.

(h)

Check for switch closure and repair or replace.

(i)

Defective automatic valve.

(i)

Replace.

Burner will not turn off

(a) Poor thermostat location.

(a)

Relocate.

Symptom and Possible Cause

(b) Defective thermostat.

(c) Limit switch maladjusted.

(d) Short circuit.

(e) Defective or sticking automatic valve.

Rapid burner cycling

(a) Clogged filters.

(b) Excessive anticipation.

(c) Limit setting too low.

(d) Poor thermostat location.

Rapid fan cycling

(a) Fan switch differential too low.

(b) Blower speed too high.

Blower will not stop

(a) Manual fan on.

(b) Fan switch defective.

(c) Shorts.

Noisy blower and motor

(a) Fan blades loose.

(b) Belt tension improper.

(c) Pulleys out of alignment.

(d) Bearings dry.

(e) Defective belt.

(f) Belt rubbing.

Blower will not run

(a) Power not on.

(b) Fan control adjustment.

(c) Loose wiring.

(d) Defective motor overload, protector, or motor.

(e) Defective or tight blower motor.

Possible Remedy

(b) Check calibration; check switch and contacts; replace.

(c) Replace.

(d) Check operation at valve; check for short and correct.

(e) Clean or replace.

(a) Clean or replace.

(b) Adjust thermostat anticipatory for longer cycles.

(c) Readjust or replace limit.

(d) Relocate.

(a) Readjust or replace.

(b) Readjust to lower speed.

(a) Switch to automatic.

(b) Replace.

(c) Check wiring and correct.

(a) Replace or tighten.

(b) Readjust (usually allow 1 in slack).

(c) Realign.

(d) Lubricate.

(e) Replace.

(f) Reposition.

(a) Check power switch; check fuses and replace if necessary.

(b) Readjust or replace.

(c) Check and tighten.

(d) Replace motor.

(e) Repair or replace.

• Check the thermostat to make sure it is set higher than the actual room temperature. If you have a programmable thermostat, make sure it has fresh batteries.

• Make sure the selector switch is on heat if the system is equipped with central air or if the system is zoned.

• Check the emergency switch (usually a red switch plate at the top of the cellar stairs or on the side of the furnace) to see that it is on.

• If you are familiar with the fuse or circuit breaker panel, see whether the fuse is burned or the breaker tripped. Correct the problem at once. If it repeats, call a serviceman.

• On standing-pilot furnaces, the burner will not light if the pilot has gone out. If you are not familiar with the function of the gas valve or lighting the pilot, call for service.

• Do not disconnect any piping to check for gas supply. An instrument is used to check for pressure. This should be done by a qualified serviceman.

• If the furnace is vented through PVC (white plastic pipe) out the side of the building, examine the ends of the pipe or pipes outside. Blockage of any kind will cause a shutdown. Many high-efficiency, condensing furnaces vented through PVC pipe are self-diagnostic. These furnaces have a steady burning red light. If the light starts blinking, the furnace is indicating there is a problem that requires attention.

Troubleshooting Charts

Some manufacturers include troubleshooting charts in their furnace manuals. A typical gas furnace troubleshooting chart for a Thermo Pride gas furnace (Thermo Products, LLC) is illustrated in Figure

11- 52. These troubleshooting charts are organized in a yes/no for­mat, guiding the technician through a list of steps that eventually leads to the specific operating problem and its remedy.

Gas Furnaces

Figure 11-52 Gas furnace troubleshooting chart. (Courtesy Thermo Pride)

Gas Furnaces

Figure 11-52 (continued).

Gas Furnaces

Figure 11-52 (continued).

Yes

Gas Furnaces

Figure 11-52 (continued).

IF LED LIGHT FLASHES:

1 FLASH, THEN PAUSE

SYSTEM LOCKOUT

2 FLASHES, THEN PAUSE

PRESSURE SWITCH STUCK CLOSED

3 FLASHES, THEN PAUSE

PRESSURE SWITCH STUCK OPEN

4 FLASHES, THEN PAUSE

OPEN LIMIT SWITCH OR ROLLOUT SWITCH

6 FLASHES, THEN PAUSE

115 VOLT AC POWER REVERSED

7 FLASHES, THEN PAUSE

LOW FLAME SENSE SIGNAL

8 FLASHES, THEN PAUSE

CHECK IGNITOR OR IMPROPER GROUNDING

CONTINUOUS FLASHING FLAME HAS BEEN SENSED WHEN

(NO PAUSE) NO FLAME SHOULD BE PRESENT (NO

CALL FOR HEAT)

THE LED WILL ALSO FLASH ONCE AT POWER-UP.

CHECK COMPLETE SYSTEM OUT LED LIGHT STAYS ON CONTINUOUSLY.

COMPLETE FAILURE — REPLACE INTEGRATED CONTROL.

TROUBLESHOOTING COMPLETE.!

Figure 11-52 (continued).

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers


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