Electric Furnaces

Electric heating is the only heat produced almost as fast as the ther­mostat calls for it. It is nearly instantaneous because there are no heat exchangers to warm up. The heating elements start producing heat the moment the thermostat calls for it. Unlike electric base­board heating systems, electric furnaces are designed to provide the same heating/cooling advantages as gas or oil furnaces.

Because there is no flame with electric heat, there is no need to vent smoke or flue gases to the outside. Furthermore, there is no chimney loss with electric heat. It is 100 percent efficient, compared with the 60 to 95 percent efficiency of furnaces using other fuels.

Electric furnaces are available in upflow, downflow, or horizontal — flow models and in a wide range of sizes. For example, Carrier elec­tric furnaces are available in 15 standard models from 5 to 35 kW in 5-kW increments. Other manufacturers offer a similar range of models (Figures 14-1 and 14-2).

An electric furnace should be listed by Underwriters Laboratories, Inc. (UL) for construction and operating safety. Furnaces approved by the agency are marked UL Approved.

Electric furnaces should be installed in accordance with local codes and regulations, the National Electrical Code, and recom­mendations made by the National Fire Protection Association.

Electrical Power Supply

Contact the local power company and make certain adequate elec­trical service is available for the furnace load plus all other appli­ances that will be on the line.

Check the National Electrical Code and the local code require­ments. All wiring (including sizing) must comply with the require­ments of these codes. When there is any conflict, the local codes and regulations take precedence. The manufacturer’s requirements are also important. For example, Janitrol discourages the use of aluminum wire, although it is acceptable to the National Electrical Code. Use of aluminum wire in this case could jeopardize the fur­nace warranty.

Be sure to consult the manufacturer’s wiring diagrams for electri­cal power requirements. Read the instructions carefully, and be sure you understand them thoroughly before you begin work.

CONTINUOUS AIR CIRCULATION SWITCH

Electric Furnaces

Figure 14-1 Lennox model E10 upflow electric

Furnace. (Courtesy Lennox Air Industries Inc.)

Planning Suggestions

If an electric furnace is being planned for a new structure, the max­imum heat loss for each heated space must be calculated in accor­dance with procedures described in the manuals of the National Warm Air Heating and Air Conditioning Association or by a com­parable method. This is very important, because these data will be used to determine the size (capacity) of the furnace selected for the installation.

Do not consider an electric heating system unless the structure is insulated properly. This insulating will be more extensive than that used with other types of heating systems. For example, ceilings should have a minimum of 6 in of blanket or loose-fill insulation, and cavities between studs in exterior walls should be filled with insulation completely. A description of the insulation requirements for a structure in which an electric heating system is used is included in Chapter 9, “Electric Heating Systems.”

If the heating or heating/cooling installation is to be approved by either the Federal Housing Authority (FHA) or the Veterans’ Administration (VA), heat loss and heat gain calculations should be

Electric Furnaces

Figure 14-2 Carrier forced-warm-air electric furnace. (Courtesy Carrier Corp.)

Made in accordance with the procedures described in the Air Conditioning Contractors of America’s Manual J.

Location and Clearance

An electric forced-warm-air furnace should be located as near as possible to the center of the heat distribution system. Centralizing the furnace eliminates the need for one or more exceptionally long supply ducts. Long supply ducts are uneconomical because they are subject to a certain amount of heat loss. The number of elbows should be kept to a minimum for the same reason.

Electric furnaces are not vented, because electric heat is not pro­duced by the combustion process. No flue gases or other toxic products of the combustion process occur with electric heat. As a result, a chimney and flue pipe are not required, and it is not neces­sary to consider these factors when locating the furnace.

A clearance of 24 to 30 inches in front of the furnace access panel should be provided for servicing and repairs. There is no min­imum clearance requirement for ductwork and combustible materi­als. Electric furnaces may be installed with zero clearance between the cabinet and combustible materials, because the heat from the furnace is not produced by a flame.

Installation Recommendations

New electric furnaces for residential installation are shipped from the factory with all internal wiring completed. These furnaces are also generally shipped as a preassembled unit. In order to install the new furnace, the electric service from the line voltage main and the low-voltage thermostat must be connected. Directions for mak­ing these connections are found in the furnace manufacturer’s installation instructions.

Familiarize yourself with all local codes and regulations that govern the installation of an electric furnace. Local codes and regu­lations take precedence over national standards.

Check the insulation of the structure to determine whether it is properly insulated for electric heat. The insulation should be installed in accordance with recommendations in All Weather Comfort Standard of Electrically Heated and Air Conditioned Homes (Electric Heating Association).

The furnace should be mounted on a level surface. If the unit is not level, it may develop serious vibrations. An insulating material can be placed under the furnace in most installations to reduce sound vibrations when the unit is operating. A noncombustible base is recommended for counterflow models.

Duct Connections

Detailed information concerning the installation of an air-duct sys­tem is contained in the following two publications of the National Fire Protection Association:

1. Installation of Air Conditioning and Ventilating Systems of Other Than Residence Type (NFPA No. 90A).

2. Residence Type Warm Air Heating and Air Conditioning Systems (NFPA No. 90B).

Additional information about duct connections can be found in Chapter 7, “Ducts and Duct Systems” of Volume 2. The comments made in Chapter 11, “Gas Furnaces” about furnace duct connec­tions and air distribution ducts apply for the most part to ducts used with electric forced-warm-air furnaces.

Basic Components

An electric forced-warm-air furnace will generally consist of the fol­lowing basic components (Figure 14-3):

1. Automatic controls

2. Heating elements

3. Safety controls

HEATING ELEMENT

подпись: heating element

HIGH LIMIT CONTROLS

подпись: high limit controls

TRANSFORMER

подпись: transformer

FUSES

подпись: fuses

TERMINAL

BLOCKS

подпись: terminal
blocks

FAN AND MOTOR

подпись: fan and motor Electric FurnacesUNIT WIRING UPPER DIAGRAM ACCESS DOOR

4. Blowers and motors

5. Air filter(s)

Some electric furnaces also include an electronic air cleaner, an air conditioning evaporator coil, a humidifier, or some combination of these accessories. An electric heating/cooling system may also include a condensate pump if the dehumidifying process produces excessive amounts of water. In a zoned electric heating system, a zone control panel and motor-actuated dampers will be attached to either the furnace or the ducts.

Each of these components is described in the sections that fol­low. Additional information is contained in Chapter 10, “Furnace Fundamentals” and the various chapters in which furnace controls are described.

Automatic Controls

The automatic controls used in an electric heating system are designed to ensure its safe and efficient operation. Detailed descrip­tions of these controls are found in Chapter 4, “Thermostats and Humidistats” of Volume 2. This section is primarily concerned with outlining the operating principles of the automatic controls used with an electric furnace. These controls include:

1. Room thermostat

2. Thermostat heat anticipator

3. Timing sequences

In a central heating system, the wall-mounted room thermostat is the control that governs the normal operation of the furnace. The operating principle is simple. The temperature selector on the ther­mostat is set for the desired temperature. When the temperature in the room falls below this setting, the thermostat will call for heat and cause the first heating circuit in the furnace to be turned on. There is generally a delay of about 15 seconds before the furnace blower starts. This prevents the blower from circulating cool air in the winter. After about 30 seconds, the second heating circuit is turned on. The other circuits are turned on one by one in timed sequence.

Note

Use a heat pump thermostat or a conventional thermostat containing an electric setting to operate an electric furnace.

Electric Furnaces

Figure 14-4 Typical room thermostats used in central electric

Heating systems. (Courtesy Lennox Industries Inc.)

Electric FurnacesWhen the temperature reaches the required level, the thermostat opens. After a short time, the first heating circuit is shut off. The others are shut off one by one in timed sequence. The blower will continue to operate until the air temperature in the furnace drops below a specified temperature.

A typical room thermostat will have a fan switch, a system switch, and a temperature selector (Figure 14-4). The temperature selector (a dial or lever device) on the thermostat is used to select the desired temperature. The actual operation of the heating system is governed by the positions of the fan and system switches. The switch positions and their functions are listed in Tables 14-1 and 14-2.

Most room thermostats contain a heat anticipator. This is a device designed to assist the thermostat in controlling closer to the desired temperature range (Figure 14-5).

When timing sequences are used (see below), the current flowing through the first time-delay sequencer (relay) must also flow through the heat anticipator. In order to obtain satisfactory opera­tion, the heat-anticipator setting must be equal to the current draw of the sequencer.

The furnace manufacturer will generally recommend the setting for the heat-anticipator adjustment for each size unit. For example,

Table 14-1 Thermostat Operation in Single-Stage Heating/Two-Stage Cooling;Two-Stage Heating/Single-Stage Cooling;Two-Stage Heating/Two-Stage Cooling

Thermostat Switch Setting

Fan System Function

Auto

Off

System completely shut down. Blower only, continuous operation.

On

Off

Provides air circulation when no cooling or heating is desired.

Auto

Cool

Blower and cooling system cycle on and off as thermostat demands.

On

Cool

Blower runs continuously; cooling system cycles on and off as thermostat demands.

Auto

Heat

Blower and furnace cycle on and off as thermostat demands.

On

Heat

Blower runs continuously; furnace cycles on and off as thermostat demands.

Auto

Auto

Unit cycles for either cooling or heating as per demand of thermostat.

On

Auto

Blower runs continuously; cooling

And heating cycle as per demand of thermostat.

Courtesy Fedders Corporation

The setting recommended for a Trane Model EUADH 07 electric furnace is 0.45. This thermostat adjustment will vary depending upon the type of time-delay sequencer used, the furnace manufac­turer, and the size of the furnace. This may be illustrated by the recommended heat-anticipator settings given by Coleman for its

10- kW, 15-kW, and 20-kW furnace models (Table 14-3). All Coleman 25-kW models require a heat-anticipator setting of 0.60.

After you have adjusted the heat anticipator to the suggested set­ting, operate the furnace several hours and observe the results. If there is insufficient heat, it may be caused by short furnace cycles. This can be corrected by moving the heat-anticipator pointer to a slightly higher setting. If there is too much heat, then long furnace cycles are overheating the structure. This can be corrected by mov­ing the heat-anticipator pointer to a slightly lower setting. After

Fan

System

Function

Auto

Off

System completely shut down. Blower only, continuous operation.

On

Off

Provides air circulation when no cooling or heating is desired.

Auto

Cool

Blower and cooling system cycle on and off as thermostat demands.

On

Cool

Blower runs continuously; cooling system cycles on and off as thermostat demands.

Auto

Heat

Blower and furnace cycle on and off as thermostat demands.

Heat

On

Blower runs continuously; furnace cycles on and off as thermostat demands.

Table 14-2 Thermostat Operation in Single-Stage Heating/Cooling

Setting

Courtesy Fedders Corporation

Thermostat Switch

подпись: thermostat switchMaking these thermostat adjustments, allow the furnace to operate several hours to determine whether further adjustment is required. Additional information about the thermostat heat anticipator is contained in Chapter 11, “Gas Furnaces.”

Electric Furnaces

HEAT-ANTICIPATOR ADJUSTMENT

Table 14-3 Recommended Heat-Anticipator Settings for Coleman 10-, 15-, and 20-kW Electric Furnaces

Sequencer

10 kW

15 kW

20 kW

White-Rogers

0.15

0.15

0.10

Klixon

0.15

0.30

Honeywell

0.40

0.40

Courtesy The Coleman Company, Inc.

Heating Elements

A heating element must operate at a temperature well above its sur­roundings to deliver heat at a useful high rate. If it is a radiant heat­ing element, it may operate red-hot or nearly white-hot over a relatively long period of time.

Wires with a high resistance, such as iron, chromium, nickel, man­ganese, and their alloys, are commonly used for heating elements. The heat output of a wire can be varied by changing its material composition or by changing its size or diameter. A smaller-diameter wire will have a higher resistance than a larger-diameter wire.

The heating elements in an electric furnace are resistance coils made from high-temperature chrome nickel heat-generating wire. Most manufacturers design their furnaces so that the entire heating element assembly can be removed for easy maintenance or repair (Figure 14-6).

Open elements are used in noncentral heating units, such as radi­ant or convective heaters, where it is desirable for the element to operate at a relatively high temperature. These elements reach oper­ating temperatures very rapidly when energized. The exact operat­ing temperature depends on the material used in the element and the type of heat desired. A radiant heater, for example, would prob­ably operate at higher temperatures than would a wall-mounted convective heater.

Another type of element used in noncentral heating is the encap­sulated, or completely enclosed, element. The simplest form of encapsulated element is ceiling cable, which has a layer of plastic insulation over it that can withstand the heat. Ceiling cable is designed to operate at very low temperatures.

A more complicated enclosed element is that used in base­board convectors. An outer sheath of ceramic or metal protects the resistance wire from damage, corrosion, and deterioration. The

Electric Furnaces

HEATING ELEMENT SUPPORT ROD

HEATING

ELEMENTS

FAN CONTROL HIGH LIMIT CONTROL

Figure 14-6 Heating elements and controls. (Courtesy Trane Co.)

Heat-dissipating fins add surface area to increase the rate of heat transfer to the surrounding medium (air or water).

Timing Sequences

If all the heating elements in an electric furnace turned on simulta­neously, there would be a momentarily excessive demand on the power supply, resulting in a temporary interruption in the electric service. This problem of power drains and surges is eliminated by timing the heating elements so that they start one at a time in pre­determined increments. Sequencers or relays are used for this pur­pose. In a typcal application the sequencer has a small electric heater powered by the 24-volt control voltage from the thermostat. The control voltage activates a thermo-disk and a timing sequence. The timing sequence is set to a predetermined number of seconds for turning on and off the heating elements. The on-off sequence is also staggered so that the heating elements do not turn on and off all at the same time. The first squencer or relay commonly turns on both the first heating element and the furnace fan.

Safety Controls

Most electric furnaces are equipped with a variety of different safety controls to protect the appliance against current overloading or excessive operating temperatures. These safety controls are:

1. Temperature limit controls

2. Secondary high-limit control

3. Furnace fuses

4. Circuit breakers

5. Control voltage transformer

6. Thermal overload protector

Temperature Limit Controls

In the line control diagram of a Trane EUADH 07A model electric furnace shown in Figure 14-7, each heating element is shown with a high-limit control device located between the heating element and the time-delay relay. These high-limit control devices are designed to limit the outlet air temperature on Trane electric furnaces to 200°F. If the temperature of the outlet air should exceed 200°F, the high-limit control will open and interrupt the supply of electric power to the heating element.

Secondary High-Limit Control

Secondary high-limit protection is provided by a fusible link in each electric heating element (Figure 14-8). This device is designed to shut off the current when temperatures in the furnace become excessive. It functions as a backup system in case of limit switch failure.

Furnace Fuses

Furnace fuses are used to provide protection against possible over­load conditions and to ensure correct, safe field wiring. Each heat­ing element circuit is protected by two branch fuses as shown in Figure 14-9. These are sized to limit the current draw of each heater and are designed to open on a short-circuit or an overloaded-circuit condition. For overload protection, the blower motor and relay are also safeguarded with a separate fuse.

Circuit Breakers

Some electric furnaces are equipped with circuit breakers and a ter­minal board. The wiring diagram for the Coleman 25-kW electric furnace illustrated in Figure 14-10 indicates that both the terminal board and the circuit breakers are located in the power supply feed line to the furnace.

240 VAC 100 AMP. BRANCH CIRCUIT

Electric Furnaces

-10—і _ I B1 240 V B2

’10 ^—’ TMIT“

O-f’30- ‘

HLC1 10 TD1 З^рЗр — 27

45 TD.341

46

Г

37|

TD1

41

РіТШТГУ-

 

Electric Furnaces

24 F1A H1

26 1

 

РМЛг

TD3

 

J9|F2A 2 H2 HLC2 ii TD2 6 _E2B,

 

Electric Furnaces Electric Furnaces

TD4

Electric Furnaces

22 F3A H3 31 гтп 3

HLC3 „TD3 , F3B 23

FC

49 48

 

TD = TIME DELAY RELAY FC = FAN CONTROL F=FUSE

HLC = HIGH LIMIT CONTROL H=HEATER CR = CONTROL RELAY TS = TERMINAL STRIP

 

T /і3

 

OUTSIDE OR 2 STAGE THERMOSTAT MAY BE ‘CONNECTED BETWEEN 1 & 2 FOR STAGING.

Figure 14-7 Line-control wiring diagram of a Trane electric furnace.

(Courtesy Trane Co.)

Figure 14-8 Fusible link connection in electric heating

Electric FurnacesElement. (Courtesy International Heating and Air Conditioning Corp.)

HEATING FAN CONTROL HIGH

ELEMENT IN BACK OF LIMIT

TERMINAL PANEL CONTROL

[//// LINE POWER FUSES

Figure 14-9 Electric furnace heating-section control panel showing location of line power fuses. (Courtesy Trane Co.)

CONTROL

POWER

FUSES

LINE POWER FUSES (TWO FOR EACH HEATING ELEMENT)

MAIN POWER TERMINAL BLOCKS

FAN

CONTROL

RELAY

CONTROL

POWER

TRANSFORMER

 

HEATING

ELEMENT

 

MOUNTING

SCREWS

 

TIME RELAY SEQUENCE

 

Electric Furnaces

Electric Furnaces

Electric Furnaces

Figure 14-10 Wiring diagram for the Coleman 25-kW electric furnace. (Courtesy Coleman Co., Inc.)

When it is desirable to run branch circuits to the circuit breakers (bypassing the terminal board), the jumper wires between the cir­cuit breakers and the wiring are connected as shown in Figures 14-11 and 14-12.

Transformer

TERMINAL BOARD

подпись: terminal board

0

GROUND

подпись: 
 0
ground

Figure 14-11 Electrical wiring for a Coleman 15- or 20-kW electric furnace with circuit breakers. (Courtesy Coleman Co., Inc.)

подпись: 
figure 14-11 electrical wiring for a coleman 15- or 20-kw electric furnace with circuit breakers. (courtesy coleman co., inc.)
A control voltage transformer is used to limit the amount of output current. Limiting the amount of output current permits the use of open control wiring.

© 3*

©

0 3*

©

0 2*

0

® 2*

0

0 1#

0

0 1*

0

* CIRCUIT NUMBER

CIRCUIT BREAKERS

JUMPER WIRES

подпись: jumper wires

TERMINAL BOARD 1

подпись: terminal board 1

GROUND

подпись: ground Electric Furnaces

Figure 14-12 Electric wiring for a Coleman 25-kW electric furnace

With circuit breaker. (Courtesy Coleman Co., Inc.)

0

 

Ф

0

 

Ф

0

 

#3

 

Electric Furnaces

Electric Furnaces

Thermal Overload Protector

The fan motor is protected against locked rotor or overheated condi­tions by thermal overload devices. When these conditions occur, the fan motor circuit is automatically opened, and the motor is shut off.

Electrical Wiring

All internal furnace wiring is done at the factory before it is shipped. At the site, the following two types of electrical connec­tions are required to field wire the unit:

1. Line voltage field wiring

2. Control voltage field wiring

Caution

DEADLY HIGH-VOLTAGE conditions exist inside the cabi­nets of electric furnaces. Only a trained HVAC technician or someone with equivalent experience should attempt to service or repair electrical components.

A typical example of line voltage field wiring is shown in Figure

14- 13. Line voltage wiring involves the connection of the furnace to the building power supply. Line voltage wiring 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.

Electric Furnaces

Figure 14.13 Typical line voltage field wiring shown for a Fedders electric furnace. (Courtesy Fedders Corp.)

The unit must be properly grounded either by attaching a grounded conduit for the supply conductors (knockouts in the side panels of Fedders electric furnaces are provided for this purpose) or by connecting a separate wire from the furnace ground lug to a suit­able ground.

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 control voltage wiring are generally shipped with the thermostat.

Some typical examples of control voltage field wiring used with Fedders electric furnaces are shown in Figures 14-14 and 14-15. Control voltage field wiring connections used with Coleman elec­tric furnaces are shown in Figure 14-16.

The National Electrical Code requires that furnaces larger than

10 kW be supplied with branch circuit fusing. Power connections on units of this size are usually made to lugs on the fuse blocks.

Blowers and Motors

The blowers and motors used with electric furnaces are identical to those used in gas-fired furnaces. Read the section Blowers and Motors in Chapter 11, “Gas Furnaces” for additional information.

Air Delivery and Blower Adjustment

It is sometimes necessary to adjust the blower speed to produce a temperature rise through the furnace that falls within the limits stamped on the furnace nameplate. Blower adjustment procedures are described in full detail in Chapter 11, “Gas Furnaces.”

Air Filter

The air filters used in electric forced-warm-air furnaces are either permanent types that can be removed and cleaned on a periodic basis or replaceable, throwaway filters.

More detailed information concerning furnace air filters is con­tained in Chapter 12, “Air Cleaners and Filters” of Volume 3. See also the comments about air filters in the section Maintenance and Operating Instructions in this chapter.

Air Conditioning

A furnace should be installed in parallel or on the upstream side of the cooling unit to avoid condensation in the heating section. Parallel installation will require dampers or some other means to prevent cool air from entering the furnace.

472

подпись: 472

TO LOW VOLTAGE

Electric Furnaces

R W, W2 G Y ON THERMOSTAT CNX220T1B OR CNX120T2A

Figure 14-14 Control voltage wiring for two-stage heating, single-stage cooling applications.

(Courtesy Fedders Corp.)

 

CONTROL VOLTAGE _____ SECTION

GROMMET-

N. E.C CLASS 2 FIELD WIRING

TO LOW VOLTAGE TERM. BLOCK IN

CONDENSING

UNIT

 

Electric Furnaces

LOW VOLTAGE FIELD WIRINGRI* TERM. BLOCK =ti

 

FUSEHOLDER

 

Electric Furnaces

RW GY ON THERMOSTAT CNX110T38

Figure 14-15 Control voltage wiring for single-stage heating, single-stage cooling applications. (Courtesy Fedders Corp.)

Electric Furnaces

R TO W2 -25 KW ItHMINAL BOAH

NO JUMPER — 10 & 15 KW

10, 15 or 20 kW Model 25 kW Model

Furnace

Electric Furnaces

Figure 14-16 Three connections for indoor and outdoor heating (only) thermostats for a Coleman model 6806 electric furnace.

(Courtesy Coleman Co., Inc.)

The air conditioning component of a typical electric heating and cooling system generally consists of an outdoor condensing unit, indoor coils, and a cabinet to house the cooling coils. Most furnace manufacturers provide detailed instructions for adding air conditioning to the heating unit. The important thing to remember is to size the ducts for the larger volume of air used in air conditioning.

Additional information about air conditioning can be found in Chapters 8, 9, and 10 in Volume 3.

Maintenance and Operating Instructions

Maintenance and operating instructions will normally be provided for the furnace by the manufacturer. If no instructions are available, try contacting a field representative or writing the company for a duplicate copy of the owner’s manual.

Caution

Always shut off the electrical power supply to the furnace before attempting to service it. This is very important because DEADLY HIGH-VOLTAGE CONDITIONS EXIST. Be sure to open all furnace fused-disconnect switches before servicing.

Furnace Air Filters, Electronic Air Cleaners, and Humidifers

The air cleaner is one of the most important parts of an electric forced-air furnace. A clogged one will cause the furnace to run longer and harder to deliver the desired amount of heat. The result will be higher energy use and higher energy bills.

Air filters should be—inspected on a periodic basis. Clean or replace the filters on a monthly basis during the heating season. Do the same every 3 or 4 months the rest of the year. If the system has an electronic air cleaner, periodically inspect and wash the grids. If the system is equipped with a humidifier, inspect and clean it on a periodic basis.

Caution

Steam-generating-type humidifers are line voltage powered. They must be shut off before servicing.

Heating Elements and Heating Control Wiring

Inspect the heating element and heating control wiring to make cer­tain connections are tight and clean. Check for burned or frayed wires. Check for any breaks or cracks in the wire insulation. These can cause shorting and are a potential fire hazard. All terminal block wiring connections should also be tight and clean.

Caution

Do not attempt to service the controls inside the furnace cab­inet unless you are a qualified HVAC technician. DEADLY HIGH-VOLTAGE CONDITIONS EXIST, which can result in serious injury or death.

Check the manufacturer’s maintenance and/or troubleshooting manual for instructions on testing the elements to see which ones are drawing current. The method (and equipment) will varying among different furnace manufacturers. For example, General Electric/Trane requires the use of either an ohmmeter or clamp on ammeter.

Check the furnace wiring diagram to make certain the fuses are of the correct type and amperage.

Blowers/Fans

Some blower/fan motors are permanently lubricated and will not require further attention. Others have ports for oiling and require periodic lubrication. See Blowers and Motors in Chapter 11, “Gas Furnaces.”

Periodically check the blower/fan belts (if used) for belt tension and adjust if necessary. If the belts are frayed or in anyway dam­aged, replace them. Brush the blower/fan blades and the entire enclosure area to keep dust from being blown through the ducts into your rooms. Blower/fan maintenance should be done every time you clean or replace the air filter. You should also inspect and clean the room registers at the end of the ducts at this time.

Ducts

Check the ducts for loose connections or damage, and correct as necessary. Seal all duct seams and joints in the ductwork, and seal the connection between the furnace plenum and the ducts. Doing so will increase the amount of air (and heat or cool air) delivered through the ducts to the rooms.

Insulate ductwork in basements and crawl spaces to reduce heat loss and lower your energy use and costs.

Thermostats

A combined heating and cooling system is designed to operate year — round without being shut down. The only changeover required are the room thermostat settings and periodic maintenance.

Check the thermostat setting against the actual temperature in a room. The actual temperature can be measured with a ther­mometer placed in the center of the room. You may find that the actual room temperature is several degrees lower than the thermo­stat setting. This is a common characteristic of electric heating systems. You will probably have to set the thermostat a bit higher to get the desired results. Just do a little experimenting. On the other hand, the thermostat may be allowing the room tempera­ture to rise above the setting. This may indicate a faulty thermo-

Stat. If you suspect a faulty thermostat, have it serviced by an HVAC technician.

Periodically check the batteries in a programmable thermostat. Your heating/cooling problem may be nothing more than an inex­pensive battery replacement. Lower the setting on a programmable thermostat when you are sleeping, away at work, or on a vacation. This will result in lower energy costs.

Furnaces without Air Conditioners

When an electric furnace is installed without an air conditioning unit, the furnace should be shut down at the end of the heating sea­son. The procedure for doing this will be found in the furnace owner’s manual. It is a simple operation that generally consists of opening the main fused-disconnect switch (or switches) in the power-supply lines serving the furnace.

Troubleshooting an Electric Furnace

Any appliance may sometimes fail to operate efficiently because of a malfunction somewhere in the equipment. The problems most com­monly associated with electric-fired furnaces are listed in Table 14-4.

Caution

DEADLY HIGH-VOLTAGE CONDITIONS exist within the cabinet (case) of an electric furnace. Do not attempt any trou­bleshooting if it might involve contact with electric controls and components unless you are a trained HVAC technician or have equivalent experience with electric furnaces.

Table 14-4 Troubleshooting Electric Furnaces

Symptom and Possible Causes Suggested Remedies

Unit fails to operate

(a)

Defective furnace transformer.

(a)

Replace.

(b)

Blown or defective transformer

(b)

Replace fuse.

Fuse.

(c)

Defective thermostat.

(c)

Replace thermostat.

(d)

Improperly set room.

(d)

Change thermostat to proper

Setting.

(e)

Open fused-disconnect switch.

(e)

Correct problem and close

Furnace circuit.

(f)

Blown or faulty furnace fuse.

(f)

Replace fuse.

Symptom and Possible Causes

Fan operates with low or no heat

(a) Blown or faulty heater element fuse.

(b) Defective time-delay sequencer or relay. Note: Some electric furnaces use relays instead of sequencers (e. g., General Electric, Trane, Rheem, Ruud).

Heats without fan operation

(a) Faulty fan-control relay.

(b) Defective fan motor.

(c) Faulty fan motor wiring or loose connections.

(d) Defective fan motor run capacitor.

Individual heater fails to operate

(a) Blown or faulty heater circuit fuse.

(b) Defective high-limit control.

(c) Defective time-delay sequencer or relay.

(d) Faulty heater element.

(e) Defective heat pump thermostat

(f) Wrong heat pump thermostat.

(continued) Suggested Remedies

(a) Replace fuse.

(b) Replace sequencer or relay. Note: Sequencers and relays are not interchangeable.

(a) Replace.

(b) Repair or replace.

(c) Repair or replace wiring.

(d) Repair or replace.

(a) Replace fuse.

(b) Replace.

(c) Replace.

(d) Replace.

. (e) Check thermostat operation.

(f) Replace with heat pump thermostat, or replace with conventional thermostat having an electric setting (the electric setting will activate the fan), or add a relay or double sequencer.

Fan operates on heating, not on cooling

(a) Defective cooling-cycle control (a) Replace. relay.

(b) Improperly connected or faulty (b) Make proper connections to

Room thermostat. thermostat.

(c) Defective or improper fan (c) Repair or replace.

Motor connections.

Furnace will not operate

(a) No power to furnace.

(b) Blown fuses.

(c) Dirty or loose connections on fuses.

(d) Heaters (elements) shorting to cabinet case.

(e) Circuit breaker incorrect size for amp load.

Low heat or no heat

(a) No power to furnace.

(b) Incorrect thermostat setting.

(c) Low air flow in and out of system.

Low humidity levels

(a) Incorrect humidifier setting

(b) Incorrect humidistat setting.

(c) Incorrect thermostat fan switch setting. Switch set on manual instead of automatic.

(d) Incorrect ventilation switch setting on furnace. Switch set on continuous (low) instead of automatic.

High humidity levels

(a) Additional means required to reduce excessive humidity.

(b) Incorrect humidistat setting.

(a) Check power source (fuses or circuit breakers in main box).

(b) Wrong size fuses. Replace with correct size fuses.

(c) Clean and/or tighten connections.

(d) Correct.

(e) Test and replace as necessary.

(a) Check power source (fuses or circuit breakers in main box).

(b) Set thermostat to desired temperature setting.

(c) Check to verify air flow matches system specifications. If not, check for a blocked air filter or duct and correct.

(a) Reset humidifier to recommended setting.

(b) Reset humidistat to recommended setting.

(c) Change thermostat fan switch setting from manual to automatic.

(d) Change ventilation switch setting from continuous (low) to automatic.

(a) Install humidistat on furnace.

(b) Change humidistat setting.

(continued)

Table 14-4 (continued)

подпись: table 14-4 (continued)Symptom and Possible Causes

(c) Incorrect thermostat fan switch setting. Switch set in automatic instead of manual.

(d) Incorrect ventilation switch setting on furnace. Switch set on automatic instead of continuous (low).

Suggested Remedies

(c) Change thermostat fan switch setting from automatic to manual.

(d) Change ventilation switch setting from automatic to continuous (low).

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


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