Oil Furnaces
Oil furnaces are available in upflow, downflow (counterflow), or horizontal-flow models and in a wide range of heating capacities for installations in attics, basements, closet spaces, or at floor level (Figures 12-1, 12-2, and 12-3). The furnace, oil burner, and controls are generally available from the manufacturer as a complete package for residential installations. All internal wiring is done at the factory.
Oil furnaces should be listed by Underwriters Laboratories, Inc. (UL), for construction and operating safety. Furnaces approved by the agency are marked UL Approved. Furnaces not made under UL standards should not be purchased or installed. Always closely follow the furnace manufacturer’s installation, maintenance, and operating instructions. Note also that any local building codes covering furnace installation will take precedence.
Other sources of information concerning standards for oil furnace selection and installation are the publications Standard for the Installation of Oil Burning Equipment, 2001 Edition (NFPA No. 31) and Installation of Warm Air Heating and Air Conditioning Systems, 2002 Edition (NFPA 90B) from the National Fire Protection Association. See also American Standard Performance Requirements for Oil-Powered Central Furnaces (ANSI Z91.1- 1972), a publication of the American National Standards Institute.
Most residential oil furnaces still in use today are the noncondensing, conventional-types installed before the mid 1980s. Since the 1980s, furnaces installed in new heating systems, and some replacement furnaces, are the newer mid-efficiency and high-efficiency oil furnaces. A conventional oil furnace with a cast-iron head burner has an AFUE (seasonal efficiency rating) of 60 percent.
The heating cycle of a conventional oil furnace begins when the thermostat calls for heat. This is accomplished in one of two ways: (1) The room temperature falls below the heat setting on the thermostat and the thermostat makes an automatic call for heat, or (2) someone turns the thermostat up manually to a warmer setting. In either case, the call for heat closes an electric circuit to a control relay, and power is sent to both the burner ignition transformer and the fuel pump motor.
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The fuel pump motor operates the oil pump, which draws fuel oil from the supply tank and sends it to the nozzle in the gun assembly, where it is combined with the combustion air and atomized. The combustion air is drawn into the combustion chamber by a blower wheel, which is attached to the pump motor shaft. The ignition transformer sends a high-voltage current to the electrodes in the gun assembly to ignite the atomized fuel-oil mixture. The oil furnace heating cycle begins.
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Figure 12-2 Lennox horizontal oil-fired forced warm-air furnace. (Courtesy Lennox Industries Inc.) |
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As soon as the heating cycle begins, a cadmium selenide photocell (called a cad cell) is activated as a safety device. The function of the cad cell is to detect the existence of a flame in the combustion chamber. If the cad cell fails to detect a flame within 15 seconds, the circuit is opened and the burner is shut off.
Some oil furnaces are equipped with a stack relay instead of a cad cell. The stack relay is designed to sense heat. If no heat is sensed, the circuit to the burner is opened and the unit is shut off. In both cases, the heating cycle is interrupted until the problem can be corrected.
As in gas-fired furnaces, a fan and limit control is used to regulate the operation of the furnace blower. The fan switch turns the blower on when there is a call for heat and warm air from the heat exchanger is forced through the heating ducts and into the structure. When the temperature reaches the setting on the thermostat, the fan control shuts off the blower to prevent overheating.
Mid-Efficiency and High-Efficiency Oil Furnaces
As was mentioned in Chapter 11 (“Gas Furnaces”), the government and furnace manufacturers have significantly improved the heating efficiency of residential gas furnaces. Similar improvements have been made in oil furnace technology. Two widely used methods, the AFUE measurement and the Energy Star program of the United States Environmental Protection Agency (EPA), are described in the sidebars. A furnace that meets the minimum efficiency requirements of these ratings is referred to as a high-efficiency oil furnace.
AFUE Rating
The energy efficiency of an oil 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 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 to 82 percent. High-efficiency furnaces have AFUE ratings ranging from 88 to 97 percent. The traditional oil furnaces have AFUE ratings of approximately 60 to 65 percent.
Energy Star Certification
The Energy Star Program is an energy performance rating system created by the EPA to identify and certify energy efficient products, including furnaces, boilers, heat pumps, and air conditioners. A broader description of the program is given in Chapter 11, “Gas Furnaces.”
Mid-Efficiency (Noncondensing) Oil Furnace
A typical noncondensing, mid-efficiency oil furnace uses 28 to 33 percent less fuel oil than a conventional oil furnace to produce the same amount of heat. Its operation is characterized by significantly lower combustion and dilution air requirements.
The internal components of a typical mid-efficiency oil furnace are illustrated in Figure 12-4. The high-static oil burner fires into the combustion chamber, commonly made of a heat-resistant ceramic material. Air for combustion is drawn into an intake opening on the burner assembly by a small motor where it mixes with the fuel oil and ignites. The hot combustion gases then pass through the heat exchanger and are eventually expelled through an insulated flue pipe to the outdoors. A separate, larger blower (sometimes called the indoor blower or furnace fan), forces air across the outer surface of the heat exchanger, extracts heat, and forces the heat out into the rooms and spaces of the house. The two air streams never mix.
Many of these mid-efficiency furnaces do not require a connection to a chimney. They pass the combustion gases though an insulated vent pipe that extends directly through the sidewall of the structure. A barometric damper is not required in the more efficient furnaces. Some use the forced draft of a high-static oil burner to vent the combustion gases. Others use sealed combustion with a high-static oil burner.
The mid-efficiency oil furnace is equipped with a safety shutoff device in case of draft problems. The other controls are similar to those found on conventional oil furnaces.
High-Efficiency (Condensing) Oil Furnace
A high-efficiency oil furnace (sometimes called a condensing oil furnace) is equipped with two heat exchangers to extract heat from the combustion gases before they leave the furnace (Figure 12-5). The second heat exchanger, which is commonly made of stainless steel, is used to recover the latent heat in the water vapor produced by combustion. This is accomplished by reducing the temperature of the combustion gases. The temperature of the flue gases exiting the furnace from the second heat exchanger drops to approximately 100 to 120°F. The lowered temperature causes the water vapor trapped in the flue gas to condense in the heat exchanger and
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Figure 12-4 Mid-efficiency oil furnace. |
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1 |
Blower: |
10 |
Flue connection |
19 |
Top panel, jacket |
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2 |
Blower motor V3 |
11 |
Fan and limit control |
20 |
Left panel jacket |
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3 |
Ter LO 2 X CO |
12 |
Wiring harness |
21 |
Right panel, jacket |
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(either side) |
13 |
Heat exchanger |
22 |
Rear panel, jacket |
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4 |
On burner motor |
14 |
Supply air opening |
23 |
Base assembly |
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5 |
Burner junction box |
15 |
Observation door |
24 |
Blower pan |
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6 |
Primary safety |
16 |
Flue collector |
25 |
Top door |
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Control |
17 |
Return air opening |
26 |
Burner access panel |
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7 |
Oil burner ignitor |
(either side) |
27 |
Blower access door |
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8 |
Oil burner pump |
18 |
Fen center (direct- |
28 |
Front divider |
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9 |
Main junction box |
Drive units only) |
29 |
Spillway baffle |
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1. Combustion chamber 2. Primary heat exchanger 3. Oil burner 4. Filter 5. Secondary heat exchanger 6. Blower |
Release its latent heat. The recovered latent heat is circulated through the house, the condensate flows out of the bottom of the furnace to a nearby drain, and the cooled combustion gases are exhausted through a plastic vent pipe installed in a sidewall.
The combustion process of an oil furnace produces only about half the water vapor that a gas furnace does. This means that there is significantly less water vapor trapped in the exhaust gases of an oil furnace and, consequently, much less latent heat to recover. The smaller amount of water vapor produced in these furnaces means that the flue gas also has a lower dew point. As a result, the furnace has to work harder to recover less energy. As a result, the highefficiency oil furnace is not much more energy efficient than the midefficiency oil furnace, in contrast to the situation with gas furnaces.
Basic Components of an Oil Furnace
The principal components of an oil furnace are illustrated in Figure
12- 6. They are listed as follows:
1. Furnace controls
2. Heat exchanger
3. Burner assembly
4. Fuel pump and motor
5. Blower and motor
6. Combustion blowers
7. Cleanout and observation ports
8. Vent openings
9. Air filter(s)
Each of these components is briefly described in the sections that follow. More detailed information is contained in Chapter 1, “Oil Burners,” and Chapter 5, “Gas and Oil Controls” of Volume 2.
A number of different controls are used to govern the operation of an oil furnace. Some, but not all of them, are shown in Figure 12-7.
Thermostat
A room thermostat controls the operation of the furnace. The thermostat senses air-temperature changes in the space or spaces being heated, closes an electrical circuit, and starts the ignition process. See Chapter 4, “Thermostats and Humidistats” of Volume 2 for a detailed description of thermostats.
Cad Cell
The cad cell is the principal safety device on an oil furnace. It is located inside the burner assembly in back of the burner access panel. The function of the cad cell is to prove (detect and verify) the burner flame at the start of the heating cycle. In this respect, it provides the same function as the thermocouple in a gas furnace. If the cad cell cannot prove the burner flame, it opens the circuit to the burner, which shuts off the burner motor and the ignition transformer. A detailed description of cad cells can be found in Chapter
5, “Gas and Oil Controls” in Volume 2.
Fan Controls
The fan and limit control is a switching and safety device commonly located in a metal box attached to the outside of the furnace
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383 |
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1 2 3 4 5 6 7 3 9 10 11 12 13 14 15 16 17 18 19 13 21 22 23 24 25 26 |
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DRAW COLLAR FRONT TOP PNEL FRONT REMOVABLE PANEL FRONT CENTER PANEL BLOWER ACCESS DOOR FLUE GASKET FAN/LIMIT MOUNTING HINGF END CAPS (INTAKE & EXHAUST) SIDE CASINGS (RIGHT & LEFT) HEAT EXCHANGER ASSEMBLY BLOWER PAN BLOWER ASSEMBLY BECKETT OIL BURNER FILTER REAR TOP PANEL REAR BOTTOM PANEL BOTTOM MOUNTING PLATE BOTTOM GASKET MOUNTING PLATE TOP GASKET MOUNTING PLATE TOP MOUNTING PLATE |
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FAN & LIMIT CONTROL AUXCONTROL CLEANOUT COVER CLEANOUT COVER GASKET |
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CHAMBER Figure 12-7 Typical automatic control system for an oil furnace. (Courtesy Honeywell Tradeline Controls) |
(Figure 12-8). Its function is to turn the blower on and off within a set temperature range and to shut off the burner if the furnace gets too hot.
The fan and limit control has two temperature settings: an upper one and a lower one. When the temperature in the house reaches the upper setting on the fan control, the fan control circuit opens and turns off the burner. The blower continues to run until the temperature in the heat exchanger reaches the lower temperature setting on the fan control, opens the circuit, and turns off the blower.
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Figure 12-8 Typical fan control. (Courtesy Honeywell, Inc.) |
Figure 12-9 illustrates the internal components of a typical fan control box. The Fan On and Fan Off pointers on the dial are used to establish the temperature range settings. The Safety Limit control is normally set by the manufacturer and should not be adjusted. Always set the Auto/Manual Switch to Auto before reinstalling the fan control cover. A detailed description of fan and limit controls is found in Chapter 6, “Other Automatic Controls” of Volume 2.
Some modern oil furnaces are equipped with electronic fan controls instead of the fan and limit control switches. The control either is a separate box attached to the furnace or is an integral part of the electronic (solid-state) control board.
Delayed-Action Solenoid Valve
A delayed-action solenoid valve fitted to a conventional oil burner will prevent oil furnace spillage (see Oil Furnace Spillage in this chapter). The valve is designed to delay the flow of oil to the
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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" |
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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" |
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Figure 12-9 Fan control with cover removed. (Courtesy Honeywell, Inc.) |
Combustion chamber a few seconds after the burner starts. The delay allows the burner time to establish a draft for a more complete and efficient combustion of the oil. It also eliminates sooty buildup and backdrafting.
The heat exchanger assembly is the section of the furnace used to transfer the heat of the combustion process to the air being circulated through the ducts of the heating system. The heat exchanger is generally constructed of heavy gauge steel (14 and 16 gauge). The wrap-around, radiator-type heat exchanger is one of the most commonly used designs. It consists of an upper and lower chamber, each with an extension, or pouch, that must be aligned when assembled. The lower part contains the combustion chamber.
The combustion chamber is that portion of the furnace within which the combustion process takes place. It is surrounded by the lower portion of the heat exchanger. The combustion chamber must be made of a material capable of withstanding the high temperatures of the combustion process. Typical combustion chamber materials are stainless steel or some sort of refractory material such as firebrick or a kiln-fired ceramic clay. As shown in Figure 12-10, the nozzle and gun assembly of the oil burner extends through an opening in the heat exchanger to another opening in the combustion chamber.
Round exchangers are the most common type, although square, rectangular, and other shapes do occur. The octagonal heat exchanger illustrated in Figure 12-11 is specific to Thermo Pride oil furnaces.
Some modern oil furnaces are equipped with sealed combustion chambers. Their purpose is to prevent spillage and backdrafts, while at the same time increasing furnace efficiency. Furnaces with sealed combustion chambers sometimes experience startup problems
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HEAT EXCHANGER
Figure 12-10 Heat exchanger and combustion chamber. (Courtesy Thermo Pride) |
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(Courtesy Thermo Pride) |
Because air delivered to the burners is too cold. The cold combustion air cools the fuel oil, causing the burner ignition problem. It can be solved by warming the air before it reaches the burner.
The oil burner assembly consists of the ignition transformer, cad cell, gun assembly, oil nozzle, and electrodes (Figure 12-12). The oil burner is a mechanical device used to combine fuel oil with the proper amount of air for combustion and deliver it to the point of ignition. Oil burners and burner assemblies are described in considerable detail in Chapter 1, “Oil Burners” of Volume 2.
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ELECTRONIC IGNITION BURNER
MOTOR Figure 12-12 Oil burner assembly. (Courtesy Lennox Industries Inc.) |
The pressure-type oil burner is the one most commonly used in residential oil furnaces, hot-water, and space heating systems. A vaporizing oil burner is sometimes used in small residential furnaces.
One of the first improvements in oil burner technology after the oil crisis in the 1970s was the introduction of more burners with flame-retention heads. These efficient designs were followed by the even more efficient high-static oil burners.
The fuel pump (Figure 12-13) draws oil from the storage tank and sends it to the gun assembly at pressures between 100 and 140 psi. The fuel pump is connected to the pump motor by the burner coupling. Both the fuel pump and the fuel pump motor are described in Chapter 1, “Oil Burners” in Volume 2.
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The blower assembly consists of the blower wheel housing, the blower wheel, the blower motor, and the blower motor capacitor. The function of the blower is to draw air into the furnace through the return air duct, force the air around the internal surfaces of the heat exchanger to pick up heat, and then deliver the warm air to the interior of the house through the supply ducts. The combustion air required for the combustion process is drawn in through an air inlet
Scoop on the side of the blower wheel housing. The amount of combustion air is regulated by an air adjustment dial located on the outside of the housing. It can be manually changed by moving the dial setting.
As shown in Figure 12-14, the blower motor is attached to the side of the blower wheel housing. The motor shaft rotates the blower wheel inside the blower wheel housing. The blower motor capacitor, which assists in starting the blower motor, is attached to the outside edge of the blower wheel housing.
Both direct-drive and belt-drive blowers are used with oil-fired furnaces. These blowers and motors are identical to those used in gas-fired furnaces. Read the section Blowers and Motors in Chapter
11, “Gas Furnaces” for additional information. The function of the blower is to blow heat through the duct system into the rooms of the structure.
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WHEEL |
Oil furnaces equipped with direct-drive blowers operated by commutating motors are capable of adjusting the furnace heat output and blower speed to the heat requirements of the structure. In other words, their operation increases with the demand for heat and decreases when there is none, thereby improving indoor comfort while reducing energy costs. These furnaces are sometimes called dual-capacity, variable-speed oil furnaces.
High-efficiency oil furnaces are often equipped with small blowers designed to provide a steady supply of combustion air to the burner. These small combustion air blowers operate on not more than V6 horsepower or less.
Pressure switches are used with combustion air blowers to sense blower speed and whether the flue is blocked. Older conventional oil furnaces used centrifugal switches instead of pressure switches.
Cleanout and Observation Ports
As shown in Figure 12-2, an oil furnace typically is provided with one or two cleanout ports to allow access to the heat exchanger for cleaning (soot removal). The observation port allows monitoring of the inside of the heat exchanger during the operation of the furnace.
The vent opening connects the furnace to a flue pipe for venting the combustion gases to the outdoors. Many of the modern oil furnaces connect the flue pipe to a sidewall of the house. Traditional oil furnaces vented these gases through a flue pipe connected to the chimney.
A forced-warm-air oil furnace is supplied with either a disposable air filter or a permanent (washable) one. A filter is not used with a gravity warm-air oil furnace, because it will obstruct the airflow.
A disposable air filter should be inspected on a regular basis and replaced when dirty. Always replace the air filter with one of the same size and type of filter media. This information is usually found on a label attached to the filter.
A permanent air filter must also be regularly inspected. When it is dirty, it must be removed and cleaned. The usual method is to vacuum it and then to wash it in a soap or detergent and water solution. Additional information about air filters is found in Chapter 11, “Gas Furnaces,” and Chapter 12, “Air Cleaners and Filters” in Volume 3.
An oil furnace must be properly set up and installed by a certified oil heat technician, a representative of the oil furnace manufacturer, or an HVAC professional with equivalent experience.
The size (Btu output capacity) of the oil furnace selected for the heating system is determined by the calculated heat loss for the structure. See Chapter 4, “Sizing Residential Heating and Air Conditioning Systems.”
Some Installation Recommendations
The installation of an oil furnace must comply with the local building codes and the manufacturer’s installation recommendations. As was mentioned at the beginning of this chapter, additional useful and important information is contained in the National Fire Protection Association’s Installation of Warm Air Heating and Air — Conditioning Systems, 2002 edition and the Standard for the Installation of Oil Burning Equipment, 2001 Edition (NFPA No. 31). The appropriate sections of the Uniform Mechanical Code, 2003 edition, should also be consulted.
New furnaces for residential installations are shipped preassembled from the factory with all internal wiring completed. The fuel supply tank must be installed and the fuel line connected to the furnace by the installer. The electrical service from the line-voltage main and the low-voltage thermostat will also have to be connected. Directions for making these connections are included in the furnace manufacturer’s installation literature.
Make certain you have familiarized yourself with all local codes and regulations that govern the installation of an oil-fired furnace. Local codes and regulations always take precedence over national standards. Read the section Installation Recommendations in Chapter 11, “Gas Furnaces” for additional information.
An oil furnace should be located as near as possible to the center of the heat-distribution system and chimney. A centralized location for the furnace usually results in the best operating characteristics because long supply ducts and the heat loss associated with them are eliminated. The furnace should be installed as close as possible to the chimney to reduce the horizontal run of flue pipe.
An oil furnace should be located a safe distance from any combustible material. Observe the minimum clearances suggested by the manufacturer for the furnace except where they may come into conflict with local codes and regulations (Table 12-1). Local standards always take precedence over the manufacturer’s specifications or national standards. Where there are no local standards specifying clearances, use those listed in Installation of Oil Burning Equipment 1972 (NFPA No. 31-1972). Contents of the 1972 edition have been approved by the American National Standards Institute (ANSI).
Allow access for servicing the furnace and oil burner. Sufficient clearance should also be provided for access to the barometric draft-control assembly.
Always follow all national and local codes when wiring a unit. The local code should take precedence when there is a conflict between
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Table 12-1 Minimum Clearances Specified for Carrier Oil Fired Upflow (58HV), Lowboy Upflow (58HL), Downflow (58HC), and Horizontal (58HH) Furnace Models
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From |
Clearance (in) |
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Top of furnace or plenum |
2 |
2 |
2 |
6 |
2a |
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Bottom of furnace |
О |
О |
— |
6 |
2a |
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Front of furnace |
15 |
24 |
15 |
24 |
4 00 A |
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Rear of furnace |
1 |
6 |
2 |
6 |
2a |
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Sides of furnace |
1 |
6 |
1 |
6 |
4a |
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Top of horizontal warm-air duct within 6 ft of furnace |
2 |
4 |
6 |
3a |
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Flue pipe: Horizontally or below pipe |
9 |
9 |
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Vertically |
18 |
18 |
15 |
18 |
9a |
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Any side of supply plenum and warm-air duct within 6 ft of furnace |
2 |
6 |
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AAlcove installation. Courtesy Carrier Corp. |
The two. If there are no local ordinances and codes, do all electrical wiring in accordance with the National Electrical Code (ANSI/NFPA 70-1999).
Closely follow the wiring diagrams provided by the furnace manufacturer and those who manufacture the automatic controls. Failure to do this may result in losing the factory warranty.
A typical wiring diagram for an oil furnace is shown in Figure
12- 15. The comments made in Chapter 11, “Gas Furnaces” concerning line-voltage and control-voltage wiring connections also pertain to the wiring of oil furnaces.
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, 2002 edition (NFPA No. 90A)
2. Residence Type Warm Air Heating and Air Conditioning Systems, 2002 edition (NFPA. No. 90B)
Additional information about duct connections can be found in Chapter 7 of Volume 2, “Ducts and Duct Systems.” The specific recommendations concerning furnace duct connections and the airdistribution ducts contained in Chapter 11, “Gas Furnaces” also apply to gas-fired furnaces.
Ventilation and Combustion Air
An oil-fired furnace should be located where a sufficient supply of air is available for combustion, proper venting, and the maintenance of suitable ambient temperature. In buildings of exceptionally tight construction, an outside air supply should be introduced for ventilation and combustion purposes.
If the furnace is located in a confined space such as a furnace room, closet, or utility room, the enclosure should be provided with two permanent openings for the passage of the air supply. One opening should be located approximately 6 in from the top of the enclosure, and the other opening approximately 6 in from the bottom. Each opening should have free area of at least 1 in2 per 1000 Btu/h (minimum size 100 in2) of the total input rating, or 1 ft2 per gallon of oil per hour.
The size of the openings must correspond to the bonnet capacity of the furnace. Furnace manufacturers generally recommend
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NOTES: 1. REPLACEMENT WIRE MUST BE TYPE 105°C 2. CONNECT HOT TO 1 3. SET HEAT ANTICIPATOR AT.2 AMPS ————————— field WIRING ————————— FACTORY WIRING |
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P/N WD-100 ECN 4363-MA LADDER DIAGRAM BLOWER IC0NTR0L MOTOR |
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WIRING DIAGRAM ELECTRICAL SCHEMATIC FAN & LIMIT _ |
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BURNER MOTOR |
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CAD CELL |
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P/N WD-101C ECN 4385-MA |
WIRING DIAGRAM 012,5,11,16,20 OH2,3,5,11,16
(BELT DRIVE)
LADDER DIAGRAM ELECTRICAL SCHEMATIC
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L1 |
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FAN CONTROL |
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IGINIIOR
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OR |
LIMIT CONTROL
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BL |
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BK |
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OR |
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Figure 12-15 Wiring diagrams for various Thermo Pride oil
Models. (CourtesyThermo Pride)
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Openings for Trane Counterflow Oil Furnace
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Courtesy The Trane Co. |
Suitable sizes for air openings in their equipment specifications (Table 12-2).
Air from an attic or a crawl space can also serve as a ventilation and combustion air supply to a confined furnace. Suitable openings near (or on) the floor and near (or on) the ceiling must be provided for the passage of the air.
When an oil furnace is installed in an unconfined area, such as a full basement, the full amount of air necessary for combustion is generally supplied by infiltration. Only when the construction is exceptionally tight is it necessary to provide openings to the outdoors.
An oil-fired furnace requires sufficient draft for the proper burning of oil. The average combustion draft is usually 0.02 in wg over the fire (the so-called overfire draft), and between 0.04 and 0.06 in wg draft in the stack (the flue draft) for units having inputs up to 2.0 gph. For higher inputs, 0.06 to 0.08 in wg will be the rule. Always consult the furnace manufacturer’s operating manual first for recommended drafts.
Oil furnaces use the same methods for venting the products of combustion to the outside air as do gas furnaces, except when the venting method is specifically limited to a particular fuel (for example, Type B gas vents). Additional information about venting methods is contained in Chapter 11, “Gas Furnaces.” The type of venting to be used will be recommended by the furnace manufacturer in the installation literature.
Most high-efficiency (condensing) oil furnaces vent their exhaust gases through a single plastic flue pipe extending from the furnace
Through a nearby sidewall. The flue vent pipe in these installations is not connected to the chimney.
In some condensing furnace installations, two vents are used. One vent consists of the plastic flue pipe running horizontally from the furnace to the sidewall; the other consists of the conventional chimney and flue liner. Both flue vents remain open during the combustion process.
The strong forced draft produced by a high-static oil burner is sufficient to propel combustion gases through the flue in sidewall- venting installations. Some oil furnaces use a small induced fan installed in the flue. The fan vents the combustion gases through a sidewall vent opening.
Flue Pipe
Use 24-gauge galvanized flue pipe (or its equivalent) for the flue connection between the furnace and the chimney. The horizontal run of flue pipe should slope upward at least V4 in per running foot. The pitch should not exceed 75 percent of the vertical vent. Locate the unit so that the furnace flue outlet is no more than 10 flue pipe diameters from the chimney connection. If the distance is greater, use the next-size-larger flue pipe.
Insulate the flue pipe when it passes near combustible material. The UL requires that any uninsulated flue pipe be installed with a minimum clearance of 6 in from any combustible materials.
Narrow plastic pipe is used to exhaust the flue gases from midefficiency and high-efficiency oil furnaces. The trade names of these plastic vent systems are ULTRAVENT® (gray plastic flue pipe with gray fittings), SELVENT® (black plastic flue pipe with black fittings), and PLEXVENT® (black plastic flue pipe with yellow or gray fittings).
Warning
The plastic venting systems of mid-efficiency and highefficiency oil furnaces installed after 1988 may crack or separate at the joints and cause carbon monoxide to leak into the house. The entire vent system should be replaced to avoid possible exposure to carbon monoxide.
Barometric Damper
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8" DIA. VENT |
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BAROMETRIC DRAFT CONTROL |
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FRICTION DAMPER |
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CONNECTOR TEE |
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Figure 12-16 Barometric draft-control assembly. (Courtesy Hydrotherm, Inc.) |
A barometric damper (also called a draft regulator or barometric draft regulator) is a device designed to maintain a proper draft in an oil furnace by automatically reducing (diluting) the chimney draft (that is, the flue draft) to the desired value (Figure 12-16). Its function is similar to that of the draft hood on a gas furnace. It isolates
the oil burner from changes in pressure at the chimney opening by pulling heated warm air into the exhaust.
Draft regulators are recommended on all installations for each oil-fired appliance connected to a chimney unless the appliance is listed for use without one.
Do not use a small draft regulator, because it will not do a satisfactory job. The draft regulator should be installed in the horizontal flue pipe as close as possible to the chimney. When this flue pipe is too short for a satisfactory installation, the draft regulator should be installed in the chimney either above or below the flue pipe entry into the chimney. Some authorities oppose installing the draft regulator in the chimney, but experience has shown that this can be a very satisfactory installation.
Chimneys and Chimney Troubleshooting
The chimney used with an oil-fired furnace should be of sufficient height and large enough in its cross-sectional area to meet the requirements of the furnace. The chimney area should be at least 20 percent greater than the area of the flue outlet. To ensure the adequate removal of flue gases, the height and size of the chimney must be sufficient to create 0.02 to 0.04 in wg draft over the fire for inputs up to 2.0 gph. For higher inputs, 0.06 to 0.08 in wg is recommended. If the stack height is limited, it may be necessary to apply mechanically induced draft to the installation.
Ideally, only the furnace should be connected to the chimney. Read the section Chimneys in Chapter 11, “Gas Furnaces” for additional information. See also Chapter 3, “Fireplaces, Stoves, and Chimneys” in Volume 3 for additional information about chimneys.
Read the section Chimney Troubleshooting in Chapter 11, “Gas Furnaces” for a description of common chimney problems and suggested remedies. The chimneys used with oil-fired furnaces are identical to those used with gas furnaces. Additional information about chimney and flues is covered in Chapter 3, “Fireplaces, Stoves, and Chimneys” of Volume 3.
A properly installed furnace will operate efficiently and economically. The following installation checklist is offered as a guide to the installer.
1. Be sure there is at least 0.01-in wg draft over the fire.
2. Check for sufficient combustion and ventilation air.
3. Eliminate downdraft or backdraft.
4. Provide sufficient space for service accessibility.
5. Check all field wiring.
6. Supply-line fuse or circuit breaker must be of proper size and type for furnace.
7. Line voltage must meet specifications while furnace is operating.
8. Ductwork must be checked for proper balance, velocity, and quietness.
9. Check the fuel line for leaks.
10. Cycle the burner.
11. Check the limit switch.
12. Check the fan switch.
13. Make final adjustments to the fire.
14. Adjust the 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 condensate lines toward a drain.
18. Check thermostat heat anticipator setting.
19. Check thermostat for normal operation. Observe at least five ignition cycles before leaving the installation.
20. Clear and clean the area around the furnace.
Fuel Supply Tank and Fuel Line
The fuel oil for an oil furnace is stored in a fuel supply tank. The tank is commonly connected directly to the fuel pump in the burner assembly of the furnace.
The fuel supply tank and fuel line must be installed in accordance with the requirements of UL and any local codes and regulations. Local codes and ordinances take precedence over national standards.
Although an existing tank probably has already met the requirements of the local codes and regulations, it would be prudent to check just to make certain. This is especially true of older, buried fuel supply tanks, because they occasionally leak. A leaking fuel supply tank violates the environmental laws in many areas of the country.
Additional information about fuel supply tanks can be obtained from Installation of Burning Equipment 1972 (NFPA No. 31), a publication of the National Fire Protection Association (NFPA). Use only an approved tank and line for the installation.
A one-pipe system is recommended for an oil-tank installation when the following conditions are present:
1. Tank is installed in the basement or similar location and/or below the pump inlet port.
2. Vacuum at pump does not exceed 2 in.
A two-pipe (return line) system is recommended for a tank installation when the following conditions exist:
1. Tank is located outside and/or below the oil-pump inlet port.
2. Vacuum at pump does not exceed 10 in.
Use a two-stage pump for installations having especially long lines or high lifts if the vacuum at the inlet port does not exceed 20 in.
A shutoff valve should be installed in the suction line close to the oil burner. This valve enables the operator to disconnect the line without draining it. Do not place a shutoff valve in the return line.
Closing a shutoff valve on the return line while the oil burner is operating could damage the pump. All joints in the fuel line should be sealed with a suitable oil-piping compound.
Note
A leak in the oil line will cause a loss of pump prime and a failure of the burner to light within the required 15 seconds of the ignition cycle.
The location of the oil supply tank is subject to local regulations. These must be consulted before a new tank is installed.
The supply tank can be located inside or outside the building, above the level of the oil burner or below it. Furthermore, outside tanks can be located underground or above it. It is recommended that larger supply tanks be located outside and underground.
Figures 12-17 through 12-20 illustrate four ways to locate oil supply tanks. When installing a fuel tank, the following suggestions should prove helpful:
1. The filler pipe should be a minimum of 2 inches in diameter; the vent pipe 1V4 inches.
2. Use wrought-iron pipe with malleable-iron fittings for both the filler and vent pipes.
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FURNACE OR BOILER WATER
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WHEN SUPPLY TANK IS LOCATED BELOW LEVEL OF THE BURNERS, INDIVIDUAL SUCTION LINES MUST BE USED Figure 12-18 Inside tank installation. (Counesy Sundstrand Hydraulics) |
3. Coat only the male thread of the pipe with a pipe compound suitable for use with oil burning equipment.
4. Oil supply lines between the oil supply tank and the oil burner should be made of copper tubing (diameters will vary depending
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WATER
Figure 12-19 Outside tank installation involving lift. (Courtesy Sundstrand Hydraulics) |
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SUPPLY TANK Figure 12-20 Outside tank installation with the tank located above The level of the burner. (Courtesy Sundstrand Hydraulics) |
On local regulations, pipe length, and the specifications of the oil burner being used).
5. Use a floor-level tee if the oil supply lines run overhead.
6. The oil burners for the water heater and furnace (or boiler) may be connected to a common feed line in conventional gravity feed installations (Figure 12-20).
7. No return line is necessary when the supply tank is installed above the level of the oil burner and the oil is fed by gravity to the burner.
8. Use a single-line system if the oil is gravity fed from the supply tank to the burner.
9. Use a two-line system if the oil tank is buried and below the level of the burner.
The filler pipe is the filling connection located on the top of the oil supply tank and terminating above the ground level at least 2 ft from the outside building wall. This is commonly a 2-in-diameter pipe of a design and material specified by the local authorities.
Generally, the specifications will mandate the use of a corrosion — resistant material.
It is important that the filler pipe be connected to one opening and the vent pipe to a separate opening on the tank. No cross-connection of vent pipe is permitted with the filler pipe or the return line from the oil burner. Some authorities demand double-swing connections at the oil supply tank.
The termination point of the filler point at ground level should be equipped with a watertight metal cap. The termination point should be of such design that oil spillage is minimized when the oil hose is disconnected.
The vent pipe is attached to the oil supply tank at a point separate from that of the filler pipe, and it should terminate at a point above the ground at least 2 ft from the outside wall of the building. The vent pipe must be equipped with an approved vent hood or weatherproof cap to prevent water and other contaminants from entering the pipe and gaining access to the oil supply tank.
A cartridge-type oil filter should be installed on the fuel line inside the building and as close to the fuel tank as possible. One filter is adequate on oil burners operating at more than 1.00 gph.
Change the filter cartridge at least once a year. The filter body should be thoroughly cleaned before a new cartridge is installed.
Note
A cartridge-type oil filter in the oil line will prevent sludge in the oil clogging the fuel pump strainer or the oil burner nozzle. Both of these problems are a common cause of system failure.
Both direct-drive and belt-drive blowers are used with oil-fired furnaces. These blowers and motors are identical to those used in gas — fired furnaces. Read the section Blowers and Motors in Chapter 11, “Gas Furnaces” for additional information.
Waste oil furnaces have been used for years in garages and other commercial establishments where a sufficient supply of waste oil is produced. Instead of paying a third party to haul away the waste
Oil, it can be cleanly burned in a waste oil furnace. Burning the waste oil on the premises not only provides heat for the structure
(at no cost) but also eliminates the expense of having the oil hauled away. A waste oil burner and furnace are designed to burn the waste oil without emitting smoke and with very little odor.
Waste oil is a surprisingly efficient heating fuel. Tests have shown that it contains from 183,000 to 240,000 Btu of energy per gallon. That is more than twice the energy potential of a gallon of natural gas or propane.
Note
Waste oil furnaces are not UL approved for use inside a residential structure. On the other hand, they can be installed in a separate structure with the heat being forced into the residence through ducts.
A typical waste oil furnace is illustrated in Figure 12-21. It consists of a multi-fuel burner, pumps, tank strainer, one-way check valve, a filter, a vacuum gauge, and a low-volt wall thermostat. Some waste oil furnaces are also provided with barometric dampers, pumps, and air compressors.
Waste oil furnaces are relatively free of operating problems. Those problems that do occur are commonly traced to incorrect installation procedures. They include (1) using fuel pipes that are too small in diameter, (2) contaminants in the fuel, and (3) improper chimney installation or parts.
Periodic maintenance includes cleaning the reusable filter by rinsing it in a solvent bath and vacuuming the ash and dust residue from inside the dual cleanout doors at the end of each day. Both procedures will maximize the operating efficiency of the waste oil furnace.
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.
Duct sizing should include an allowance for air conditioning, even though it may not be initially installed with the furnace. Air conditioning involves a greater volume of air than heating.
All ductwork located in unconditioned areas (attics, crawl spaces, and so on) downstream from the furnace should be insulated to prevent unwanted heat gain.
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NONCOMBUSTABLES FLOOR Figure 12-21 KAGI waste oil furnace. (Courtesy Kagi)
The procedure for starting an oil burner may be summarized as follows:
1. Open all warm-air registers.
2. Check to be sure all return air grilles are unobstructed.
3. Open the valve on the oil supply line.
4. Reset the burner primary relay.
5. Set the thermostat above the room temperature.
6. Turn on the electric supply to the unit by setting the main electrical switch to the on position.
7. Change the room thermostat setting to the desired temperature.
The oil burner should start after the electric power has been switched on (Step 6). There is no pilot to light as is the case with gas-fired appliances. The spark for ignition is provided automatically on demand from the room thermostat. Allow the burner to operate at least 10 minutes before making any final adjustments. Whenever possible, use instruments to adjust the flame.
In oil-fired furnaces, the inefficient incomplete combustion of the oil will cause sulfur dioxide (SO2) gas to spill from the furnace. Spillage commonly occurs during the few seconds during the initial startup of a conventional oil furnace, which is not a problem, but it can occur more often, and well beyond the initial startup period, in a poorly maintained furnace. Sulfur dioxide gas in large quantities can be harmful to the lungs. It has a strong, unpleasant odor. The problem can be eliminated by:
1. Fitting a delayed-action solenoid valve to the gas burner (see Delayed-Action Solenoid Valve in this chapter) to increase the efficiency of the combustion process.
2. Install a well-balanced barometric damper in the flue pipe.
3. Seal all flue pipe joints against leaks.
Oil Furnace Inspection, Service, and Maintenance Tips
Oil 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:
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 the power is switched off, the gas valve is then turned off.
1. Inspect the vent pipe for water accumulation, sagging piping, dirt, loose joints, and damage.
2. 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.
3. Inspect the furnace wiring for burnt or damaged wires and loose connections.
4. Inspect interior and exterior furnace surfaces for dirt or water accumulation.
5. Make sure the blower access door is tightly closed.
6. Inspect the burners for dirt, rust, or signs of water.
7. Make sure the fresh air grilles and louvers are open, clean, and unobstructed.
8. Inspect and clean the condensate traps and drain to prevent water accumulation in the furnace (condensing furnaces).
9. Inspect the blower wheel and remove any debris.
10. Check the furnace support and base. The base of the furnace must form a tight seal with the support.
11. Run the furnace and observe its operation. The furnace should operate smoothly and quietly.
12. Check the vent pipe and return duct to make sure they are not leaking.
13. Analyze the combustion gases to make sure they meet the furnace manufacturer’s specifications.
14. Clean or change the air filter once a month or as needed, if less than a month.
15. Clean and adjust oil burner before the start of the heating season.
16. Inspect oil supply tank for water and possible contaminants daily or at the beginning of the heating season.
17. Drain oil supply tank at the end of the heating season and check for cracks or other damage.
18. Clean heat exchanger and combustion chamber at the beginning of the heating season and whenever there appears to be a
Significant accumulation of soot and dirt. Remove soot and dirt with a long-handled brush and a vacuum hose.
19. Inspect the inside and outside of the chimney for damage.
20. Inspect and clean the chimney before the beginning of the heating season.
21. Clean the furnace flue pipe, barometric damper, and chimney base.
22. Clean the fan/blower blades.
23. Clean or replace the air filter.
24. Fill the small oil cups of a belt-driven blower motor once or twice during the heating season. Direct-drive blowers with internal motors do not require periodic oiling.
25. Check the oil pressure in the burner.
26. Check all burner fittings for leaks.
27. Clean the oil filter bowl and replace, if required.
28. Clean the burner.
29. Check the condition of the burner nozzle and replace if dirty. Dirty oil burner nozzles cannot be cleaned and returned to service.
30. Check the operation of the cad cell flame detector.
31. Check the operation of the thermostat and fan controls.
After inspecting the heating system and making any necessary service and repairs, the oil furnace should be adjusted for maximum operating efficiency. This is accomplished by taking four measurements through a small (V2 inch diameter) hole in the flue pipe very close to the furnace. First, run the furnace for at least 15 minutes to establish a steady and uniform flue pressure. Then, take the following measurements:
1. Check the smoke contact of the flue gases.
2. Check the draft pressure of the flue gases.
3. Check the temperature of the flue gases.
4. Check the carbon monoxide or oxygen content of the flue gases.
Adjustments to the oil furnace and the burner will be made on the basis of the four measurements listed above. Adjustment must be made in accordance with the furnace, burner, or control
Manufacturer’s procedures listed in its service, maintenance, and repair manuals.
Table 12-3 contains the most common operating problems associated with oil-fired furnaces. Each problem is given in the form of a symptom, the possible cause, and a suggested remedy. This list is intended to provide the operator with a quick reference to the cause and correction of a specific problem.
Table 12-3 Troubleshooting an Oil-Fired Furnace
Symptom and Possible Cause Possible Remedy
Change in size of fire
(a) Dirty nozzle.
(b) Low pressure.
(c) Plugged strainer.
(d) Cold oil.
No oil flow
(a) Oil level below intake line in the supply tank.
(b) Clogged strainer.
(c) Clogged nozzle.
(d) Air leak in intake line.
(e) Restricted intake line (high vacuum reading).
(f) Two-pipe system air bound.
(g) Single-pipe system air bound.
(h) Slipping or broken coupling.
(i) Frozen pump shaft.
Oil spray but no ignition
(a) Dirty electrodes.
(b) Improper spacing.
(c) Cracked porcelain.
(d) Dead transformer.
(e) Loose connection.
(f) Faulty relay.
(a) Clean or replace.
(b) Adjust at pump.
(c) Clean.
(d) Adjust pressure.
(a) Fill tank.
(b) Remove and clean strainer.
(c) Clean or replace.
(d) Tighten fittings and plugs; check valves.
(e) Replace kinked tubing; check valves, filters.
(f) Check bypass plug.
(g) Loosen gauge port and drain oil until foam is gone.
(h) Tighten or replace coupling.
(i) Replace.
(a) Clean or replace.
(b) Reset.
(c) Replace.
(d) Replace.
(e) Tighten.
(f ) Replace.
Symptom and Possible Cause
Burner motor does not start
(a) Defective thermostat.
(b) Fuse burned out.
(c) Limit control open.
(d) Contact dirty or open on primary relay.
(e) Relay transformer burned out.
(f) Motor stuck or burned out or overload protector out.
(g) Primary relay off on safety. Noisy operation
(a) Bad coupling alignment.
(b) Loose coupling.
(c) Air in oil line.
(d) Pump noise.
(e) Hum vibration.
(f) Combustion noise.
(g) Furnace too small.
(h) Burner noisy.
(i) Blower noisy.
Burner will not run continuously
(a) Lockout timing too short.
(b) Poor flame due to too much air; too little oil.
(c) Water or air in oil.
(d) Control wired wrong. Pulsation
(a) Air adjustment.
(b) Pressure over fire.
(a) Replace.
(b) Replace.
(c) Check setting and correct.
(d) Clean or replace relay.
(e) Replace relay.
(f) Replace if burned out.
(g) Push reset button.
(a) Loosen fuel unit or motor.
(b) Tighten setscrews.
(c) Bleed oil line; look for leaks.
(d) Continued running sometimes works in gears. If not, replace.
(e) Isolate pipes from structural members.
(f) Adjust noise.
(g) Check heat loss to be sure furnace properly sized.
(h) Check mounting and position; adjust air.
(i) Oil bearing. Tighten shaft collars; adjust belt tension; align and tighten pulleys; position rubber isolators.
(a) Replace primary control.
(b) Check nozzle, air adjustment, oil pressure, and size of nozzle.
(c) Look for leak in supply.
(d) Check and rewire.
(a) Readjust air.
(b) Correct draft to 0.02 in W. C. negative.
(c)
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Possible Remedy |
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Symptom and Possible Cause |
Dirty or improperly set electrodes.
(d) Too much oil impingement.
Short cycling of fan
(a) Fan control setting.
(b) Input too low.
(c) Temperature rise too low due
To excessive speed of blower.
Short cycling on limit control
(a) Limit setting low.
(b) Input too high.
(c) Temperature rise too high due to blower running too slow.
(d) Temperature rise too high due to restricted returns or outlets.
(e) Fan control setting too high.
(f) Control out of position.
High fuel consumption
(a) Input too high.
(b) Flue loss too great.
Not heating
(a) Low input.
(b) Insufficient air circulating.
(c) Clean and reset; wire primary control for continuous ignition.
(d) Check nozzle and pump pressure; check nozzle size and angle and position of drawer assembly.
(a) Set lower turn on (115°F).
(b) Check burner input.
(c) Slow blower down and check ventilation.
(a) Reset to maximum.
(b) Check burner input.
(c) Increase blower speed.
(d) Open dampers or add additional outlets or returns.
(e) Reset lower (115°F).
(f) Place cad cell in proper position.
(a) Check burner input.
(b) Measure CO2 and flue-gas temperature; if loss is more than 25%, reset air, check input, and speed up blower. Check static pressure in return and outlet plenum and correct to recommended values.
(a) Check nozzles and input.
(b) Speed up blower. Check size and location of ducts and outlets. Set fan control and blower for continuous air circulation.
Some manufacturers include troubleshooting charts in their furnace manuals. A typical oil furnace troubleshooting chart for a Thermo Pride oil furnace is illustrated in Figure 12-22. These troubleshooting charts are organized in a yes/no format, guiding the technician through a list of steps that eventually lead to the specific operating problem and its remedy.
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Figure 12-22 (continued). |
Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers