Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Coal and wood furnaces burn solid fuels. The former burns coal or coke; the latter, various types of wood. Solid-fuel forced-warm-air furnaces are rated in accordance with formulas provided by the for­mer National Warm Air Heating and Air Conditioning Association (now the Air Conditioning Contractors of America). Check the lat­est edition of the publication Commercial Standard for Solid-Fuel — Burning Forced Air Furnaces. Some solid-fuel furnaces are designed to burn coal in combination with one or more other fuels, such as gas, oil, or wood (see Multi-Fuel Furnaces in this chapter).

The operating principle of coal, wood, or multi-fuel furnaces is relatively simple. The combustion process takes place in a sealed firebox located inside the furnace cabinet. A blower forces the heated air over an exchanger and then through the ducts to the liv­ing areas inside the house or building. Automatic controls are used to control its safe operation.

Coal Furnaces

Coal furnaces are either hand fired or fired with a coal stoker. A stoker is an automatic coal feeding device that carries the coal from the storage bin to the furnace as needed.

Early coal furnace models were gravity-feed types (Figure 13-1). The heat rose from the furnace through registers in the floor. No fan or blower was used to force the heated air through ducts. Later furnace models were equipped with blowers so that they could be used in forced-warm-air heating systems (Figure 13-2). The com­pact design of a modern coal furnace is illustrated in Figure 13-3.

Sizing Requirements

The first step in planning a heating system is to calculate the maxi­mum heat loss and gain for the structure. This should be done in accordance with procedures described in the manuals of the Air Conditioning Contractors of America (e. g., Manual J, Residential Load Calculation) or by a comparable method (see Chapter 4 in this volume). This is very important, because the data will be used

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-1 Oneida coal-fired gravity warm-air

Furnace. (Courtesy Oneida Heater Co., Inc.)

To determine the size (capacity) of the furnace selected for the installation.

Location and Clearance

A coal furnace should be located a safe distance from any com­bustible materials. Consult the local codes and regulations for the required clearances.

Never obstruct the front of the furnace. Access must be provided to the fire and ashpit doors in order to operate the furnace.

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.

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-2 Oneida coal-fired forced-warm-air furnace.

(Courtesy Oneida Heater Co., Inc.)

Installation Recommendations

Make certain you have familiarized yourself with all local codes and regulations that govern the installation of coal furnaces and coal stokers. Local codes and regulations take precedence over national standards. See Installation, Operating and Maintenance Instructions for Coal, Wood, and Multi-Fuel Furnaces in this chapter for additional information.

Duct Connections

The furnace supply plenum contains air at very high temperatures. Because of these high temperatures, make certain there are safe clearances between the ductwork and any combustibles (including wall and ceiling materials). Check the furnace manufacturer’s recommendations.

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-3 Yukon/Eagle coal-fired furnace.

(Courtesy Yukon/Eagle)

Note

The same fire prevention precautions should also be taken for combustible materials near air conditioning and evaporator coils.

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 of Volume 2 (“Ducts and Duct Systems”). The comments in Chapter 11, “Gas Furnaces” about furnace duct connections and air distribution ducts also apply to ducts used with coal-fired forced-warm-air furnaces.

Electrical Wiring

A wiring diagram specifying the electrical connections between the various controls should be provided by the furnace and stoker manufacturers.

Caution

All electrical wiring must be done in accordance with the National Electrical Code and the local authorized code.

Warning

Turn off the electric power at the circuit breakers (or fuse box) before making any line voltage connections.

As shown in Figure 13-4, a room thermostat operating through the stoker time relay starts and stops the stoker in response to tem­perature conditions. The time relay operates the stoker to keep the fire alive during periods when heat may not be required by the room thermostat. A stoker time relay should include a device to shut down the stoker immediately after a shutdown call from the thermostat. This action eliminates fuel waste by preventing the overshooting of room temperature.

A high-limit control is used to protect the system against exces­sive temperatures. This control takes the form of an air switch in a warm-air heating system.

A snap switch should be installed in the wiring between the stoker time relay and the stoker motor to open the stoker motor cir­cuit when the firebox or ashpit is being cleaned (Figure 13-4).

A wiring diagram for a combination wood-oil furnace is shown in Figure 13-5. Note that the wood controls and oil controls are wired as separate systems. A similar method of wiring the controls as two separate systems is used for a combination wood-gas furnace

G H

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-4 Wiring diagram for coal-fired furnace. The snap switch, located between the stoker time relay and the stoker motor, is used to open the stoker motor circuit when cleaning the fire.

(Figure 13-6). This is a common practice on furnaces designed to burn more than one fuel.

The wiring diagrams shown in Figures 13-7 and 13-8 are for gas and oil heating systems with air conditioning.

Ventilation and Combustion Air

The total draft requirement of a coal-fired furnace is greater than those of furnaces that burn gas or oil because the chimney draft must overcome the resistance of the fuel bed.

Both a primary and secondary air supply are necessary for the combustion of solid fuels. The primary air passes through the fuel bed, generally on an upward path from the ashpit. The secondary air is usually admitted through slots in the furnace fire

FAN LIMIT SWITCH FAN

подпись: fan limit switch fan

HONEYWELL

T8000C

THERMOSTAT

подпись: honeywell
t8000c
thermostat

PUSH-IN CONNECTION

SCREW TERMINAL

PUSH-ON TERMINAL

WIRE-NUT SOLDERLESS CONNECTOR

подпись: push-in connection
screw terminal
push-on terminal
wire-nut solderless connector

HONEYWELL T8400 THERMO STE AT

подпись: honeywell t8400 thermo ste at

LEGEND

подпись: legend

MAXIMUM RATING OF CIRCUIT PROTECTOR DEVICE — 20 AMPS-

подпись: maximum rating of circuit protector device - 20 amps- Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

BLK-BLACK BLU-BLUE WHT-WHITE GRN-GREEN RED-RED

————- SOLID FACTORY WIRED LINE VOLTAGE CONDUIT FURNISHED

BROKEN FIELD WIRED LINE VOLTAGE CONDUIT NOT FURNISHED DOTTED 24 VOLT FIELD WIRING

подпись: blk-black blu-blue wht-white grn-green red-red
 solid factory wired line voltage conduit furnished
broken field wired line voltage conduit not furnished dotted 24 volt field wiring

.b.

K>

Vn

подпись: .b.
k>
vn
Figure I 3-5 Wiring diagram for a wood-oil combination furnace. (Courtesy Yukon/Eagle)

• PUSH-IN CONNECTION 0 SCREW TERMINAL m PUSH-ON TERMINAL /ffl WIRE-NUTSOLDERLESS CONNECTOR

LEGEND

MAXIMUM RATING OF CIRCUIT PROTECTOR DEVICE -20 AMPS-

Figure 13-6 Wiring diagram for a wood-gas combination furnace. (Courtesy Yukon/Eagle)

BLK-BLACK BLU-BLUE WHT- WHITE GRN-GREEN RED-RED

SOLID FACTORY WIRED LINE VOLTAGE CONDUIT FURNISHED BROKEN FIELD WIRED LINE VOLTAGE CONDUIT NOT FURNISHED DOTTED 24 VOLT FIELD WIRING

DIL

T

 

BURNER

T

 

FUME SWITCH

 

K>

O>

 

FAN LIMIT SWITCH FAN

 

HONEYWELL

T8400

THERMOSTEAT

 

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

LEGEND BLK-BLACK

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

AC

CONDENSING

 

AIRCOND.

 

UNIT

 

COMPRESSOR

 

RELAY

 

подпись: ac
condensing 
 aircond.
unit 
 compressor
 relay

GAS

T

 

BURNER

T

 

подпись: gas t
burner t
PUSH-IN CONNECTION BLU-BLUE

0 SCREW TERMINAL WHT-WHITE

■ PUSH-ON TERMINAL GRN-GREEN

Fifi WIRE-NUTSOLDERLESS CONNECTOR fED-RED

———- SOLID FACTORY WIRED LINE VOLTAGE CONDUIT FURNISHED

BROKEN FIELD WIRED LINE VOLTAGE CONDUIT NOT FURNISHED

———- DOTTED 24 VOLT FIELD WIRING

MAXIMUM RATING OF CIRCUIT PROTECTOR DEVICE

— 20 AMPS-

Figure 13-7 Wiring diagram for gas heating system with air

Door and passes over the fire to complete the combustion process (Figure 13-9).

On residential furnaces, the secondary air slots in the fire door should be kept closed. There is usually enough air leakage to

подпись: door and passes over the fire to complete the combustion process (figure 13-9).
on residential furnaces, the secondary air slots in the fire door should be kept closed. there is usually enough air leakage to
Conditioning. (Courtesy Yukon/ Eagle)

LEGEND BLK-BLACK

FAN LIMIT SWITCH

FAN r~

подпись: fan limit switch
fan r~
Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

AC

CONDENSING

UNIT

AIRCOND.

COMPRESSOR

RELAY

подпись: ac 
condensing
unit 
 aircond.
compressor ---
 relay 
PUSH-IN CONNECTION BLU-BLUE

0 SCREW TERMINAL WHT-WHITE

_ PUSH-ON TERMINAL GRN-GREEN

M WIRE-NUTSOLDERLESS CONNECTOR RED-RED

——— SOLID FACTORY WIRED LINE VOLTAGE CONDUIT FURNISHED

BROKEN FIELD WIRED LINE VOLTAGE CONDUIT NOT FURNISHED

——— DOTTED 24 VOLT FIELD WIRING

MAXIMUM RATING OF CIRCUIT PROTECTOR DEVICE

— 20 AMPS-

Figure 13-8 Wiring diagram for oil heating system with air

Conditioning. (Courtesy Yukon/ Eagle)

SHAPE \* MERGEFORMAT Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Admit a suitable amount of secondary air without having to open the fire door. An excessive amount of secondary air (particularly after the fire has taken hold) will actually reduce the efficiency of the furnace.

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

The draft will determine the rate at which the fuel is burned. The draft in furnaces using coal or coke will depend upon the following factors:

1. Type of fuel (anthracite or bituminous, coke, etc.)

2. Size of fuel

3. Fuel-bed thickness

4. Ash and clinker accumulation

5. Flue resistance to the flow of gas

6. Soot accumulation in flue

7. Grate area

The rate at which the fuel is burned will depend upon the heat­ing load demand of the specific location. The burning rate can be varied by regulating the draft. One of the most effective methods of doing this is by placing a damper in the flue outlet. This method proportionally reduces both primary and secondary air.

An ashpit damper is required for low combustion rates. A cold — air check damper is necessary when there is excessive chimney draft.

Venting

Provisions must be made for venting the products of combustion to the outside in order to avoid contamination of the air in the living or working spaces of the structure. Masonry chimneys and low — heat Type A prefabricated chimneys are the most common methods of venting coal-fired furnaces.

Flue Pipe

Coal furnaces require flue passages—larger than those used—for gas and oil furnaces. This is due to the far greater volume of smoke, soot, and other products of combustion associated with burning solid fuels.

The flue pipe extending vertically up through the chimney should have a constant cross-sectional area throughout its length (Figure 13-10). The chimney flue (i. e., the passage between the furnace and the chimney) must have a cross-sectional area no less than that of the furnace flue collar. A diameter 1 inch larger is recommended. Consult the local building code for flue sizing recommendations if more than one furnace or appliance is to be connected to a single flue.

The chimney thimble, which connects the smoke pipe to the chim­ney, should be made of heat resistant fire clay. Its inside diameter should be roughly equal to the outside diameter of the smoke pipe to ensure an airtight fit at the chimney. Do not allow the chimney thimble to extend beyond the chimney flue lining.

Chimneys and Chimney Troubleshooting

A chimney used with a coal furnace should be of sufficient height and area to meet the requirements of the furnace. The top of the chimney should be at least 2 ft higher than the highest portion of the roof within 10 ft horizontally. There should also be a minimum of 3 feet between the top of the chimney and the point at which it passes through the roof (see Figure 13-10). For best results, only the furnace should be connected to the chimney. Read the section Chimneys in Chapter 11, “Gas Furnaces.”

A chimney connected to a coal or wood burning furnace must have a cleanout door at the bottom. This door allows ashes and other residue to be removed from the bottom of the chimney. The cleanout door must have a tight fit. It must never be left open dur­ing the combustion process.

The chimneys used with coal furnaces are basically the same as those used with gas and oil units. Read the section Chimney Troubleshooting in Chapter 11, “Gas Furnaces” for a description of common chimney problems and suggested remedies.

The cap should be such that it helps to prevent downdraft while providing adequate exhaust draft.

Top of chimney must be at least 2" higher than peak or highest portion of roof within 10" hori­zontally.

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Min. 3" from top of chimney to point at which it passes through the roof.

подпись: min. 3" from top of chimney to point at which it passes through the roof.

Recommended min. height approx. 20"

подпись: recommended min. height approx. 20"

Cleanout door should be pro­vided and it must have an air­tight fit. It should not be left open for any length of time when stove is in operation.

подпись: cleanout door should be provided and it must have an airtight fit. it should not be left open for any length of time when stove is in operation. Coal Furnaces, Wood Furnaces, and Multi-Fuel FurnacesA chimney that rises within house is better insulated than a chimney located outside the house & exposed to weather.

The chimney flue must have a cross-sectional area at least equal to, or 1" larger than the diameter of the stove’s flue collar. If more than one stove or appliance is to be connected to a single flue, consult your local building authority for flue sizing and ap­proval.

Coal Furnaces, Wood Furnaces, and Multi-Fuel FurnacesThe chimney thimble should be constructed of fire clay and should have an inside diameter which is nominally equal to the outside diameter of the smoke pipe to ensure a reasonably air­tight fit.

The thimble must not extend beyond the flue lining.

H

подпись: hIf the chimney flue is to serve more than one stove, the thimbles should be separated vertically. Again, consult your local build­ing authority.

Furnace Components

The basic components of a coal furnace used in a forced-warm-air heating system are:

1. Cabinet or jacket

2. Firebox

3. Grate

4. Heat exchanger

5. Blower and motor

6. Access door or doors for stoking and cleaning

7. Small blower to fan fire when more heat is required

8. Coal stoker (for automatic feeding)

9. Automatic controls

Each of these components is described in the sections that fol­low. Most of these components are also found in wood and multi­fuel furnaces. Additional information is contained in Chapter 10, “Furnace Fundamentals” and the various chapters in which furnace controls and fuel-burning equipment are described.

Note

Solid — and multi-fuel furnaces may also include backup devices such as a gas burner, an oil burner, or electric heating elements.

Automatic Controls

The automatic controls of a coal furnace equipped with a stoker are shown in Figure 13-11. This control system is very similar to the one used for an oil burner except for the automatic timer included with the stoker relay and transformer. During the heating season, the fire of a coal furnace must burn continually. The timer is a device designed to operate the coal stoker for a few minutes every hour or half-hour in order to keep the fire alive during those peri­ods when little heat is required.

A stack thermostat or similar device is recommended for use with stokers in areas subject to electric power failures. An electric power failure will shut down the stoker. If the shutdown period is long enough, the fire will die for lack of fuel. When the electricity comes on again, the stoker will feed coal to a cold firepot. A stack thermostat or light-sensitive electronic device will monitor the stack heat or fire and prevent the stoker operation when the fire is out.

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-11 Automatic controls for a coal-fired furnace fed by a

Stoker. (Courtesy U. S. Department of Agriculture)

A blower fan control is generally included with most forced — warm-air furnaces. Many manufacturers will also provide a hand­operated draft control with a coal-fired furnace. Electrically operated dampers for draft control are generally available, but at extra cost.

The operation of a furnace is controlled by the room thermostat. In heating systems in which a coal-fired furnace is used, the ther­mostat opens or shuts dampers to increase or decrease the supply of air to the fire. When the supply of air is increased, the fire becomes hotter, and the amount of heat generated by the furnace is increased. Decreasing the supply of air to the fire has the opposite effect. Older coal-fired furnaces were not controlled by a room thermostat. Hand-operated dampers were used instead.

Furnace Grate

An essential part of any coal-fired furnace is the metal grate on which the fuel is burned (Figure 13-12). The grate should be designed to allow sufficient primary air to pass upward through the fuel bed for the combustion process. The ashes will drop to the ashpit below through the same openings when the grate is shaken.

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-12 Cutaway view of a coal-fired furnace showing grate and combustion chamber details.

(Courtesy Oneida Heater Co., Inc.)

There should be a metal heat transfer surface (heat exchanger) of sufficient size above the fire to transfer the heat from the combus­tion process to the water or air in the heating system.

Coal Stoker

A coal stoker is a device used to feed coal to a coal-fired furnace or boiler automatically. See Chapter 3 of Volume 2, “Coal Firing Methods” for additional information about coal stokers.

Hand-Firing Methods

The method used to hand-fire a furnace will depend largely upon the type of solid fuel used in it. Coke and the various types of coals each have their own special hand-firing methods. Some of these firing methods are described in Chapter 3 of Volume 2, “Coal Firing Methods.”

Blower and Motor Assembly

The blowers used on coal furnaces are the same centrifugal types found on gas, oil, and electric forced-warm-air furnaces. The blowers are usually installed on either side of the furnace or at the back. The latter arrangement is especially recommended for wood-burning and multi-fuel furnaces. A detailed description of blowers and blower motors is included in Chapter 11, “Gas Furnaces.”

Blower Adjustment

Instructions for making blower adjustments can usually be obtained from the furnace manufacturer. A brief description of methods used to make blower adjustments is included in Chapter 11, “Gas Furnaces.”

Air Filter

The number, size, and type of air filter will be recommended by the furnace manufacturer in the specifications. See also Chapter 12, “Air Cleaners and Filters” in Volume 3.

System Accessory Devices

A coal furnace used in a central forced-warm-air heating system may include some or all of the following accessories:

1. Electronic air filter

2. Humidifier

3. Dehumidifier

4. Electronic air cleaner

5. High performance media filter

6. Central air evaporator coil

7. Domestic hot-water coil

8. Condensate pump

Each of these accessories is covered in Volumes 2 and 3. They may also be used with wood and multi-fuel furnaces.

One method of adding summer air conditioning to a coal-fired heating installation is to install a separate and independent air con­ditioning system. This is expensive, because it includes the equip­ment and separate ducts, but it avoids many complications.

If you are considering the idea of adding air conditioning at some future date, you should select a solid-fuel furnace capable of meeting your cooling needs. For example, Oneida All-Fuel and Two-in-One furnaces are equipped with blowers large enough to handle up to 4 tons of air conditioning and electronic air cleaning.

Wood Furnaces

A wood furnace is very similar in design to a coal furnace except that it burns wood. The components are identical (sealed firebox, exchanger, blower, etc.), and it can be combined with the same types of accessories (electronic air cleaner, humidifier, etc.) Figure

13- 13 shows a wood furnace.

Decorative

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Wood Add-On Furnaces

Wood add-on furnaces are designed to be added to existing oil fur­naces as a backup heat source (Figure 13-14). Their construction details are identical to those of a standalone wood furnace.

Caution

Make sure the existing furnace can accept an add-on unit. Not all furnaces can do this.

There are two types of add-on installations: independent and parallel. The parallel installation uses the existing blower in addi­tion to the blower on the wood burning furnace (Figure 13-15).

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces Coal Furnaces, Wood Furnaces, and Multi-Fuel FurnacesTYPICAL

INSTALLATIONS

O

SERIES ADD-ON INSTALLATION A PARALLEL ADD-ON W INSTALLATION 0 PRIMARY INDEPENDENT INSTALLATION

Figure 13-15 Wood add-on furnace (parallel installation).

(Courtesy Yukon/Eagle)

Multi-Fuel Furnaces

Furnaces designed to burn more than one fuel are called multi-fuel furnaces or combination furnaces. These furnaces are designed to burn oil or gas in one combustion chamber, wood or coal in the

Other, and automatically switch between the fuels when the situa­tion warrants (Figures 13-16 and 13-17).

The solid-fuel combustion chamber of these furnaces burns either coal or wood and does so much more efficiently than stan­dard coal or wood stoves. The other chamber uses an oil burner, gas burner, or electric resistance heater to generate heat in the same manner as in a single-fuel furnace. Either combustion chamber is capable of heating an entire house.

The operation of the furnace shown in Figure 13-16 is controlled by a Honeywell two-stage thermostat. The thermostat is equipped with indicator lights to indicate which fuel is being burned.

Wood or coal can be used as the primary fuel with oil, gas, or electricity as the automatic backup; or oil, gas, or electricity can be used as the primary fuel with wood or coal serving as the backup.

FAN/LIMIT

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

1

Smoke baffle.

10

Thread cutting screw

19

Ash pan weldment

2

Hex nut (V2 inch)

11

Secondary air tube

20

Ash door assembly

3

Baffle bracket

Weldment

21

Fire door assembly

4

Top brick retainer (rear)

12

Fire brick (12 x 6 x

22

Machine screw

5

Top brick retainer

2-inch)

23

Base weldment

(side)

13

Secondary air tube

24.

Door latch

6

Fire brick (9 x 4/2 x

Gasket

25

Refractory pot liner

2-inch)

14

Connecting rod assembly

26

Combination chamber

7

Fire brick (9 x 6 x

15

Coal grate

Weldment

2-inch)

16

Coal grate handle

8

Tube support bracker

17

Coal grate frame

9

Fender washers ([3]/i6-

18

Coal grate shaker

Inch I. D. x 9-inch O. D.)

Assembly

Coal Furnaces, Wood Furnaces, and Multi-Fuel Furnaces

Figure 13-17 Exploded view of Yukon/Eagle oil/gas solid fuel combination furnace. (CourtesyYukon/Eagle)

подпись: figure 13-17 exploded view of yukon/eagle oil/gas solid fuel combination furnace. (courtesyyukon/eagle)

©

подпись: ©

Installation, Operating, and Maintenance Instructions for Coal, Wood, and Multi-Fuel Furnaces

Make certain you have familiarized yourself with all local codes and regulations governing the installation, operation, and mainte­nance of coal furnaces and coal stokers, wood furnaces, and multi­fuel furnaces. Local codes and regulations take precedence over national standards.

Assembly and Installation Recommendations

Coal, wood, and multi-fuel furnaces are shipped disassembled with complete assembly instructions. The installer must assemble the furnace in accordance with these instructions. If the furnace is fired by a coal stoker or contains some automatic controls, the installer must connect the electrical service from the line voltage main. The low-voltage thermostat must also be connected.

Caution

NEVER attempt to assemble and install a furnace unless you have the necessary qualifications. Furnace assembly is a task best left to those with the necessary training and experience. It is not a do-it-yourself project. An improperly assembled and installed furnace is a fire hazard and could lead to serious injuries or even death.

Note

Always check the local building codes first before installing a furnace. The installation must comply with the local code requirements.

NEVER install a wood furnace so that the supply air from the wood furnace feeds into the return air of the conventional furnace. Doing so may result in the following problems:

Locate the furnace as close to the chimney or flue as possible and near the center of the heat distribution system (i. e., the ductwork) or next to a primary furnace. Be sure to maintain the minimum clearances specified in the local building code. The NFPA (National Fire Protection Agency) Standard No. 90B also lists and describes the minimum standard clearances from combustible surfaces. Recommended clearances are listed in Table 13-1.

Reduced clearances are permitted if the area between the furnace and combustible ducts are protected according to NFPA Bulletin 90B. In any case, the furnace must be so located that it receives suf­ficient combustion air.

Caution

Shielding a wall with sheet metal or masonry will not protect it. These materials are excellent heat conductors, and any com­bustible wall material behind them will still be a fire hazard.

Maintenance Instructions

Always follow a regular service and maintenance schedule to ensure safe and efficient operation of the furnace. Keep a service and main­tenance list next to the furnace and check off the date of each inspection with comments on what was done.

1. Make certain the duct connections to the furnace are tight and secure.

2. Follow a regular service and maintenance schedule of the fur­nace and chimney for safe and secure operation.

Table 13-1 Recommended Minimum Clearances

3. Frequently inspect and clean the heat exchanger, smoke pipe, and chimney of soot and/or creosote.

4. Periodically check the furnace cabinet (jacket) for cracks.

5. Make certain all access doors fit tightly.

6. Inspect and clean the air filter monthly. Change the air filter at least twice a year.

7. Regularly inspect and clean the air cleaner grids on furnaces equipped with electronic air cleaners.

8. Periodically clean the humidifier (if so equipped).

9. Oil the blower motor twice a year (blowers with oil caps).

Wood and Coal Furnace Maintenance

1. Remove ash and other debris left over from the combustion process on a daily basis during the heating season.

Note

Never spread ash on your garden or lawn. It contains toxic substances that will kill plants or grass.

Caution

Never allow ashes to build up to the grate level in the furnace. Doing so will shorten the service life of the grate.

2. Keep the heating surfaces and flues clean. Soot will reduce heating efficiency. A dirty furnace requires much more fuel to produce the required heat than a clean one does.

3. Hard clinkers lodged between the grate bars should be removed with a poker or slice bar.

Multi-Fuel Furnace Maintenance

Multi-fuel furnaces, or combination furnaces as they are also called, burn coal or wood with oil, gas, or electricity. Because they burn solid fuels (wood or coal), the maintenance instructions for coal and wood furnaces will also apply to the solid-fuel burning sections of a multi-fuel furnace.

1. Inspect and perform any necessary maintenance on a gas or oil burner at least twice a year, preferably at the end and beginning of the heating system. Gas burner and oil burner service and maintenance is covered in the second volume of the Heating, Ventilating, and Air Conditioning Library.

Caution

Maintaining, adjusting, and repairing oil and gas burners requires special instruments and training. These tasks are best left to a trained HVAC technician or someone with equivalent experience.

2. Remove and clean the oil burner strainer at the end of the heating system.

3. Fill the oil storage tank at the end of the heating season to prevent water vapor from collecting.

4. Lubricate the oil burner every 3 months.

5. Have a qualified HVAC technician check and service the oil burner at the beginning of the heating season.

6. Clean, inspect, and service the furnace, smoke pipe, and chim­ney at the beginning of the heating system.

7. Check the furnace and burner electrical system before the begin­ning of the heating system (primary relay, limit control, thermo­stat, electrodes, electrode terminals, and transformer terminals).

8. Clean the solid fuel combustion chamber according to the procedures for cleaning a coal or wood furnace.

Wood and Coal Furnace Operation

Any solid fuel has approximately 12,000 Btu per pound if its mois­ture content is zero. This holds true for coal as well as any of the woods burned in a furnace. The Btu per cord of air dried wood are listed in Table 13-2. Ignition temperatures of coal and wood are listed in Table 13-3.

Wood Furnaces

The wood burning furnace shown in Fig. 13-13 is designed to allow the primary air under the grate to create the initial burning. As the wood burns, gases containing as much as 40 percent of the energy of the wood rise to the top of the flame. The secondary air system (i. e., the round tubes between the firebrick) draws room air into the tubes and provides oxygen to the firebox to burn these gases before they can escape up the chimney flue. The procedure for starting and maintaining a wood fire in a furnace may be outlined as follows:

1. Open the manual draft spinner three or four turns.

2. Make sure the smoke pipe damper is open.

3. Place several pieces of crumpled paper in the center of the fur­nace firebox.

Table 13-2 Btu Per Cord of Air Dried Wood

Type

Pound Weight Per Cord

Btus Per Cord of Air Dried Wood

Equivalent Value #2 Fuel Oil Gallons

White Pine

1800

17,000,000

120

Aspen

1900

17,500,000

125

Spruce

2100

18,000,000

130

Ash

2900

22,500,000

160

Tamarack

2500

24,000,000

170

Soft Maple

2500

24,000,000

170

Yellow Birch

3000

26,000,000

185

Red Oak

3250

27,000,000

195

Hard Maple

3000

29,000,000

200

Hickory

3600

30,500,000

215

Courtesy Yukon/Eagle

4. Place a few handfuls of kindling wood in a criss-cross pattern across the paper, and then add a few pieces of firewood.

Caution

Never start a coal or wood fire with gasoline, kerosene, thin — ners, or any other type of volatile substance.

5. Ignite the paper and close the door to the firebox. Resist the desire to open the door immediately after igniting the paper to see how the fire is doing. Opening the door at this point may cause a flame flash out. It will take a few minutes for the fire to establish itself.

Table 13-3 Ignition Temperatures of Coal and Wood

Type

Ignition Temperature

Paper

350°F

Wood

435°F

Western lignite coal

630°F

Low volatile bituminous coal

765°F

High volatile bituminous coal

870°F

Anthracite coal

925°F

6. Start adding larger pieces of wood when you see some red-hot burning embers. Build the fire slowly. Smaller pieces of wood burn cleaner because they have more surface area exposed to the fire. Use wood a maximum of 4 to 6 inches thick and about 2 to 4 inches narrower than the width of the furnace firebox.

7. Once the fire is established, adjust the draft according to your needs. The draft control on the furnace controls the burning time.

Caution

Always open the door to the firebox slowly. Rapidly opening the door could cause flames to leap out of the firebox.

Coal Furnaces

Burning coal is more difficult than burning wood. It requires patience and a regular procedure. If you are not careful, a coal fire can extinguish itself in a relatively short time. Once a coal fire starts to go out, it is next to impossible to reverse it. If the coal fire goes out, remove all the coal from the furnace and then start the process all over again.

Coal comes in various sizes and types. As a rule, anthracite coal is recommended, because it burns with little smoke when burned properly. Select a coal size that will not fall through the air spaces in the furnace grate. These air spaces are */2 inch wide, so the coal size must be larger than that. Coal is available in three sizes: pea size, nut size, and stove size. Nut-size coal (l3/i6 to l5/s inch) is recom­mended for coal furnaces.

How to Start a Coal Fire

1. Place a small amount of crumpled paper and sticks of kin­dling wood on the ash-covered grates of the furnace.

2. Ignite the paper and wait until the wood starts to burn well.

3. Cover the burning paper and wood with a thin layer of evenly distributed coal. The pieces of coal must touch each other to ignite.

4. Soon after the first layer of coal is ignited, gradually add more coal until the fire bed is built up to about 6 inches deep. As you add the fresh coal always leave some of the burning coal uncov­ered. Be careful not to smother the fire when adding fresh coal.

5. Draw the top red coals toward the front of the firebox and pile fresh coals toward the back.

Note

Protect the furnace grate from direct contact with the fire by a 1- to 2-inch thick layer of ash. This will help prevent the cast — iron grates from overheating and coal from falling through the grate openings.

How to Maintain a Coal Fire

1. Shake the grates once or twice a day to prevent excessive ash accumulation on the grates.

2. Shake the grates with a few short strokes and stop as soon as you see red coals appearing in the ash pan.

Note

Shaking the grate too much may extinguish the fire; shaking it not enough may restrict the amount of air reaching the fire.

3. Never poke or break up a coal fire. Poking the fire tends to bring ash to the surface of the coal bed, where it fuses into lumps or clinkers. Clinkers interfere with efficient burning of the coal.

How to Bank a Coal Fire

Coal fires are reduced (banked) during the night, when less heat is required. If you are using anthracite coal, pile the coal higher toward the back of the firebox and allow it to slope forward toward the firebox door. Leave some red or burning coal uncovered in the front of the firebox.

If you are using bituminous coal, shake the fire and add coal, forming the center cone. Allow enough time for the volatiles to burn off before closing the door to the firebox.

Multi-Fuel Furnace Operation

Begin by loading the coal or wood into the furnace firebox. The auto­matic controls of the furnace will then take over, and the oil burner or gas burner will ignite the coal or wood when heat is required.

Note

Electric heating is not automatic (i. e., thermostat controlled) and must be manually started.

Set the two-stage thermostat. The thermostat controls both com­bustion chambers at separate temperature settings. The oil or gas burner will automatically start and provide supplemental, backup

Heat if the wood or coal fire cannot maintain the temperatures set on the thermostat.

The wood or coal fire is automatically controlled by the wood or coal thermostat setting. After the wood or coal is ignited and the furnace begins delivering the heat called for by the thermostat, the oil or gas burner automatically shuts off. The thermostat automati­cally maintains the heat levels inside the structure by controlling the amount of combustion air to the wood or coal fire.

As the wood or coal fire dies down, the temperature inside the structure declines until it reaches the thermostat temperature setting selected for the oil, gas, or electric heat. When this tempera­ture setting is reached, the oil burner, gas burner, or electric heaters automatically start, and the backup combustion chamber then functions as a conventional furnace.

Troubleshooting Coal, Wood, and Multi-Fuel Furnaces

The troubleshooting sections that follow list the most common problems associated with coal, wood, and multi-fuel furnaces. Each problem is given in the form of a symptom, the possible cause or causes, and some suggested remedies (Tables 13-4, 13-5, and 13-6).

Table 13-4 General Heating Problems

Symptom and Possible Causes Suggested Remedies

Inadequate fire

(a) Insufficient draft

(a) Clean ashpit and remove obstruction to primary air supply; adjust damper control.

(b)

(b) Dirty furnace

(c) Dirty flue

(d) Poor-quality fuel

(e) Grate clogged with slate, clinkers, or other combustion residue

Excessive coal in firebox

(a) Stoker feeding too much coal

(b) Insufficient air

подпись: (b) dirty furnace
(c) dirty flue
(d) poor-quality fuel
(e) grate clogged with slate, clinkers, or other combustion residue
excessive coal in firebox
(a) stoker feeding too much coal
(b) insufficient air
Clean furnace.

(c) Clean flue

(d) Replace with better-quality fuel.

(e) Dislodge with poker, or dump grate and rebuild fire.

(a) Reduce coal feed rate on stoker.

(b) Open manual draft.

Symptom and Possible Causes Suggested Remedies

(c) Stoker wind box full of siftings

(d) Accumulations of clinkers in fire

Excessive fuel use

(a) Dirty furnace

(b) Dirty flue

(c) Incorrect draft

Not enough heat or no heat

(a) Thermostat set too low

(b) Thermostat in wrong location

(c) Thermostat out of calibration

(d) Lamp or other heat source too near thermostat

(e) Dirty air filter

(f) Limit set too low

(g) Fan speed too low

(h) Oil or gas burner is not firing properly

(i) No oil in storage tank

(j) Valve in oil line is closed

(k) No power to furnace

(c) Clean out wind box.

(d) Clean fire.

(a) Clean furnace.

(b) Clean flue.

(c) Adjust dampers for correct rate of combustion.

(a) Raise thermostat setting.

(b) Relocate thermostat.

(c) Recalibrate or replace thermostat.

(d) Remove heat source. Relocate thermostat if heat source cannot be moved.

(e) Clean or replace air filter.

(f) Reset or replace thermostat.

(g) Check for loose fan belts and tighten if too loose; check motor and repair or replace.

(h) Contact your local serviceman.

(i) Check oil tank gauge. If empty, fill tank and start the burner.

(j) Open valve in oil line and start burner.

(k) Check fuse or circuit breaker. Replace blown fuse, or reset tripped circuit breaker. Check the shutoff switch to make sure it is in the ON position.

Symptom and Possible Causes

Suggested Remedies

Blower will not run

(a) No power

(a)

Check blower/fan power switch; check fuses, circuit breakers. Make corrections to re-establish power.

(b) Fan control adjustment too high

(b)

Readjust or replace.

(c) Loose wiring

(c)

Tighten connections.

(d) Defective wiring

(d)

Replace wiring.

(e) Defective motor overload, protector, or motor

(e)

Replace motor.

Blower will not stop

(a) Fan set on manual

(a)

Switch to automatic.

(b) Defective fan switch

(b)

Replace fan switch.

(c) Short in wiring Rapid fan cycling

(c)

Check wiring for damage or loose connection and correct.

(a) Fan switch differential too

(a)

Readjust or replace fan

Low

Switch

(b) Blower speed too high

(b)

Readjust to lower speed

Noisy blower motor

(a) Loose fan blades

(a)

Tighten or replace fan blade assembly.

(b) Incorrect belt tension

(b)

Check specifications and readjust to allow the required slack (commonly about 1 inch).

(c) Pulleys out of alignment

(c)

Realign pulleys.

(d) Dry pulley bearings

(d)

Lubricate bearings.

(e) Defective belt

(e)

Replace belt.

(f) Belt rubbing another surface

(f)

Adjust belt tension.

Symptom and possible causes Suggested remedies

Coal stoker stops

(a) No power

(b) Obstruction in feed screw

(c) Dirty fire

Stoker motor fails to start

(a) No power

(b) Overload condition

(c) Blown fuses

(d) Limit control contacts open

Stoker operates continuously

(a) Controls out of adjustment

(b) Dirty fire

(c) Fire out

(d) Dirty furnace

(a) Check main power switch, fuses/circuit breakers, and correct to restore power.

(b) Remove obstruction.

(c) Clean fire.

(a) Check main power switch, fuses/circuit breakers, and correct to restore power.

(b) Push reset button on transmission; push reset buton on stoker.

(c) Replace fuses.

(d) Allow furnace to cool off.

(a) Readjust controls or call local sales representative for adjustment.

(b) Clean fire.

(c) Rebuild fire.

(d) Clean furnace.

Caution

Gas odors around a multi-fuel (gas/coal or gas/wood) furnace may indicate a safety problem. If you smell gas, (1) immedi­ately open some windows; (2) extinguish any open flame; (3) do not touch any electrical switches; and (4) immediately call the local gas company for advice.

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


Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *