Furnace Fundamentals

The American Society of Heating, Refrigerating, and Air — Conditioning Engineers (ASHRAE) defines a furnace as “a com­plete heating unit for transferring heat from fuel being burned to the air supplied to a heating system.” The Standard Handbook for Mechanical Engineers (Baumeister and Marks, seventh edition) provides a definition that differs only slightly from the one offered by the ASHRAE: “a self-enclosed, fuel-burning unit for heating air by transfer of combustion through metal directly to the air.” Contained within these closely similar definitions are the two basic operating principles of a furnace: (1) Some sort of fuel is used to produce combustion, and (2) the heat resulting from this combus­tion is transferred to the air within the structure. Note that air—not steam, water, or some other fluid—is used as the heat-conveying medium. This feature distinguishes warm-air heating systems from the other types; see Chapter 6, “Warm-Air Heating Systems.”

Most modern furnaces are used in warm-air heating systems in which the furnace is a centralized unit, and the heat produced in the furnace is forced or rises by means of gravity through a system of ducts or pipes to the various rooms in the structure. This is what one commonly refers to as a central heating system. In other words, the furnace is generally in a centralized location within the heating system in order to obtain the most economical and efficient distri­bution of heat (although this is not an absolute necessity when a forced-warm-air furnace is used).

Ductless or pipeless furnaces are also used in some heating appli­cations but are limited in the size of the area that they can effec­tively heat. They are installed in the room or area to be heated but are provided with no means for distributing the heat beyond the immediately adjacent area. This is a far less efficient and economi­cal method of heating than the central heating system, but it is found to be adequate for a room, an addition to an existing struc­ture, or a small house or building.

Classifying Furnaces

There are several different ways in which furnaces can be classified. One of the more popular methods is based on the fuel used to fire the furnace. Using this method, the following four types of furnaces are recognized:

1. Gas-fired furnaces

2. Oil-fired furnaces

3. Coal, wood, and multi-fuel furnaces

4. Electric furnaces

The first three categories of furnaces use a fossil fuel to produce the combustion necessary for heat transfer. The last one, electric furnaces, uses electricity. Whether or not electricity can be justifi­ably called a fuel is not of great importance here, because in this particular instance it functions in the same manner as the three fos­sil fuels: It heats the air being distributed.

Furnaces can also be classified by the means with which the heated air is distributed to the room (or rooms) in the structure. This method of classifying furnaces establishes two broad cate­gories: (1) gravity warm-air furnaces and (2) forced-warm-air fur­naces. The gravity warm-air furnaces rely primarily upon the principle of gravity for circulating the heated air. Because warm air is lighter than cold air, it will rise and pass through ducts or directly into the rooms to be heated. After passing off its heat, the air, now cooler and heavier, descends through returns to the furnace, where it is reheated. Gravity-type furnaces represent the earliest designs used in warm-air heating systems. They were sometimes equipped with fans (integral or booster) to increase the rate of air flow. They have been largely replaced in popularity by forced-warm-air fur­naces, which are equipped with integral fans.

Forced-warm-air furnaces are often divided into three principal classes, based primarily upon the location on the furnace of the warm-air discharge outlet and the return-air inlet. Furthermore, additional design considerations dependent upon the planned location of the furnace also enter into the classification of warm­air furnaces, resulting in three types, or classes, of warm-air furnaces:

1. Upflow furnaces

A. Upflow highboy furnaces

B. Upflow lowboy furnaces

2. Downflow furnaces

3. Horizontal furnaces

Upflow Highboy Furnace

A typical upflow highboy furnace (also referred to as an upflow fur­nace or a highboy furnace) is shown in Figure 10-1. These are com­pact heating units that stand no higher than 5 or 6 ft and occupy a floorspace of approximately 4 to 6 ft2 (2 ft X 2 ft or 2 ft X 3 ft).

The heated air is discharged through the top of the upflow furnace (hence the name), and the return air enters the furnace

Furnace Fundamentals

Through air intakes in the bottom or sides. Cooling coils can be easily added to the top of the furnace or in the duct system.

Upflow Lowboy Furnace

The upflow lowboy furnace (also referred to as an upflow furnace or lowboy furnace) (Figure 10-2) is designed for low clearances and stands only about 4 to 41i2 ft high. Although shorter than either the upflow highboy or downflow types (see below), it is longer from front to back.


подпись: air flowFigure 10-2 Upflow (lowboy) furnace.

Furnace Fundamentals


Furnace Fundamentals

Both the return-air inlet and the warm-air discharge outlet are usually located on the top of the lowboy furnace. The lowboy fur­nace is found in heating installations where the ductwork is located above the furnace.

Downflow Furnace

Figure 10-3 shows an example of a downflow furnace (also referred to as a counterflow furnace or a downdraft furnace). It is very sim­ilar in size and shape to the upflow furnace, but it discharges its

Furnace Fundamentals

Furnace Fundamentals

Figure 10-3 Downflow furnace.

Warm air at the bottom rather than the top. The return-air intake is located at the top.

A downflow furnace is used primarily in heating installations where the duct system is either embedded in a poured concrete slab or suspended beneath the floor in a crawl space.

Horizontal Furnace

Horizontal furnaces (Figure 10-4) are designed for installation in low, cramped spaces. They are often installed in attics (and referred to as attic furnaces), where they are positioned in such a way that a minimum of ductwork is used. This type of furnace is also fre­quently installed in crawl spaces.

Although dimensions will vary slightly among the various manu­factures, the typical horizontal furnace is about 2 ft wide by 2 ft high and 41i2 to 5 ft long.

The terms used in this classification system (upflow furnace, downflow furnace, etc.) are commonly employed by furnace man­ufacturers in the advertising literature describing their products. This is equally true of their installation and operation manuals. Because of the widespread usage of these terms, they will be employed in the more detailed description of furnaces in the fol­lowing chapters.

Furnace Fundamentals

Furnace Fundamentals

Gravity Warm-Air Furnaces

Gravity warm-air furnaces rely upon the fact that warm air is lighter than cold air. As a result, the warmer air rises through the ducts or pipes in the structure, gives off its heat to the rooms, and descends to the furnace as it becomes cooler and heavier. It is then reheated and rises once again to the rooms. A continuous circulation path is thus established through the heating and cooling of the air. Sometimes a fan is added to increase the rate of flow, but the primary emphasis is still the effect of gravity on the differing weights of air.

Depending upon the design, a gravity warm-air furnace will fall into one of the following categories:

1. A gravity warm-air furnace without a fan

2. A gravity warm-air furnace with an integral fan

3. A gravity warm-air furnace with a booster fan

Each of these three categories represents a different type of warm-air furnace used in central heating systems.

Any gravity warm-air furnace not equipped with a fan relies entirely upon gravity for air circulation. The flow rate is very slow, and extreme care must be taken in the design and placement of the ducts of pipes. Sometimes an integral fan is added to reduce the internal resistance to airflow and thereby speed up air circulation. A booster fan provides the same function but is designed not to interfere with air circulation when it is not use.

The round-cased, gravity warm-air furnace illustrated in Figure

10- 5 is a coal-fired unit that can be converted to gas or oil; see

Chapter 16, “Boiler and Furnace Conversions.” Depending upon the model, these furnaces are capable of developing up to 108,266 Btu at register and up to 144,319 Btu at bonnet.

Floor, wall, pipeless furnaces, and some unit (space) heaters also operate on the principle of the gravity warm-air furnace. They are distinguished by the fact that the warm air is discharged directly into the room (or rooms) without the use of ducts or pipes.

Selecting a Furnace for a New House

One of the many decisions involved in building a new house is the se­lection of the heating, ventilating, and air conditioning system. This is probably one of the most important decisions, because the cost of operating and maintaining the system and its efficiency will be a daily fact of life for as long as you live there. It is therefore in your best in­terest to select the most efficient heating system that you possibly can.

Furnace Fundamentals

The type of heating system selected (forced-warm-air, hydronic, steam, electric baseboard, etc.) will depend upon such factors as the design of the house, the climate where the house is located, fuel costs, and personal preference. Comments on the advantages and disadvantages of the various types of heating systems are found in Chapters 6 to 9.

Assuming that, like the majority of home owners, you have decided to install a forced-warm-air heating system, you should pay particular attention to selecting a suitable furnace. Should this deci­sion be left entirely to your building contractor? Certainly not. For convenience, friendship, or other reasons, he may prefer to subcon­tract the work to a heating firm with which he customarily does business. As a result, you may not be getting the best furnace for your needs. For example, John Doe Heating, Ventilating, and Air Conditioning Company may deal in furnace Brand X (or Brands X,

Y, and Z). It may be that none of these furnaces is as efficient and economical to operate as the one you might wish to use.

There are many manufacturers of forced-warm-air furnaces doing business in the country. The furnaces produced by these manufacturers differ from one another in a variety of ways, including:

1. Installation cost

2. Design factors

3. Type of fuel used

4. Cost of furnace

5. Furnace gross output (Btu/h)

6. Furnace net output (Btu/h)

7. Furnace efficiency

The installation cost depends upon the area in which you are liv­ing and can vary widely for the same furnace model from the same manufacturer. It is not something you can easily control, because it depends upon local labor costs, delivery charges, and availability of equipment.

Design factors involve developments in the heating equipment that improve (or detract from) their performance. For example, some electric-fired furnaces use a direct-drive blower motor instead of a belt-driven one. The direct-drive motor results in a smaller, more compact furnace. The belt-driven blower motor has the advantage of being more easily maintained (it is simply a matter of replacing the standard motor used in the unit). Other design devel­opments include improvement in flame retention (e. g., in oil-fired furnaces), more efficient combustion chambers, or improvements in other components. It may take a little research, but several profes­sional journals in the heating field carry articles reviewing new prod­ucts. Consumer magazines also provide this service. Do not take the manufacturer’s (or the sales representative’s) claim at face value. Go to your local library and read what the experts have to say.

The type of fuel used will be an important factor in determining future operating costs. What is the cost of the various fuels in your area? Remember, the cheapest may not necessarily be the best for your purposes. Operating costs depend on the total amount of fuel necessary to heat your home on a month-to-month basis. As a gen­eral rule, electricity is more expensive than the fossil fuels, and gas slightly more expensive than oil. Read Chapter 5, “Heating Fuels,” for additional information.

In considering the cost of a furnace, remember that the most expensive furnace is not necessarily the best one, and the cheapest is not always that bargain you expect it to be. Your first consideration should be furnace performance. Although the initial cost of a suit­able furnace may be high relative to others, it will soon pay for itself with lower operating costs.

Furnace efficiency is expressed as a percentage and is found by subtracting furnace net output (the actual amount of heat delivered to the rooms) from furnace gross output. For example, an oil-fired furnace with a gross output of 150,000 Btu/h and a net output of 120,000 Btu/h will have an approximate efficiency of 80 percent.

By way of review, the three basic steps involved in selecting a suitable furnace for a new house are as follows:

1. Determine the type of heating system you want for the house.

2. Estimate the amount of heat loss from the structure; see Chapter

4, “Sizing Residential Heating and Air Conditioning Systems.”

3. Select a furnace with a heating capacity capable of replacing the lost heat.

Selecting a Furnace for an Older House

Sometimes an older house or building will have a furnace that needs replacing. If you are planning to install a newer model of the same kind of furnace, you should have no serious difficulties. However, be sure that the newer model develops a similar Btu rating at both reg­ister and bonnet. In a forced-warm-air heating system, the ducts are sized in accordance with the overall requirements of the system, and a furnace is selected with a capacity to meet these requirements.

Switching from coal to gas or oil also should not present any great difficulties. Conversion burners have been designed and manufac­tured for just this purpose; see Chapter 16, “Boiler and Furnace Conversions.”

Installing an electric-fired furnace in an older structure is not generally recommended unless the construction is particularly tight and well insulated.

Furnace Components and Controls

Most forced-warm-air furnaces have the following basic compo­nents and controls:

1. Heat exchanger

2. Blower

3. Air filter

4. Primary control

5. Fan and limit controls

6. Thermostat control

The heat exchanger (Figures 10-6 and 10-7) is a metal surface (0.05- to 0.06-in steel) located between the burning fuel and the circulating air in the furnace. The metal becomes hot and transfers its heat to the air above, which is then circulated through the ducts by the blower.

Furnace Fundamentals

Figure 10-6 Heat exchanger for a horizontal gas-fired furnace.

(Courtesy Meyer Furnace Co.)

Furnace Fundamentals

Figure 10-7 Heat exchanger for an downflow gas furnace.

(Courtesy Meyer Furnace Co.)

Most forced-warm-air furnaces are equipped with either a dispos­able or permanent (and washable) air filter to clean the circulating air. An air filter is not recommend for a gravity warm-air furnace, because it tends to restrict the air flow.

Most warm-air furnaces are thermostatically controlled. In addi­tion, there will be a primary control as well as fan and limit controls. Among the functions of the primary control is the regulation or stop­page of the flow of fuel to the combustion chamber when the fire is out or the thermostat indicates that no further fuel is needed. The fan and limit controls are also actuated by the thermostat and are designed to start or stop the fan when the temperature in the furnace bonnet reaches predetermined and preset temperatures. In warm-air furnaces, it is suggested that the limit-control switch be placed in the warm-air plenum, with a recommended setting of 200°F for a forced — warm-air furnace and 300°F for a gravity warm-air furnace.

These and other controls are offered by furnace manufacturers in a variety of different combinations. Their primary function is to provide safe, smooth, and automatic operation of the warm-air fur­nace. More detailed descriptions of the controls used in furnaces and heating systems are found in Chapter 4, “Thermostats and Humidistats”; Chapter 5, “Gas and Oil Controls”; Chapter 6, “Other Automatic Controls”; and Chapter 9 of Volume 2, “Valves and Valve Installation.” (Other chapters also include sections on automatic controls. Check the Index).

Most warm-air furnaces (except hand-fired coal furnaces and electric-fired furnaces) are provided with devices that automatically prepare the fuel for combustion or to feed it directly to the fire. These devices (oil burners, gas burners, and automatic coal stokers) are described in the appropriate chapters. Electric-fired warm-air furnaces utilize electric resistance heaters.

Cooling coils, electronic air cleaners, and humidifiers are exam­ples of optional equipment that can be added to most forced-warm — air furnaces to provide total environmental control. It is best to include this optional equipment when the furnace is installed rather than add it at a later date. By doing so, you avoid complications that might arise in the overall design of your heating system. For exam­ple, adding air conditioning capability is not simply a matter of installing cooling coils. Duct sizes must also be considered, and they are not necessarily the same as those used for heating. This optional equipment is considered in detail in Chapters 7 and 8 in Volume 3.

Pipeless Floor and Wall Furnaces

A pipeless furnace (Figure 10-8) is commonly a gravity warm-air furnace installed in a central location beneath the floor. A single grille for the warm air and a return is provided for air circulation. This type of pipeless furnace is sometimes referred to as a floor fur­nace, although the latter is actually a permanently installed room heater and should be distinguished as such. Wall furnaces also belong to this category.

Both gas — and oil-fired furnaces are manufactured for installa­tion in recesses cut from the floor. They are available in a wide range of Btu ratings for different types of installations. Some are thermostatically equipped for automatic temperature control and with blowers for forced-air circulation (although those that operate on the gravity principle of air circulation are more common). There


Furnace Fundamentals

Is also a choice between electric ignition and pilot flame. All gas — or oil-fired floor furnaces must be vented.

Another type of pipeless furnace is the gas — or oil-fired vertical furnace installed in a closet or a wall recess. The counterflow types discharge the warm air from grilles located at the bottom of the fur­nace, as shown in Figure 10-9.

Duct Furnaces

A duct furnace (Figure 10-10) is a unit heater designed for installa­tion in a duct system where a blower (or blowers) is used to circu­late the air. It is commonly designed to operate on natural or propane gas, although electric duct heaters are also available (see Chapter 7 of Volume 2).

Geothermal Furnaces

Calling this heating appliance a furnace is a bit of a misnomer. A geothermal furnace is actually a heat pump, not a furnace. It is designed to use water instead of air to deliver the heat. Geothermal

Furnace Fundamentals

Figure 10-9 Counterflow vertical furnace. (Courtesy U. S. Department of Agriculture)

Furnace FundamentalsAIR FLOW

Furnaces are described in Chapter 12, “Heat Pumps” in Volume 3 of the Heating, Ventilating, and Air Conditioning Library.

Furnace Installation

No attempt should be made to install a warm-air furnace until you have consulted the local codes and standards. The American Gas Association and the National Fire Protection Association have also established codes for installing warm-air furnaces; these are avail­able to the public through their publications (see, for example, NFPA No. 31, “Installation of Oil Burning Equipment 1972”; nFpA No. 90B, “Residence Warm Air Heating 1971”; and NFPA No. 204, “Smoke and Heat Venting 1968”). These publications can be ob­tained free or at a modest price from these organizations. Their ad­dresses appear in Appendix A, “Professional and Trade Associations” of this book.

As soon as the heating equipment is shipped to your building site or existing structure, check it for missing or damaged parts. The manufacturer should be notified immediately of any discrepancies so that replacements can be made.

Follow the manufacturer’s instructions for installing the heating equipment, but give precedence to local codes and regulations should any conflict arise. Most manufacturers base their installa­tion instructions on existing codes formulated by the American Gas Association and the National Fire Protection Association.

Locate the furnace so that it has the shortest possible flue run containing as few elbows (turns in the run) as possible. Clearances around the furnace should be kept to the required minimum but within the regulations established by local codes and standards. Air ventilation must be adequate for efficient operation.

After the furnace has been installed, check all gas or oil lines for possible leaks. Make the necessary repairs if any leaks are found.

More detailed instructions for installing furnaces are found in Chapter 11, “Gas Furnaces”; Chapter 12, “Oil Furnaces”; Chapter 13, “Coal, Wood, and Multi-Fuel Furnaces”; and Chapter 14, “Electric Furnaces.”

Furnace Maintenance

Furnace maintenance is a very important part of the efficient oper­ation of a warm-air heating system and should never be neglected.

The manufacturer of the heating equipment will provide recom­mendations for proper maintenance, and these recommendations should be carefully followed. By doing so, you will extend the life of the equipment, improve its efficiency, and reduce operating costs.

Maintenance recommendations specific to gas, oil, electric, coal, wood, and multi-fuel furnaces are found at the end of Chapters 11 through 14. Always read and closely follow the furnace manufac­turer’s service and maintenance instructions. Maintain a periodic service and maintenance schedule and post the schedule near the furnace. The schedule should include the dates of any service, maintenance, and repairs performed, and the telephone numbers of the local furnace manufacturer’s representative and/or a local technician qualified to work on the furnace.

Troubleshooting Furnaces

The automatic controls of all furnaces, as well as the heating ele­ments in electric furnaces, are operated by electricity. If there is no electricity, the furnace will not operate and there will be no heat. Many times the problem of a furnace failing to operate can be traced to a blown fuse or a tripped circuit breaker caused by an electrical surge in the main power supply line. The fuses or circuit breakers protect the furnace (and other equipment on the circuit) from potentially damaging high voltages and currents by shutting off the power supply before the excess can harm the equipment. Replace a blown fuse or reset a tripped circuit breaker and restart the furnace. If the problem persists and you do not have the train­ing or experience to make furnace repairs, call the local representa­tive of the furnace manufacturer or a qualified HVAC technician for a service call.

Chapters 11 through 14 contain troubleshooting recommen­dations specific to gas, oil, electric, coal, wood, and multi-fuel furnaces, respectively.

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