Forced air furnaces are the most common heating source in America (EIA 2009). Forced air furnaces are typically fueled by natural gas. Propane and fuel oil are used for new homes in some locations where natural gas is not readily available. Oil furnaces have slightly lower efficiencies than gas furnaces. Electric furnaces are not described here; although they achieve high operating efficiencies, other fuels are typically used at the electric generating plant to produce the electricity and there are transmission losses so they are considered inefficient from an electricity generation and transmission standpoint. In homes with very low loads, electric resistance heaters might be an option.

Forced air furnaces are often configured with an add-on indoor refrigerant cooling coil installed in the air handler on the supply air side of the burner. Some hybrid systems combine a gas furnace with a heat pump for heating and cooling. As ducted and ductless heat pumps with variable refrigerant flow technology reach higher efficiencies over broader temperature ranges and at operating capacities ranging from 25% to 100% to better match low-load conditions, they offer an increasingly plausible alternative to combustion furnaces.

Furnaces are classified in the International Mechanical Code and in the National Fuel Gas Code. Understanding the descriptions of these furnace types based on both codes is important with respect to safety and efficiency.

The 2009 (and 2012) International Mechanical Code classifies furnaces based on vent type: direct, mechanical, or atmospheric, as described in the 2009 and 2012 IMC, Chapter 2, Definitions:

• "a direct-vent appliance is one that is constructed and installed so that all air for combustion is derived from the outdoor atmosphere and all flue gases are discharged to the outside atmosphere;
• a mechanical draft system is a venting system designed to remove flue or vent gases by mechanical means consisting of

- an induced draft portion under non-positive static pressure; or,

- a forced draft portion under positive static pressure;

• a natural draft system is a venting system designed to remove flue or vent gases under non-positive static vent pressure entirely by natural draft".

The 2012 National Fuel Gas Code (NFPA 54) puts furnaces in four categories based on flue vent pressures, flue gas temperatures (relates to condensing or non-condensing), and vent pipe materials, as shown in Table 1.

A Category I furnace operates with the flue at negative pressure with respect to the combustion appliance zone (CAZ), i.e., the room in which the furnace is located, and the stack temperature is hot enough to avoid condensation in the vent. The burner draws its combustion air from the CAZ. The combustion chamber is also open to the CAZ; i.e., if you are standing next to the furnace, you can peer in and see the burner and the flames.

Older Category I furnaces use an open draft hood that allows dilution air to enter the vent pipe and mix with the exhaust gases (Figure 1). A draft diverter at the base of the flue protects the flame from downdrafts coming down the chimney or flue. These older furnaces are not mechanically drafted but are called natural draft (or atmospheric draft) because they rely entirely on high flue temperatures (relative to outside temperatures) to draw exhaust gases up and out of the flue. Because so much of the heat goes up the chimney, natural draft furnaces have very low Annual Fuel Utilization Efficiency (AFUE) ratings, usually 70% or less.

A newer type of Category 1 furnace replaced the draft hood with a small fan, referred to as an induced draft fan, which pulls air through the combustion chamber, although the furnace still relies on flue temperatures to lift the combustion gases up the flue stack (Figure 2). The induced draft fan helps to prevent back drafting on startup and assists in getting the draft started. Once the vent pipe gets up to temperature (140°F+) and a draft is established, the pressure inside the vent pipe becomes negative with respect to the CAZ. Depending on the model, the induced draft fan may turn off, but will continue to spin due to airflow. Category 1 furnaces that incorporate an induced draft fan typically have cleaner or more complete combustion than their older counter parts and therefore expel less pollutants into the air. The byproducts of an 80%+ furnace are CO2, N, and H2O. Category 1 induced draft furnaces typically have efficiencies of 80% to 86%.

An induced draft fan-equipped furnace is considered a mechanically drafted furnace, according to the IMC. However, because it relies on negative flue pressure to carry away combustion byproducts, it can, like the naturally drafted furnace, have the potential to backdraft. Back drafting, when combustion gases spill down into the CAZ rather than going out the flue, can occur if the CAZ becomes depressurized with respect to the flue. This could occur if multiple exhaust fans and the dryer or the fireplace are operating at the same time.

Never install a Category I furnace using the CAZ as the return air plenum; duct the return plenum to return registers in other parts of the house. The return air side of the forced air furnace should have no communication with the CAZ at all. The blower in Category I furnaces is meant to move a high volume of air against a relatively high pressure (approximately 0.5 inch water column (IWC) static pressure or 125 Pascals). The pressure in the vent pipe in a gas, forced-air Category I furnace when it is 30°F outdoors is about -4 Pascals (0.016 IWC). Figure 3 shows a poorly configured Category I furnace that has no return ducts, the return is open at the base of furnace and draws in air from the room around it. Because Category I furnaces have an open combustion chamber (the burners are visible in the photo), the big blower fan can easily overcome the small induced-draft fan and back draft the furnace pulling carbon monoxide into the open return on the furnace and distributing it throughout the house via the supply ducts.

ENERGY STAR Single-Family New Homes (Version 3, Rev 11) permits naturally drafted furnaces in IECC climates zones 1 through 3 and  DOE Zero Energy Ready Home program permits them in climates zones 1 and 2, if they have an AFUE of ≥80%. However, all ≥80% AFUE furnaces available today are induced draft fan equipped or direct vent furnaces. When naturally drafted or induced draft fan equipped furnaces are installed, a combustion safety test must be performed. (See Direct Vent Equipment for more on combustion safety testing of combustion heating equipment.)

Category II applies to some commercial furnaces but no residential furnaces.

A Category III furnace has a vent pipe that is under positive pressure and the furnace is non-condensing, meaning its flue gases only go through one heat exchanger then exit through the vent at temperatures above 140°F. A Category I appliance vents through the wall or roof and is forced draft, meaning it is equipped with a combustion fan that is located before the burner to push air through the combustion chamber and out of the vent. The fan is continually operating when the burner is firing so the vent stack pressure is always positive.

Most Category III furnaces are high-efficiency oil furnaces with gun type burners that force the fuel oil through a nozzle that emits the oil in an atomizing spray that mixes well with air for a more efficient burn (Figure 4). These oil furnaces have an efficiency range of 82% to 88%.

Category III furnaces vent their exhaust gases outside through a sealed pipe so they cannot be back drafted. They are typically installed as sealed combustion/direct vent appliances, meaning they draw their combustion air directly from outside. The pipe for the incoming air may be a separate pipe from the exhaust pipe or it may be the outer circle of a concentric pipe-within-a-pipe where the inner pipe is the exhaust vent. Although it is not recommended, Category III furnaces are sometimes installed as non-direct vent appliances where the combustion air is drawn from the CAZ and enters the furnace through a port on the combustion chamber, while exhaust gases vent to the outside via the single vent pipe.

Category IV furnaces are combustion appliances that have a vent pipe under positive pressure and flue gases under 140ºF. The vent exhaust is so low temperature because all Category IV appliances are equipped with a secondary heat exchanger where heat is further extracted from the combustion air as the water vapor (a byproduct of combustion) cools and condenses to liquid water. This liquid is drained to the outside through a condensate drain. The condensate is highly acidic (pH ≤3) so local code may require that it be pretreated before disposing to the sewer. Because the combustion gases are directed through a secondary heat exchanger, more heat is extracted, enabling gas-fired Category IV furnaces to achieve efficiencies of >90% AFUE. Category IV oil-fired furnaces can achieve efficiencies of 95% AFUE. Category IV furnaces with two-stage motors for high and low capacity can achieve AFUEs greater than 94%.

Like Category III furnaces, Category IV furnaces are forced-draft, meaning they are equipped with a fan to pull air through the combustion chamber and push the byproducts of combustion out of the furnace through a vent pipe; the vent pipe is sealed so they cannot be back drafted (Figure 5).

Category IV furnaces should be installed as sealed-combustion/direct vent appliances, which means their combustion chamber is sealed off from the CAZ and they draw their combustion air from outside via a second vent pipe that brings combustion air directly to the combustion chamber from outside the home. Because nearly all of the heat in the combustion gases is removed by the two heat exchangers, the vent pipe for Category IV furnaces can be made of PVC (see Figure 6). Manufacturers do not recommend installing Category IV furnaces as non-direct vent furnaces that draw their combustion air from the compliance zone. Always install the second vent pipe to bring combustion air in from outside.

ENERGY STAR Single-Family New Homes (Version 3, Rev 11) requires furnaces in IECC Climate Zones 4 through 8 to be direct vent or mechanically drafted and requires that gas furnaces have an AFUE > 90% and that oil furnaces have an AFUE of > 85% and be ENERGY STAR-certified. 

The DOE Zero Energy Ready Home program permits > 80% gas furnaces in Climate Zones 1 and 2. In Climate Zones 3 and 4, furnaces must have an AFUE of > 90%. In Climate Zones 5 through 8, gas furnaces must be AFUE > 94%, in other words, a direct-vent, two-stage furnace since these are the only kind that can reach AFUEs > 94%. See the Compliance tab for more details.

How to Select and Install a Furnace

1. Choose the highest performing furnace that projects costs will allow, to meet the design heating load of the project. If you are participating in an energy-efficiency program, select a furnace that complies with the requirements for your climate zone, as described in the Compliance tab.
2. Install in accordance with the manufacturers’ instructions and relevant standards including ACCA Standard 5: HVAC Quality Installation Specification and the ACCA’s Technician's Guide for Quality Installations and ACCA Standard 9: HVAC Quality Installation Verification Protocols.
3. Select a furnace that is properly sized to meet the design heating and cooling load of the home, based on the ACCA Manual J heating and cooling load calculations and Manual S sizing guidelines. If the design load is equal to or lower than the furnace’s lowest output rating, consider alternative heating equipment options that better match the design load of the home.
4. Design an efficient air distribution system with a compact layout in accord with the ACCA Manual D. Install ducts properly for maximum airflow and efficiency in accord with ACCA Manual D. See also the Building America Solution Center guides on duct installation, insulation, and air sealing.
5. Install the furnace per the manufacturer’s instructions. A direct-vent installation where combustion air is piped directly to the furnace combustion chamber from outside is preferable. If the furnace must use the CAZ for combustion air, verify that required combustion air is provided in the CAZ and perform a combustion safety test after installation. If a Category I furnace is installed, do not use the CAZ as the return air plenum; duct the return to another room. If the combustion appliance (furnace, boiler, water heater) is a Category I appliance, verify that the minimum amount of combustion and dilution air is provided in the CAZ. For an explanation of how to calculate the combustion air needed for the CAZ, see the guide Combustion Appliance Zone (CAZ) Testing.
6. For condensing furnaces, ensure that the condensate drains properly. The condensate line can be routed to an existing sewer or septic system. Ideally, this can be done with a simple gravity line, but many applications will require a condensate pump. Either way, it is important to protect the condensate line from freezing. The condensate is fairly acidic, with a pH of 3 or 4. To ensure that the condensate won’t corrode metallic fittings downstream, some local codes require passing it through a neutralizer containing granular calcium carbonate or a similar material to raise its pH before discharge. Provide a secondary (emergency) drain pan constructed of durable material.