Forced air furnaces are the most common heating source in America (EIA 2020). Forced air furnaces are typically fueled by natural gas, but propane and fuel oil furnaces are used for new homes in some locations where natural gas is not readily available. Oil furnaces have slightly lower efficiencies than gas furnaces. 

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 (also sometimes referred to as “dual-fuel” systems) combine a gas furnace with a heat pump for heating and cooling. 

Fully electric heating systems, such as electric furnaces, traditional split heat pumps, ductless heat pumps, and ground-source heat pumps can offer an alternative to fuel-fired furnaces.

Furnace Classifications

The National Fuel Gas Code (NFPA 54) categorizes furnaces based on pressures in the exhaust flue (the pipe or chimney that exhausts combustion gases to the outdoors) and flue gas temperatures (condensing or non-condensing) as shown in Table 1.

Category I Furnaces 

Category I furnaces are low- to standard-efficiency, non-condensing (meaning the vent gas temperature is hot enough to avoid excess condensation in the flue), open combustion/non-direct vent (meaning they draw combustion air from the room they are in) appliances with either a natural draft or an induced draft. The flue of a Category I furnace operates at negative pressure with respect to the combustion appliance zone (CAZ), i.e., the room in which the furnace is located. 

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). These older furnaces are called natural draft (or atmospheric draft) because they rely entirely on high flue temperatures to draw exhaust gases up and out of the flue due to the stack effect. Because so much of the heat goes up the flue, 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 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 backdrafting on startup and assists in getting the draft started. Category 1 induced draft furnaces typically have efficiencies of 80% to 86%, and also tend to have cleaner combustion than their older counterparts, therefore releasing less airborne pollutants.

An induced draft fan-equipped furnace is considered a mechanically drafted furnace under the International Mechanical Code (IMC). However, because it still draws combustion air from inside the home or apartment, it can still have the potential to backdraft 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 or apartment. The return air side of the forced air furnace should have no communication with the CAZ at all. Because Category I furnaces have an open combustion chamber, the blower fan can easily overcome the small induced-draft fan and backdraft the furnace, pulling carbon monoxide into the open return on the furnace and distributing it throughout the house or apartment via the supply ducts.  

Category II Furnaces

Category II applies to some commercial furnaces but not residential furnaces. For this reason, they will not be discussed here. 

Category III Furnaces

Category III furnaces are standard- to high-efficiency, non-condensing, forced-draft appliances (meaning they are equipped with a combustion fan that is located before the burner to push air through the combustion chamber and out of the flue vent). Because the draft fan is continually operating when the burner is firing, the flue stack pressure is always positive. 

A Category III furnace vents through the wall or roof, and the flue gases exit through the flue at temperatures above 140°F. Most Category III furnaces are 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. These oil furnaces have an efficiency range of 82% to 88%.

Category III furnaces can be installed as sealed combustion/direct-vent appliances, meaning the combustion air is supplied entirely from the outdoors through a sealed pipe. Properly installed direct-vent appliances cannot backdraft into the living space. The pipe used for the intake vent for a direct vent appliance may be separate from the pipe used for the exhaust flue, 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 can also be 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 (Figure 3). It should be noted that if Category III furnaces are installed in this manner, they have the potential to backdraft into the living space if the return air is not ducted. 

Category IV Furnaces

Category IV furnaces are high-efficiency, condensing, sealed combustion/direct-vent, forced-draft appliances (meaning they are equipped with a fan to pull air through the combustion chamber and push combustion byproducts out of the furnace through the exhaust flue). 

The vent pipes of Category IV furnaces are under positive pressure with respect to the outdoors. Because Category IV furnaces have two heat exchangers, more heat is extracted from combustion gases than in other furnace types. This lowers the combustion gas temperature, allowing water vapor in the gases to condense on the secondary heat exchanger. As it condenses, the water releases extra heat, enabling gas-fired Category IV furnaces to achieve efficiencies of >90% AFUE. Due to the low exhaust temperatures, water vapor can also condense inside the vent pipe, so the pipe must be designed for contact with condensate. 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. Additionally, because the exhaust gas temperature is low (under 140°F) after passing through two heat exchangers, the vent pipe for Category IV furnaces can be made of PVC (Figure 4). 

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 the outdoors (Figure 5). Because of this, Category IV furnaces cannot backdraft into the living space. Manufacturers do not recommend installing Category IV furnaces as non-direct vent furnaces that draw their combustion air from the CAZ. Always install a second vent pipe to bring combustion air in from outside. Similar to Category III furnaces, the intake vent for a Category IV furnace can either be separate from the pipe used for the exhaust flue, or the outer circle in a concentric pipe-within-a-pipe configuration, where the inner pipe is the exhaust vent. 

ENERGY STAR Single-Family New Homes, ENERGY STAR Multifamily New Construction and DOE Zero Energy Ready Home program permit naturally drafted furnaces in some circumstances. See the requirements for each program for more details. However, all ≥80% AFUE furnaces available today are induced draft fan equipped or direct-vent furnaces. When naturally drafted or induced draft furnaces are installed, a combustion safety test must be performed. See Direct Vent Equipment for more on combustion safety testing of combustion heating equipment. 

See the Compliance Tab for more information on specific program requirements regarding combustion furnaces. 

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, the ACCA Technician's Guide for Quality Installations and ACCA Standard 9.
3. Select a furnace that is properly sized to meet the design heating and cooling load of the space, based on the ACCA Manual J calculations and Manual S sizing guidelines. 
4. Design and install an efficient duct system with a compact layout in accord with the ACCA Manual D. See the guides on duct installation, insulation, and air sealing for additional information. 
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. Regardless, it is important to protect the condensate line from freezing. The condensate is fairly acidic, and some local codes require neutralizing it. Provide a secondary (emergency) drain pan constructed of durable material.