When installing combustion furnaces, boilers, and water heaters inside the home’s pressure boundary (i.e., within the conditioned space of the home), the safest (and usually most energy efficient) type of equipment to install is a direct-vent, sealed-combustion appliance.
A direct-vent sealed-combustion furnace, boiler, or water heater brings combustion air directly from outdoors to the sealed combustion chamber through one sealed pipe and exhausts the byproducts of combustion (CO2 and N) to the outdoors through a second sealed pipe (Figure 1). Because of this sealed combustion, the risk of backdrafting is eliminated. Combustion air (oxygen) enters the furnace through the sealed pipe and goes directly from the outdoors to the burner assembly. There it mixes with the fuel and is electronically ignited in the sealed combustion chamber. The heated air passes through a primary heat exchanger, which extracts most of the heat, and then through a secondary heat exchanger, where more heat is extracted, cooling the exhaust gases to the point that they condense. The condensed water and carbon dioxide exit through a condensate drain, while the remaining flue gases exit to the outside through the sealed exhaust pipe, which can be made of PVC because of the low temperature of the gases (Figure 2). Condensing furnaces have annual fuel utilization efficiencies (AFUE) of 90% to 98%.
The National Fuel Gas Code identifies four categories for combustion furnaces and water heaters (see Figure 3). The categories are based on combustion type (sealed or unsealed) and vent pipe temperature. Direct-vent sealed-combustion furnaces are Category 4 appliances. The flue and vent pipe are under a positive pressure, flue gases are under 140°F, and water vapor (a byproduct of combustion) is condensed into water liquid and drained to the outside through a condensate drain. Combustion air is piped directly to the burner, which is sealed off from the CAZ (sealed combustion) and the byproducts of combustion (carbon dioxide and nitrogen) are power exhausted to outdoors.
A Category 2 appliance is a natural aspirating furnace with a vent temperature below 140°F that does some condensing of the flue gases and has negative pressure in the vent; category 2 appliances are huge commercial furnaces. A Category 3 appliance is a furnace with a vent temperature above 140°F (high enough to avoid condensation in the vent) and the vent has a positive static pressure.
Figure 1 - A direct-vent sealed-combustion furnace has dedicated pipes for combustion air and exhaust
Figure 2 - Because the flue temperatures are cool, intake and exhaust ducts on a Category 4 direct-vent sealed-combustion condensing furnace can be made of PVC
Figure 3 - The National Fuel Gas Code (NFPA 2012) identifies four categories for combustion furnaces and water heaters based on combustion type (sealed or unsealed) and vent pipe temperature
Atmospheric (or naturally) drafted furnaces and water heaters and mechanically drafted (or draft-induced) furnaces and water heaters are both considered Category 1 appliances (Figure 4). A Category 1 combustion appliance has a flue gas temperature over 140°F. The flue and vent pipe pressure will be negative with respect to the space in which the appliance is located (referred to as the combustion appliance zone or CAZ). With a Category 1 furnace, there is no barrier between the burner and the CAZ. A Category 1 furnace or water heater draws its combustion and dilution air from the CAZ and the byproducts of combustion (carbon dioxide, nitrogen, and water vapor) are transported outside through the flue. Because both types of appliances draw their combustion air from the CAZ, both are susceptible to backdrafting.
Naturally drafted (also called atmospheric draft or draft hood) appliances use the stack effect to draw combustion gases up the chimney. Natural draft furnaces are low-efficiency (60%-78%) older model appliances that are no longer sold in the United States but are still found operating in existing homes. Mechanically drafted (also known as induced draft) combustion appliances use a fan to pull air through the combustion chamber and to push exhaust gases out of the flue. Mechanically drafted (also called induced draft) furnaces have AFUEs of 78% to 83%. Natural draft water heaters are still sold, although safer and more efficient water heaters are also available, such as power-vented models, which use a fan to push combustion gases out the flue, and direct vent models, which use a sealed intake for combustion air and a sealed flue for exhaust gases.
Naturally drafted appliances are not recommended inside the home, especially in air-tight, high-efficiency homes, where exhaust fans or other combustion appliances such as fireplaces could depressurize the home and overcome the natural draft of an atmospherically vented appliance, thus backdrafting the appliance and pulling combustion gases into the living space.
Although most building scientists would not recommend them, naturally drafted furnaces, boilers, and water heaters are permitted by ENERGY STAR Version 3.0 in Climate Zones 1 through 3. If they are installed, the builder must ensure that two outside fresh air supply ducts are installed in the CAZ: one 12 inches from the floor and one 12 inches from the ceiling or as required by NFPA 54, Sections 5.3.3 and 5.3.4. These ducts provide combustion and dilution air for safe operation in each combustion appliance zone that exists in the home. If atmospheric vented furnaces, boilers, or water heaters are installed, the rater must test the equipment using combustion safety test procedures for depressurization, spillage, draft pressure, and CO concentration in the flue and in the ambient air. Ambient CO concentration should be < 25 ppm (< 8 ppm is preferred). This combustion safety testing should be conducted according to the procedures of the Building Performance Institute (BPI 2012) (or RESNET when approved).
Although most building scientists would not recommend them, ENERGY STAR Version 3.0 permits the installation of mechanically drafted furnaces, boilers, and water heaters inside conditioned space in all climate zones. Although not required in ENERGY STAR Version 3.0, the rater should test any Category 1 equipment using combustion safety test procedures for worst case depressurization, spillage, draft pressure, and CO concentration in the flue and in the ambient air. Ambient CO concentration should be < 25 ppm (< 8 ppm is preferred). This combustion safety testing should be conducted according to the procedures of the Building Performance Institute (BPI 2012) (or RESNET when approved).
The method described below is adapted from BPI and the U.S. Department of Energy’s Weatherization Assistance Project requirements.
Figure 4 - The natural draft and induced draft furnace are both Category I appliances that receive combustion air from the combustion appliance zone
Figure 5 - The raised hood at the base of the vent stack on this water heater shows that it is an atmospheric vented gas water heater
How to Install Furnaces, Boilers, or Water Heaters
Select high-efficiency, direct vent, sealed combustion furnaces, boilers, and water heaters for installation inside the home. Use certified installers and install the appliances according to the manufacturer’s instructions.
If the builder chooses to install a Category I combustion appliance (i.e., one that is naturally or mechanically drafted) the rater should test the equipment using combustion safety test procedures for depressurization, spillage, draft pressure, and CO concentration in the ambient air and in the flue (≤ 25 ppm). (This is an ENERGY STAR requirement if the home is located in IECC Climate Zone 1, 2 or 3, and if the builder chooses to install a naturally drafted furnace, boiler, or water heater within the home’s pressure boundary (an action which is not recommended).
ENERGY STAR recommends the rater follow BPI test procedures (or RESNET when approved). The following combustion safety testing steps are based on BPI and U.S. DOE Weatherization Program Procedures: