The Size of a Furnace Heat Duct

Considerations

• According to the Department of Energy, older air conditioning systems, such as from the 20th century, are “often unreliable and much less efficient than a modern system.” If they were sized too small, they did not heat spaces adequately. If they were sized too large, they did keep homes warmer, but only by cycling on and off more frequently, creating uncomfortable energy swings and using more energy. Most older systems were oversized, especially since advancing technology has allowed homeowners to use less energy by insulating, weather-stripping and adding new windows.

Standards

• Contractors consult local or state building codes to ensure that their furnace and heat duct installations meet the requirements to pass inspection. Without building authority approval, their systems cannot be used legally. Regulations in the U.S. generally come from the same source: the International Residential Code, which the International Code Council streamlined from three national codes in 1994. The Council is also responsible for managing other codes that can affect heating installation, including the International Fire Code, the International Energy Conservation Code and the International Building Code.

Sizes

• According to IRC section 1601.1.1, the heating ducts must take into account their size, shape, thickness and material. For round ducts and enclosed rectangular ducts that are aboveground, those that are 14 inches and less must have a minimum thickness of 0.0157 inches for galvanized steel or 0.0145 inches for aluminum; with 16 to 18 inches, the thickness for galvanized steel is 0.0187 inches and for aluminum, it’s 0.018 inches. And for those 20 inches and larger, the thickness is 0.0236 or 0.023 inches, respectively. For exposed rectangular ducts, those 14 inches or less need a minimum thickness of 0.0157 for galvanized steel and 0.0145 inches for aluminum. For those larger than 16 inches, the thickness is 0.0187 or 0.018 inches, respectively.

Contractors

• The most accurate way to size furnace heat ducts is to hire a contractor who uses Manual D, “Residential Duct Design,” produced by the Air Conditioning Contractors of America. He typically uses this document with Manual J, “Residential Load Calculation,” which is used to size furnaces. Both manuals take into account the local climate; size, shape and orientation of the house; insulation level; windows, doors and other openings; number and ages of occupants; and the efficiency of lights and major appliances. The calculations are typically complex enough that most contractors use either electronic spreadsheets or software to come up with accurate estimates.