Size of Air Returns for Central Heating Systems

Sizing

• Sizing of the air return ducts in a central heating system is essential to the system's proper operation. The furnace needs air return ductwork that allows the airflow rate to keep up with the output of the furnace. The size of the ductwork is determined by conducting a room-by-room examination of the home to determine exactly how much you will need and how much it costs. Measurements are generally made in terms of cubic feet per minute. High-flow heating systems will distribute cool air much more quickly when there is additional space to do so.

Typical Size

• The size of the furnace ducts will vary based on the output of the furnace itself. The typical size of an air conditioner return duct is 12 inches by 6 inches or 10 inches by 8 inches, according to John D. Wagner of This Old House. Most times this type of ductwork is in the form of squared metal conduit, but some areas may call for the use of flex duct also, which is usually rounded, insulated, foil-wrapped conduit.

Type

• Another factor that helps determine the exact size of heating returns in your home is the specific type of heating system you utilize. The reason that different furnaces need differently sized HVAC returns is because some run more efficiently than others when the air returns at a faster rate through the ductwork. Gas furnaces require a flow rate of about 1 cubic foot per meter per 100 British thermal units per hour. Electric furnaces require 50 to 70 cfm per kilowatt of output, although this can vary based on the size of the equipment.

Considerations

• Aside from the overall dimensions of the duct itself, you should consider other factor into the equation when considering the size of the air return and other ducts. The length or distance of the return duct is a major consideration. Air flows more freely in return ducts that are a straight shot back to the furnace unit than in those where the air has to make a series of turns.