HVAC Duct Size

Unit Output

• The process of sizing ductwork starts with the output of the unit. You get this number from a label on the unit or a user's manual. There should be numbers for the heating side and the cooling side, given in terms of airflow by cubic feet per minute. Next you need to determine the cubic feet per minute needed for each room in the house. This is outlined in Manual J of the Air Conditioning Contractors Association. The size of the house will determine the size of the unit.

Duct Length

• Then you must determine the duct length, both the basic trunk duct, which supplies air from the unit, and the branches, which distribute air to the vents. The length affects the airflow, because there is friction loss as air travels through ducts. Type of duct affects this; smooth round sheet metal ducts have the least friction. Air also moves more efficiently through round ducts than through rectangular ones.

Number of Ducts

• The number of ducts influences duct size, especially of the main supply duct. A system to feed 10 6-inch round ducts to vents would need a central rectangular duct of 8 by 22 inches based on an output of 1,000 cubic feet per minute, for instance. Changing any of the elements changes the equation.

Balance

• Return ducts are usually much larger than supply ducts because they must handle all the airflow from multiple supply outlets. One or two returns may handle all the airflow from a dozen vent ducts, but the system must be balanced for the system to operate efficiently. Imbalance will reduce the effectiveness of the unit.

Calculations

• Duct sizes are usually calculated by the installer of the basic unit. Homeowners who want to try their own can use one of three methods: velocity, based on airflow through ducts of a given size; constant pressure, based on the pressure in the ducts; and static pressure, based on maintaining the same pressure at all outlets and inlets. Or they can use websites like The Engineering ToolBox or devices like the Ductulator, which will take basic figures and translate them into duct specifics. The ACCA also provides manuals to guide calculations.