How to Determine HVAC Duct Sizes

Begin with the output of the unit, using a basic airflow measurement of cubic feet per minute; ducts are designed around a basis of 100 cubic feet per minute. A unit with 85,000 British Thermal Units of heating and 30,000 BTUs of cooling, a 2 1/2 ton unit, will require 1,000 CFM of airflow capacity. This equates to an 8-by-22-inch main supply feeding ten 6-inch round vent ducts.

Use a device called a Ductulator to calculate specifics or visit an online site that offers tools for calculating duct sizes. Enter the desired airflow, based on the unit output, and it will calculate the size of duct, varying with the type of duct. A round sheet metal duct has the most efficient airflow, round flexible ductwork, the next best. Round ducts move air better than square or rectangular ones, sheet metal better than duct board.

Try one of three types of mathematical formulas as an alternative. A velocity method uses an equation of the duct cross section, airflow rate and air speed. A constant pressure or equal friction loss method starts with the speed in the main duct at the unit's output fan and figures duct size based on pressure loss as air travels through the ducts. A static pressure method aims to keep the same pressure at all outlets but is so complicated it is rarely used.

Rely on manuals from the Air-Conditioning Contractors Association, which simplify all these calculations into various standards for sizing ducts based on unit output, room-by-room heating and cooling requirements, length of ducts, type of ductwork used and other factors. At least three different manuals deal with various options; all are available from ACCA or contractors.