How to Design AC Ducts

Assess the house and look for spaces to install ductwork, in basements or sealed crawl spaces under floors or in attics. Calculate the maximum size of duct needed from the air flow of the air conditioning unit; the manufacturer should provide guidelines for duct size. Start the central duct with the size needed for maximum air flow, then decrease it as branches pull air out. If you start with an 18-inch duct, for instance, and have two 6-inch branches at the first intersection, drop the central duct to 12 inches.

Use a house plan or measure distances with a tape measure and make a rough drawing of the house, showing lengths of each individual duct and open areas where ducts can run. Mark takeoff points for branches from the main duct; stagger these by at least 12 inches to maintain air pressure in the central duct. Put the last takeoff or vent at least 18 inches from the end of the supply run. Include areas for return ducts, to take "used" air back to the unit.

Consult tables and manuals from the Air Conditioning Contractors of America (ACCA) for specifics on calculating air flow, resistance from ductwork, effects of turns and branches and other factors. These publications can help in special calculations of cooling needs, based on such variables as average temperatures, roof style and insulation.

Use flexible ductwork -- flexduct -- for most designs. It is easier to install and air flows more efficiently through round ducts than through square or rectangular metal ducts. Metal ductwork may be needed to fit special small spaces; flexduct should never be compressed, as it will restrict air flow. Consult ACCA tables when combining flex and fixed duct, so the rigid duct does not become an obstacle to air flow.

Design a return system to match the airflow of the supply side. That means to start with a large duct at the collection point, typically in a central area like a hallway and at the bottom of a wall. Design return collection points with filter grills, to hold filters that remove dust and other foreign material from the air before it is sent back through the cooling unit. The total airflow of the return system must equal the total output from the cooling unit.