Sizing ducts for a heating system is a complex task. It involves the heat output of the unit, measured in British Thermal Units, the air flow from its blower, the number of vents and their location and the length of ducts needed to reach those vents. The purpose of the return vents is to return the air from the house to the heating or cooling unit, to be recirculated. Those elements must be balanced for an efficient, effective system.
A furnace return system must collect all the air being put out by that system. If the furnace is putting out 1,000 cubic feet per minute of air flow, the return system must collect 1,000 cubic feet and take it back to the unit. An insufficient return system or one too large will reduce the efficiency of the heating and increase energy bills. A return system also filters air, to cleanse it of dust and debris before it goes back through the furnace.
There typically are as many supply vents as there are rooms in a house. There usually are fewer return vents, but they are much larger. A typical supply vent is 4 by 10 to 12 inches and a typical return vent is 16 by 20 inches or larger. Houses often have two or more return collecting points, each with a filter, which join before re-entering the heating unit. Two-story houses will have one return downstairs and another upstairs.
Ideally, every room with a supply vent would have an equal return vent, but practically, this is impossible in most houses. Return entries are usually placed in hallways or other central locations, in areas where cold air flows naturally. They are called cold air returns, although they also return "used" air in combined heating and cooling systems.
There are mathematical formulas for calculating the air flow to and from a heating unit, but the Air Conditioning Contractors Association simplifies the process in a series of manuals. These take into account not only the output of the unit and the length of duct work, but the type of duct material, the loss of air flow due to friction and other influences. A website called "The Engineering Toolbox" offers a calculation method.
Start With the Furnace
The starting point for sizing returns is always the heating unit. A label or manual for the unit will give the maximum output in cubic feet per minute. The basic standard is 100 cubic feet per minute representing 6,000 BTUs. Ducts are calculated in multiples of this standard, with adjustments for friction; air flows most efficiently through metal and through round ducts. The BTU capacity of the unit and its airflow will determine how large a return system must be to collect that air.