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How to Identify Supply & Return Ducts

Nathan Fisher

Forced HVAC systems require a constant supply of air to continually recirculate through the system, creating a balance of pressure between intake and supply. Identifying air supply and air return ducts is not always a simple matter; both duct types may look identical on the outside. For most systems the return duct, which supplies air to the system, is equal to or somewhat larger than the supply duct that delivers the heated or cooled air.

HVAC systems require a balanced exchange of air between supply and return ducts.
  1. Locate the furnace, air conditioner or combination HVAC unit.

  2. Turn on the system at the thermostat. If you are working with a combination heating and cooling system you can test the unit in either AC or heating mode.

  3. Allow the unit to run for 3 minutes. Place your hand near either duct. As hot air rises, for heating or combination systems the supply duct is usually the higher of the two ducts.

  4. Feel for a difference in temperature between the two ducts. The supply duct will become hotter for heating or cooler for air conditioning whereas the temperature of the return duct will remain fairly constant.