HVAC Duct Design and the Importance of Return Air

Return air is an often overlooked part of great duct design. Air return duct systems can be configured in two ways:

  1. Each room can have a return duct that sends air back to the heating and cooling equipment. This is the ideal configuration.
  2. Return grills can be located in central locations on each floor.
  3. Air can not blow if can not suck.   Some 75% of Kansas City 2 story homes do not have working  return on 2nd floor

In the second case, either grills must be installed to allow air to pass out of closed rooms, or short “jumper ducts” can be installed to connect the vent in one room with the next, allowing air to flow back to the central return grilles.

Door undercuts help, but they are usually not sufficient for return airflow. I am not a big fan on doing this.

Here is a really quick way to test for adequate return air capacity.  I like to Measure air presser just to make sure there not taxing.

  1. Close all exterior doors and windows
  2. Close all interior room doors
  3. Turn on the central air handler
  4. “Crack” interior doors one by one and observe if the door closes or further opens “on its own.” (Whether it closes or opens will depend on the direction of the air handler-driven air flow.) Rooms served by air-moved doors have restricted return air flow and need pressure relief as described above.

Source: Energy.gov

Categorized in: Heating and Cooling