The box that collects the returning air from one or more return air ducts at the inlet side of a furnace or air conditioner is the return air plenum. It acts as a manifold and is usually, but not always, at the bottom of the unit.

    Sometimes a return air register (vent grille) is cut into the side of the plenum and air returns directly to it from the adjacent room, without any additional ducts. Today a return air plenum is required to be insulated and sealed, and they are usually constructed of pieces of insulated duct board, taped and sealed. There are also ready-made plenums of sheet metal.

    The photo above is a view of an older return air plenum. You are looking through an opening in the side, with the hinged return air register opened. The interior is only the back side of drywall and wood stud framing, which was allowed years ago. Because these older plenums were not sealed, unfiltered air leaking into them can cause a heavy coating of dust/dirt over time. The refrigerant lines and condensate drain piping run across the top of the image, and an air handler sits above it all. This one has a collection of pans to catch dripping water due to a problem with the drainage of the water that condenses on the evaporator coils above it.

    Mold growth, as shown below, can develop when condensate water dripping into the return air plenum goes unnoticed for even a few weeks.

    And here’s a typical, modern sealed plenum.

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.