Last Updated on March 23, 2021 by HVAC

What Is HVAC Plenum Anyway?

A simple HVAC Plenum definition is that they are the part of the duct work or the air path for different kinds of HVAC systems and can handle either supply or return. Plenums are used throughout the heating ventilation and air conditioning industry.

The plenum or plenums are use in residential and commercial for supply air, return air and exhaust air. You can have custom made HVAC plenums made up at your local HVAC sheet metal fabrication shop or purchase ready made ones online. For residential applications, it’s more common to use flex pipe rather than all metal work.

HVAC Plenum – Residential application

When speaking about residential HVAC systems, a plenum is one of the most important parts of the entire forced air heating or cooling system. The more accurate term would be “plenum box” rather than just plenum. This is where all of your air either enters, or exits your HVAC system. In a residential application the supply air plenum is usually on the top of the furnace.

You will (or should) have at least one plenum box, but for best results you should have two plenum boxes. These boxes are the connections to and away from your forced air HVAC system. Depending on which way your air is flowing, you either have a supply plenum or a return plenum. From these plenums your air is usually circulated through flex piping or in some cases sheet metal duct work, depending on how large your home and unit is.

The supply plenum, regardless if it is a heating, cooling or a duel unit, is where the heated or cooled air comes out and where the duct work is connected that carries the air to all of the parts of your house. Your duct work in your home is most likely Flex piping. That’s the soft but metal looking round “piping” that runs through your home bringing cool or heated air. So when you place your hand over a vent and feel the air coming out, this air has run from your HVAC unit, through the plenum box, then through your duct work to get there.

The supply plenum is also where you should have your filter. This ensures that the air that is being forced into your home is clean. If you want to know more about air filters or the best furnace filters, check out my other articles. If desired or required an air conditioning coil may or can be incorporated into the supply air plenum. Another device to look into is a condensate pump to help remove any water that collects due to condensation in the duct work.

The second plenum in a residential application is the return air plenum or the return airdrop. This plenum or drop is usually attached to the side of the furnace or appliance with a filter rack and filter. The return plenum box is where the air is sucked into your HVAC unit to be either cooled or heated.

Again, it’s best to have a filter for this part of your unit as well. The air is being sucked from different rooms in your home, and even though it is filtered air, normal everyday airborne particles are still present and should be filtered out so that they don’t clog your HVAC unit.

You should always make sure you have clean filters on both ends of your unit. Once the filters begin to get dirty, the unit must work a lot harder causing undo stress and energy consumption. If you don’t keep your air filters clean, you will end up shortening the life of your expensive HVAC system.

Dirty filters also cause the entire duct system to also become dirty. Which means it all just being blown back into your home. Not what you want happening.

And a side note here, when inspecting your plenums and filters, also be sure to check your duct-work for leaks. It’s estimated most homes using flex piping for their duct work loose as much as 20% of their heated or cooled air to leaks. Again, this is a waste of energy and can become expensive. You don’t want to heat or cool your basement or attic.

A Ventilation Contractor or Sheet Metal Mechanic will size the return air plenum and filter to match the required airflow of the furnace or appliance.

These ducts after installation should be checked for water vapor and cleaned regularly. Furnace duct cleaning companies do a good job of this, or you can find a reputable company where you live by looking on the internet.

HVAC Plenum Used On The Roof Top

Plenums are also parts of a roof top unit installation. In this application there is usually a supply and return air plenum. Typically the equipment manufacturer will also supply a roof curb. The manufactured roof curb will indicate the required sizes of the supply and return air plenums on their shop drawings.

Once the size is determined, a Ventilation Contractor or a Sheet Metal Mechanic will fabricate the plenums to the required size. The supply and return plenums may require insulation.

These plenums may be insulated with acoustic insulation for sound or thermal insulation to protect the conditioned air from the climatic or building environment.

Having fabricated the plenums to the required specification the Sheet Metal Mechanic will install the roof curb on a structural Engineered roof with the plenums.

Another HVAC Plenum Application – Exhaust Plenums

There are also exhaust plenums. These plenums are usually installed on a roof exhauster.

The manufacture of the exhaust fan or fans usually provides a manufactured curb suitable for the application. Shop drawings for the fan are available for each fan.

From the drawings you can determine the curb internal an external size as well as the required plenum size. Once the size of the plenum is determined, the Ventilation Contractor or a Sheet Metal Mechanic can fabricate it.

If required it can be acoustical insulated and a back draft damper installed in it. With the specified requirements for the fabrication met the plenum is now ready to be installed.

The standard installation of a return air plenum or drop is a duct that is usually attached to either side of the furnace at the bottom. There is a filter rack installed between the furnace and this drop. The drop is capped at the bottom.

The top is typically attached to a return duct that is running tight to the underside of the joists. The top of the return duct is cut out between the joists where required.

From this opening the bottom of the joist or joists are lined with an appropriate material to a hole in the floor sheeting and in line with a wall above. The wall usually has a framed opening to accommodate the return grill.

In most cases a return is required in the basement as well as some supply.

Depending on where you live there may be different requirements. A local building inspector or HVAC inspector would be able to give you more details about these requirements.