What Is HVAC Plenum Anyway?

HVAC Plenum is part of the ductwork and the air path, 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 custom made HVAC plenums at your local HVAC sheet metal fabrication shop or purchase ready made ones online.

HVAC Plenum – Residential application

In a residential application the supply air plenum is usually on the top of the furnace. A Ventilation Contractor or Sheet Metal Mechanic will measure and fabricate the plenum to fit the application, taking into consideration the required clearances as specified by the furnace or appliance manufacture for front, back, sides and top.

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.

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 in Calgary does 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.