HVAC Term You Need To Know-Plenum

As a home or business owner, your HVAC system makes your space comfortable to all who visit, and you have your plenum to thank for that. You may be asking yourself, “what’s that?” – and you are not alone. Even though the number of sold HVAC units in 2024 is projected to surpass 151 million, many users are unaware of the delicate intricacies of their system.  After reading this article, you will understand what a plenum is, how it differs from the air duct, why you need it, its common issues, and how to repair or replace it.

The HVAC Plenum: What Is It?

This box made of sheet metal joins your HVAC’s air handler with its ductwork.  Depending on the time of year, your HVAC generates cool or warm air. Once the air has been either cooled by the air conditioner or warmed by the furnace, it transfers into the air handler.  The plenum takes seasonally appropriate air from the supply half of the air handler and moves it into the ductwork, warming or cooling your home to the desired temperature.  On the air handler’s return half, the plenum delivers air from your home to be sent back into the air conditioner or furnace.  In summary, your plenum is the glue that holds the entire operation together, connecting your air handler to your air ducts – it is a very important part of your HVAC system.

Plenum vs. Air Duct

Most people immediately conflate the plenum and the air duct to be the same piece of the HVAC, but though they work together, they are unique parts.  While some air duct systems are also box-shaped and made of steel, the plenum is always a box made of steel, regardless of whether the air duct system is box-shaped or cylindrical, steel, aluminum, or wire coil.  Another major difference is that the plenum is merely a connector between the HVAC system and the air ducts, while the air ducts are responsible for heating or cooling your entire home or building.  Because a plenum is a unique piece of equipment, every HVAC system must have one to operate.

Four Common Plenum Issues

Though plenum malfunctions do not frequently occur, it never hurts to be aware of potential issues that may require repair. 

Dirty Plenum

A build-up of dust and dirt in your plenum can diminish airflow in your home. To prevent this, plan to clean out your air ducts frequently and change your air filters at least every three months.

Leaky Plenum

Rarely, a leak will occur in your plenum box, preventing air from moving between the air handler and the air ducts. One of the clearest symptoms of this type of HVAC issue is the inability of your home’s temperature to match that of the thermostat. Luckily, Energy.gov provides a step-by-step guide to accessing this type of issue.  Should you find a leak in your plenum, you will be pleased to know that you can usually fix this yourself by closing off the leak with silicone caulk and aluminum tape.

Incorrect Installation

Some of the ways a plenum can be incorrectly installed include a plenum not fully joined to either the air handler or the air ducts, poorly cut duct holes, or a plenum positioned at an awkward angle that stalls airflow. If you think your HVAC system has been incorrectly installed, it is best to have a professional check your plenum and schedule any necessary repairs.

Incorrect Size

Sometimes your HVAC installation is spotless, but the plenum is either too big or too small for the system it is matched with. When it is too small, the airflow is stunted. If a plenum is too big, the system has to work harder than it should to heat or cool your home.  As with an incorrect installation, if you expect your system to have an issue of this degree, we always recommend consulting with a professional to prevent any further damage to your HVAC system.

What To Do If You Need a New Plenum

As we stressed in the previous section, it is so important when it comes to your HVAC system to hire a professional. Your HVAC contractor will know exactly what size plenum you need and how to properly install it, so leave acquiring a new plenum to them rather than scouring Amazon or Lowes.  The average plenum ranges from $100-$200 depending on your system, and a contractor will charge $50-$150 an hour for the labor. You can expect to pay anywhere from $150-$500 for your installation.

Prioritize the Plenum

While the plenum appears to be nothing more than a measly metal box, it is the connecting force that powers your HVAC. Now that you know your plenum, you can begin to prioritize HVAC maintenance, an often overlooked necessity for your home or business.