What is a plenum? HVAC terms explained

An HVAC plenum box is a piece of ductwork attached to the air handler. The plenum moves air through the house and your HVAC system.

In your home, the HVAC plenum helps reduce humidity and improve airflow. You likely have two plenum boxes, one on the supply side and one on the return side of your ductwork.

This article will answer the question, “What is a plenum?”. In addition to explaining its function in your HVAC system, we’ll provide tips on plenum repair and things to consider when purchasing a new plenum.

What is an HVAC plenum?

A plenum is a box made of sheet metal. It connects your HVAC system’s air handler to your ductwork. 

Your HVAC system creates cool or warm air depending on the season. That air goes from your furnace or air conditioner to the air handler. 

On the supply side of the air handler, the plenum takes the newly warmed or cooled air and leads it into your ductwork. This is how the air from your HVAC system is distributed throughout your home.

On the return side of the air handler, the plenum takes air from inside your home and sends it to your air conditioner or furnace to be heated or cooled.

What’s the difference between an air duct and a plenum?

Plenums can be considered part of your ductwork. They serve as the piece that connects your ductwork to your central HVAC system.

Plenums are usually box-shaped and made of sheet metal. Residential air ducts are sometimes also box-shaped and made of sheet metal.

Some homes have cylindrical air ducts made of galvanized steel, aluminum, or wire coil covered in durable plastic. No matter the type of air ducts you have, the plenum is usually always a sheet metal box.

Your air ducts are the vehicle that distributes air from your HVAC system throughout your home. Your plenums are the pieces that join your HVAC equipment and your ductwork, allowing circulation between the two.

Do you need a plenum?

A plenum is a necessary component if you have a central heating and cooling system. In fact, you need two for your system to work correctly and circulate air within your home.

Common HVAC plenum issues

While a broken plenum isn’t a frequent cause of HVAC problems, there are occasions when plenums may malfunction and require repair.

Leaky plenum

Sometimes, your plenums may leak air. This can cause your HVAC equipment to work overtime, as the heated or cooled air never makes it into your home.

If your system cannot achieve the temperature you’ve set on your thermostat, examine your system for a plenum leak. Simply locate your air handler and check to see if you feel air flowing out of the plenums. Energy.gov has good tips on detecting home air leaks. 

If you find an air leak, the repair is DIYable. Use silicone caulk and aluminum tape to seal the plenum at its joints. 

Dirty plenum

Over time, your plenums may experience a build-up of dust or dirt. This may cause them to operate inefficiently, diminishing airflow in your home.

To keep your plenums clean, change your air filters every three months. Clean your air ducts regularly. 

Though DIY duct cleaning is important and useful, it won’t reach your plenums. We recommend supplementing your DIY cleans with professional air duct cleaning every 3-5 years.

Poorly sized plenum

An incorrectly sized plenum will negatively impact your whole HVAC system. If the plenum is too small, it will inhibit airflow. If it’s too big, it may cause your system to work harder than necessary, increasing your utility bills.

Contact a licensed HVAC technician if you suspect one of your plenums is the wrong size. They’ll be able to test it and suggest an appropriate replacement.

Since your plenums are such a vital piece of your home comfort equipment, we don’t recommend attempting repairs on your own – always seek the help of a professional.

Incorrectly installed plenum

If your plenums weren’t installed properly, you may be experiencing air circulation issues and a frequently malfunctioning HVAC system. The installer might have connected the plenum inefficiently, cut duct holes poorly, or made attachments at awkward angles that halt airflow.

If you think this is the case with your plenums, consult with an HVAC professional. They’ll be able to identify the exact issues and remedy them, improving the overall performance of your heating and cooling equipment.

HVAC plenum repair or replacement

We don’t recommend attempting a plenum repair or replacement on your own. This piece should be specially fit to your HVAC system and installed securely by an expert.

When hiring an HVAC contractor, you can expect to pay $100-200 for a new plenum, assuming your system requires one of standard size and material. Pros typically charge $50-150 per hour. This job should take a few hours at most.

Purchasing a new plenum

We don’t recommend buying your plenum from sites like Amazon or a home improvement store before installation. Let your HVAC contractor take care of securing the materials for you.

Most plenums are made of sheet metal and can be custom-sized by your technician to fit your space and HVAC system perfectly. They’ll likely need to cut holes to fit and connect your ductwork.

Brand names don’t matter when it comes to plenums. They generally don’t have special features or any differentiating bells and whistles.

HVAC plenums: the bottom line

Plenums ensure air cycles through your home and your heating and cooling equipment. One plenum is as good as the next as long as it’s effectively connecting your HVAC system and ductwork.

Contact one of our top-rated local HVAC professionals if you suspect your plenum needs cleaning, repair, or replacement.