What is Plenum in HVAC? (Explained In Simple Terms)

A plenum in the HVAC world is simply a box that connects the HVAC system. Its job is to circulate air for cooling and heating purposes by providing a path for heated or cooled air to pass.

The plenum is important because it is where all supply and air ducts come together. In other words, a properly installed plenum can make your house comfortable, but a poorly installed one can be inefficient or even dangerous.

If you’re thinking about getting a plenum for your house or simply looking to understand how one works, this article is for you.

Homes are typically heated or cooled using an air conditioning and heating system. A network of ducts is used to distribute the cold or heated air through the house, and lying at the heart of this system is the plenum which brings in, distributes, and removes air, keeping the whole system functioning.

The ducts that supply air all around the house are each connected to the plenum. They are often made from different materials but are primarily sheet metal, fiberglass, or plastic.

4 Types of Plenum in HVAC Explained

Check out the four common types of plenum you may find in your home:

  • System Supply Plenum – supplies all zones in a multi-zone HVAC system. It works by taking the cool or warm air from the air-conditioning or heating system and then distributing it throughout the home using a network of ducts.
  • System Return Plenum – receives used air from the return vents to the HVAC unit and then heats or cools it again for distribution. Air from the return plenum has a filter to ensure that only clean air reenters the A/C or heating vent.
  • Zone Supply Plenum – serves only one zone of your home.
  • Zone Return Plenum – supplies a specific zone by receiving and redistributing clean air back to the said zone.
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Common HVAC Plenum Designs

DIY fans should familiarize themselves with the different plenum designs so that their homes are cooled and heated efficiently.

Here are some standard HVAC plenum designs and how they work:

  • Upflow Return Air Configurations
    Upflow configurations work by bringing in air at the bottom of the unit and expelling cool or warm air from the stop. These tend to be more efficient, especially when heating as they work with hot air’s tendency to rise.
  • Straight Plenum with Take-Offs (Straight & Flared)
    Take-offs help to redirect air around your home. In a 2-way duct system, the supply plenum usually sits on top of the AH/furnace or A/C coil in an upflow configuration. When the take-offs are straight, they form a right angle which can result in poor airflow. Instead of straight upflow, a flared one can work better.
  • Plenum Ell for 1-Way Duct System
    For 1-way duct systems, a rounded plenum airflow works better for airflow, by removing awkward 90-degree angles in the ventilation system.

Importance of Proper Plenum Installation

Although you may be tempted to fit a plenum yourself, getting a professional installation from an HVAC expert can help put your mind at ease. Plenums supply cool or warm air from your HVAC, and improperly installed ductwork or poor workmanship can be a fire hazard.

Poorly constructed systems can also lead to higher electric bills due to leaks or improper fittings.

If you’re interested in finding out more about how your HVAC works, check out our recommended reading list.

Here are other reasons why proper plenum installation is essential:

  • Improper air distribution – a poorly installed plenum can cause parts of your home to be poorly cooled or heated. This is mainly due to the wrong duct location from the plenum itself.
  • Airflow issues – are usually due to undersized ducts.
  • Elevated humidity – is caused by your HVAC unit doing a poor job of controlling temperature. High humidity in your home can cause accelerated mold and mildew growth.
  • Poor indoor air quality – a proper air filtration system is needed to ensure high-quality air. Poorly installed plenums could fail to filter air effectively, resulting in a reduction in air quality.
  • Airflow leaks – are usually caused by poor ductwork.
  • Increased operational costs – caused by airflow leaks around the plenum and ducts.
  • Shortened equipment life – Your home HVAC can work harder due to airflow leaks, poor ductwork, or improper temperature control, decreasing its useful life.

How to Properly Size and Build a Plenum Box

So, how big should your plenum be?

HVAC experts typically use the below formula to compute the size of the plenum required for a particular home.

Q= V X A


Q = flow rate measured in cubic feet per minute (CFM)
V = velocity measured in feet per minute (ft/m)
A = area of plenum box measure in square feet (ft2)

While the calculation looks simple, the fact is it can be a little tricky getting all figures together. The good news is, we built this easy-to-use calculator in case math isn’t your strong point.

People Also Ask (FAQ)

How much does a plenum cost?

Homeowners generally spend $50-$100 depending on the size of their HVAC unit. Labor for installing a plenum and unit usually costs more, around $290 to $370.

What is the difference between a plenum and a duct?

Ducts are pipes, tubes, or vents that carry cool or warm air to different areas of the home. A plenum, on the other hand, is a box that connects to the HVAC system, working to regulate the movement of air.

How do I seal my HVAC plenum?

Use a mastic sealant to seal your HVAC plenum. You can also use aluminum tape, but avoid using duct tape.



An HVAC air plenum can help reduce energy costs while keeping your home comfortable no matter the weather.

In a typical home HVAC, you will find a plenum box that helps your unit heat or cool your home efficiently, so understanding how it works can help you troubleshoot problems without calling in a professional.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.