What is a Plenum in an HVAC System?

by Steve Coffey  // in Construction, General Contractors, HVAC


HVAC systems have become standard housing features, but not because of their status-symbol appeal. In fact, it’s the exact opposite – air conditioning has become a landlord’s tool for attracting tenants or convincing buyers to shell out more money for a home. However, these systems do more than keep houses warm or cool; they also provide air quality control essential for everyone’s health.

HVAC contractors play a key role in ensuring that a home’s HVAC System is properly-installed and functions well. But what makes them work so well? One of the components of an HVAC is a plenum. What exactly is it, and what does it do? Read on to find out.

What is a Plenum?

An HVAC plenum hanging on an red brick wall. A trash can with a yellow X sits underneath.

HVAC plenum also goes by the name plenum boxes, especially when dealing with residential systems. They are an essential part of the HVAC ductwork of an airflow system which comes in the shape of an enclosed box.

A plenum is a box that distributes air designed for an HVAC system for residential homes and business premises. Ductwork installation goes through the building’s walls and ceiling.

The plenum is attached to the air conditioner or the furnace. 

HVAC technicians, installers and contractors should be aware that a plenum box is on either side of the fan or heat exchanger.

Why Is A Plenum Used In An HVAC System?

The heating, ventilation, and air conditioning system relies on a series of ducts. These ducts provide a regulated path for cooled or heated air distribution in a building.

A residential AC, furnace, or heat pump generates hot or cold air, moves the air through the ductwork, and circulates it. After supplying the cooled or heated air, the ductwork returns it to the HVAC unit, drawing it again, cooling or heating it accordingly. Any HVAC technician assigned an inspection or routine repair job should be able to ensure that this circulation remains flawless.

Plenum boxes on either side of the heat exchanger or fan play a crucial role in taking in air, distributing, and taking it out. They are the initial air stop after heating or cooling and the last stop before returning to the furnace, heat pump, or AC.

When performing an installation, HVAC technicians must connect a plenum box to the return duct or supply outlet, depending on the type.

Types of Plenum

Most HVAC systems have two primary types of plenum boxes: supply and return plenum boxes.

Supply plenum

The supply plenum is an air distribution box that directly connects to the supply outlet of the AC unit, furnace, or heat pump. It receives cooled or heated air from the system unit and distributes it to the ducts regulating the temperatures of an apartment.

The supply plenum is air ducts that spread out chilled or heated air into respective rooms of a home. The HVAC system has a blower that blasts heated or chilled air into the supply plenum, then to the individual household rooms through the supply duct system.

Return plenum

As the name suggests, the return plenum box returns filtered cooled or heated air back to the HVAC system for reconditioning and redistribution.

As conditioned air blows into an apartment through the supply ducts, ‘used’ air from the rooms gets sucked in by the return ducts. The returning air must go through a filter for cleaning before entering the return plenum.

The return plenum box connects to the AC, furnace, or heat pump’s return outlet. 

How Much Does It Cost To Repair The Plenum?

HVAC contractors usually have an idea of how much it would cost to change a plenum for a client. However, there are several considerations to look out for.

The plenum boxes on their own aren’t costly, but the associated labor costs definitely push the overall costs up. As an expert, you also need to know the commonly used materials today.

The cost of plenum box repair or reinstallation depends on factors such as;

  • Whether it’s return or supply plenum
  • The state in which the unit is in
  • Whether the HVAC professional discovers other faults

Regardless of the factors above, costs can be anything between $200 and $500 for the plenum box repair.

However, a cheaper alternative is the plenum duct board kits, which have a more straightforward installation process yet are less durable. 

There are also HVAC plenums in the “trim to fit” class that is easier to handle, and dual filter plenums are some of the best.

Some of the best plenums are in this class. The 28-gauge galvanized steel is well-known for durability. But, most ductwork companies can custom-build plenum boxes from heavier material.

What Are The Risks Of Using The Plenum?

The benefits of having the plenum in an HVAC system definitely outweigh the risks. But, we cannot look down on the dangers that using a plenum may pose. 

For instance, the plenum may become blocked, causing the whole system failure. Dirty air ducts can restrict unrestricted airflow and distribution. As a result, the HVAC system has to run longer trying to reach the required temperatures on the thermostat.

The complete system failure means your clients will not get the freshly supercooled or warm air they usually get.

Again, improper ductwork installation may cause future problems; the ductwork may not function effectively. 

Lastly, a damaged plenum must get a replacement or fixation as soon as possible from an HVAC technician. About 30% of conditioned air leaks into unoccupied spaces. 

The good thing about this issue is that technicians can solve all these problems with a proper HVAC system installation. 

Again, routine maintenance may reduce the likelihood of the HVAC system’s failure. And by adhering to these ways guarantees that;

  • Clients do not incur high costs on system repairs
  • Clients get efficient distribution of well-conditioned air
  • Clients experience reduced airflow problems
  • Clients get improved quality indoor air
  • Clients are guaranteed a longer HVAC system lifespan

Tips on Maintaining the Client’s Plenum

The condition of ductwork and plenum is critical as they determine the effectiveness and efficacy of AN HVAC system. For an HVAC technician to maintain the plenum correctly, we recommend the following:

Keep air filters clean

Regular cleaning of any filters inside or within the air conditioning plenum box is critical.

When working on the equipment, technicians must grab a screwdriver, take the air filters out, and run warm water over them. Warm water washes away dust and any other debris. When done, restore the air filters.

Get rid of moisture and contaminants

Installers can use a stiff sponge or wash cloth and little elbow grease. To remove moisture and contaminants present in the plenum box, scrub well. 

Always clean the supply screens

The supply screens have air contaminants as well. 

  • To remove them, technicians must check the ends of the ducts, where they usually filter out pollutants from the air circulation. 
  • Unsnap the hinge and use a clean rug or a soft piece of cloth to clean the screens.
  • When done, return them and snap the hinge close. Do this regularly.

Listen out for motor problems

Run the HVAC system; is there grinding noises? If yes, the technician has to open the plenum from the sideways and check any gears in its motor, then add a few drops of oil to stop the grinding.

Fix the positioning

The plenum can shift at any moment, causing air leakages which are not good. Periodic inspection of the position of the plenum and adjusting them accordingly can prevent air leakage. 

Also, avoid future shifts by tightening the screws linking the ducts and plenums.

Pro Tip: Put duct tape around the HVAC plenum ducts’ edges. The tapes hold them in place and reduce loss of air.


A plenum is an indoor air distribution box attached to the supply side of an HVAC system, and it distributes conditioned air throughout the house.

Knowing how a plenum works and how to maintain it, HVAC technicians are better positioned to understand how to keep the whole HVAC system in good shape. Feel free to sign up today for Construction Fanatics newsletters to learn more about the current construction insights.

About the author Steve Coffey

I am passionate about the building materials industry helping companies all throughout the channel see success and exceed the expectations of their audiences

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