What is Plenum in HVAC and Why It is so Important

With well over a dozen components that go into an HVAC system's ductwork, the plenum is the most critical part. While take-offs, reducers, and dampers can’t be overlooked, an HVAC plenum serves two important purposes. We’re going to discuss what the component does, and what you need to look for when a plenum needs to be replaced in your home.

What does a plenum do?

If you were to Google the term “Plenum” or look it up in a Merriam-Webster dictionary, you might be surprised by the results. It’s a broad word with several different meanings, although it only refers to one thing in the world of heating, ventilation, and air conditioning… airflow.

A plenum is an air distribution box designed for forced-air systems on residential homes and commercial businesses. It is part of the ductwork, but far different than an air duct as these important boxes handle air as it enters the ducts and helps to recirculate it within a system.

The Two Types of HVAC Plenums

Many homeowners may have heard the term plenum when their HVAC technician makes a trip to their home. It may even be something they’ve serviced although you may not be aware that there are not one, but two of these air distribution boxes in a home.

  • Supply Plenums – This metal box is placed at the beginning of the ductwork where it’s attached to the supply outlet of HVAC equipment like central heating and air units and furnaces. The supply plenum “supplies” the air that’s forced into the trunk lines and branches spread throughout a home.
  • Return Plenums – The other type of plenum found in homes with HVAC equipment is called a return plenum. As the name implies, this air distribution box is used to return air into your system. It’s also a part most people will be familiar with – the return plenum is where an air filter is located.
Supply Plenums

Plenum Maintenance

In some homes, the supply plenum can be challenging to access, but that isn’t the case with the return plenum. In some cases, you can find filters in both of these air distribution boxes, but normally the only one most people have to deal with is the HVAC filter found in the return plenum.

It’s recommended that you clean or replace the filter in the plenum at least once every three months. Consumers that live in dusty areas or homes with poorly sealed ductwork may need to clean the filters once a month. When they become clogged, your system has to work harder and the air quality in your home decreases as well.

If you’d like to learn more about the types of HVAC filters available on the market today, take a look at this guide which breaks down MERV, MPR, and FPR filter ratings. While changing the filter is simple the area around the filter in the plenum can also become dirty. For dust and loose debris, you can simply use a vacuum cleaner with a hose attachment but will need to find a suitable cleaner for grime on the plenum itself.

Aside from keeping your plenum clean and the filter replaced, most maintenance is handled by professionals when something is amiss with these air distribution boxes. With that in mind, be mindful of placement in areas with exposed ductwork. Plenums can be damaged or moved if bumped hard enough which can weaken or break the seal with your ductwork.

HVAC Plenum Cost and Installation

Whether you have a damaged plenum in your home or are considering a change, it’s a good idea to know how much one will cost. If you intend to use a professional HVAC technician, which we recommend, then you simply need to familiarize yourself with the materials and options commonly used today.

Plenums purchased from hardware stores can run between $50 to $200 depending on the size and style. The cheapest alternative are plenum duct board kits, which are simple to install, but not exactly durable. HVAC plenums in the “trim to fit” class are also easy to deal with while dual filter plenums are considered top of the line.

Some of the best plenums are in this class with an R6 insulation rating. 28-gauge galvanized steel is the most popular option for durability, but ductwork companies can build custom boxes from thicker material as well. To get a better idea of what a professional plenum installation will cost, use our quote tool for an estimate from a certificated technician in your area.

If you are considering saving money by doing the job yourself, there are several things to consider beforehand. Unless everything lines up perfectly and you’re replacing an old plenum with an identical one, you may have to make some tricky cuts in the sheet metal. Many plenums are also fabricated directly on site.

Seals are of the utmost importance as well, and these boxes can be difficult to access. The video below shows just how challenging it can be…

FAQ

Q: Is a plenum necessary?

A: In modern forced-air HVAC systems, you must have a plenum installed.

Q: What are HVAC plenums made from?

A: Most plenums are made from galvanized steel, but there are duct board kits and many contractors that fabricate plenums on-site from both materials as well.

Q: Do furnaces have plenum boxes?

A: Yes, a furnace will have a plenum box just like a central air conditioning system.

Q: What is the best way to clean inside a plenum?

A: Most return plenums can be wiped down and vacuumed around the inside and outside of the box. Supply plenums can be more challenging, but the same simple process applies. 

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