What is a plenum in HVAC

Air conditioners, heat pumps, furnaces, and other HVAC systems have become a need of almost all households in the world, especially for the people who live in regions with extreme temperatures. An HVAC system has plenty of small to major components that have some specific jobs to perform and if you look into its design layout, you may ask, what is a plenum in HVAC system? What does a plenum do in HVAC?

what is plenum in hvac

Compressors, fans, pipes, and other related things are usually heard by common people but plenum is a thing that does not usually come into consideration. But if you ask a professional, he will immediately tell you that a plenum is like the heart of any HVAC system.

Continue reading this article to get all the necessary information about supply outlet, plenum ceiling, return plenum box, supply plenum box, special plenum cabling and cables in plenum spaces.

What is the use of plenum in HVAC

What is a plenum? The plenum is one of the essential components in any HVAC system connected with the ducts and comes in the shape of an enclosed box. It is usually located around the major components of an HVAC system such as filters, furnaces, or air handlers.

The filter is placed right in between a couple of plenum boxes. This is the reason that you need to take the filter out of a box while changing or cleaning, that very box is the plenum box.

Why do we use plenum in hvac? The main function for which a plenum box is used in an HVAC system is to regulate or distribute the cool or hot air coming from the system throughout the required area. After the air in the HVAC unit has been cooled or heated, the plenum box takes that air into the outside body and then pumps it into the ductwork. This process helps in taking the air out of HVAC and rotating it in the whole area.

How much does a plenum cost

The cost of a plenum can vary depending on the area you live in and the quality of the product. You may be amazed to hear that the cost of installation is much more than the actual cost of a plenum box and this is the only reason that telling an exact amount is a bit difficult but here is the best estimation of the plenum box along with its installation and other total costs.

Plenum Material$50 to $60$70 to $80$90 to $110
Installation Cost$100$100 to $110$150 to $200
Average Total Cost$150$175$275

Where is the plenum located

The plenum hvac box is usually equipped around the heat exchangers or fan while it is directly connected to the ductwork on the HVAC system.

As its main function is to distribute the air in the area or HVAC unit, it is placed at the point where it forms a connection of the unit to the air distribution ducts.

In terms of building construction, a plenum is usually placed between the actual ceiling and the drop-down ceiling of the room. If it needs to be placed on the floor, it will be equipped in a raised floor area.

Where is the plenum cable required

Plenum cable is required for almost all kinds of air handling spaces as it is mandatory to use plenum-rated cables if you are going to install an HVAC unit with a plenum box.

It is essential to only use CMP plenum-rated cable for all kinds of your air ducts and plenum spaces.

Although this cable is often considered necessary for plenum HVAC boxes, it should be installed in large areas such as hospitals or schools even if there are no plenum spaces in the building.

HVAC plenum design theory

If we briefly go through the design theory of an HVAC plenum box, it can be said that it is just a box that has one, two, or sometimes more openings where ducts are attached.

The basic design starts with a Supply Plenum Panel which is followed by Blower Access Cover, Indoor Coil Cover, and Secondary Drain Pan.

On the outer cover, a few passages are equipped that are mostly known as Suction Line Stub Out, Liquid Line Stub Out, and Condensate Drain Stub Out.

As all these things are equipped in the central box, it ends with filter access that is connected to the last part of the plenum HVAC box known as the Return Air Plenum.

Should the plenum be insulated

In simple words, all kinds of HVAC working, plumbing, fittings, plenums, and ducts should be insulated to get the best out of your HVAC unit. If plenums are equipped indoors in an unconditioned place, it will be enough to just insulate them with R-6 insulation as they are not exposed to extreme weather or temperatures.

When it comes to plenums, supply, or return air ducts installed in an outdoor environment, it is essential to have a minimum of R-8 insulation. If the outdoor climate remains extreme for almost the whole year, it will be better to insulate your plenum HVAC box in an HVAC unit with R-12 insulations.

How to calculate HVAC plenum size

Calculating the size of an HVAC plenum box is not that difficult if you know the right formula and have the values needed to reach the final point. Here is the brief and step-by-step procedure to measure the perfect size of the plenum HVAC box that is necessary for your HVAC unit to meet your requirements.

To get started, you need to gather the values which are denoted as Q, V, and A. Here:

  • Q is the flow rate (Cubic Feet per Minute)
  • V is the velocity (Feet per Minute)
  • A is the area of the plenum box usually conducted in square feet.

In general standards, an air handling unit in HVAC requires about 800 feet per minute of velocity which will be about 8 TR in terms of tonnage. It is the pre-determined rule, that 1 TR of the HVAC system needs 400 CFM while the 8 TR will require 3200 CFM.

  • So, taking A = Q/A as our equation 1, we get:
  • A = Q/V
  • A= 3200/800
  • Area = 4 Square Feet
  • Now taking Area of Plenum = Width x Height as our equation 2, we get:
  • Assume height is 2 feet and width will be:
  • W = A/H
  • W = 4/2
  • Width = 2 feet
  • Taking D = Width x d as our equation 3 to calculate the depth of plenum box, we get:
  • Here d is the diameters of the fan or blower in your HVAC unit.
  • Assume d = 12 inches
  • D = W x d
  • D = 2 x 12 inches
  • Depth = 24 inches or 2 feet

Going through all these steps and solving these 3 equations will get you the exact measurement of the required plenum box in all angles and aspects.

How to build an HVAC plenum

Building an HVAC plenum box can be done as a DIY project. Metal sheets are usually used in creating this masterpiece while filters, sometimes a grill, stub outs, and other related items are equipped inside a rectangular box of metal sheets.

The sheets are attached using screws while other equipment is either attached using nails or fixed with mastic tape as well. Having a round opening is necessary so that the air ducts can be connected to the box.

How to seal the HVAC plenum

Having a leaked ac plenum can harm you in various aspects of using an HVAC unit where the wastage of energy and getting less performance are most prominent.

If air is leaking from the plenum, it will bring less cooling or heating to the room and the unit will take more time to meet the desired temperature that will eventually add up in your monthly utility bills.

You can save this all by spending only a little amount with a 10-minute process, which can be done at home as a DIY project.

  • Remove all the dirt and debris from the sheet of plenum box so that the mastic and other materials can get glued better.
  • Apply mastic directly on the plenum sheets using a small brush that can be found in any paint shop.
  • If the seal is done with a sheet, using sheet screws will be a huge help and should be done efficiently.
  • Once you have glued two sheets with mastic, apply some more on the outside and then put the fiberglass tape on the areas.
  • Now cover with mastic, the whole of the fiberglass tape and other areas which can get leaked using the same procedure.
  • Let it dry for some time and your air conditioner plenum box will be sealed for your whole life as combing fiberglass tape with mastic makes the band permanent.

How to clean plenum HVAC

Clearing HVAC plenum is a combination of various activities that are briefly mentioned below:

  • The air filters inside or around the air conditioning plenum box should be cleaned on a regular basis.
  • Simply take it out and run warm water from it so that dust and debris can get out of it.
  • Use a plastic dull scraper to remove molds from the a/c plenum box’s inside.
  • Use a sponge to remove moisture from the plenum box.
  • Supply screens usually have air contaminants attached to them, remove them off using a rug or a soft cloth on a periodic basis.

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