HVAC plenum box

Having an HVAC unit is essential in almost all households. Although they have the ability to provide excellent results, you should work on it and equip some additional structures that can enhance their performance. One such structure is the HVAC supply plenum box.

What is the plenum box used in HVAC systems/ducts

A plenum box is an air conditioner component that is directly connected to the ducts of an HVAC system. You will most probably find it right behind the air handler or furnace. It is an enclosed box that has the purpose to distribute the cool or hot air inside the room in an efficient manner.

It is extremely necessary for the air conditioner to distribute its cool air in the whole room area via their ductworks. The plenum box acts as a pusher and puller. It takes the cool air inside from the air conditioner and then pushes it into the ducts of the system. In this way, the cool air can be distributed in a much effective way reaching every single corner of the room.

HVAC plenum box design

Although there are four basic types of HVAC plenum boxes, the return air plenum box is probably the most popular and maybe effective as well.

They are usually designed in two different kinds, one is known as universal design, while the other is called custom design.

As the name implies, universal design plenum box’ panels can be trimmed to match with any size of air handlers and furnace.

The custom one is manufactured in a single specified size with equal dimensions so that it can fit all furnaces of the same size without requiring any alteration.

It mixes the return air so that the temperature can be balanced that will eventually result in better cooling performance by the air conditioner.

Basically, 1 to 3 circular openings are attached to ducts or fan coils that are followed by a rectangular box. The major parts of plenum box design may include:

  • Supply Plenum
  • Blower Access Cover
  • Indoor Coil Cover
  • Filter Access
  • Secondary Drain Pan
  • Suction Line Stub Out
  • Liquid Line Stub Out
  • Condensate Drain Sub Out
  • Return Air Plenum

What is an HVAC plenum box made of

Most commonly HVAC plenum boxes are manufactured by using galvanized steel, plastics, or some other materials as well.

HVAC plenum box sizing

Most commonly, HVAC plenum boxes almost have the same dimensions like the air conditioner unit outlet itself but could be sized small or large depending upon conditions and requirements.

For Calculating Area:

In a mathematical way, the area of an HVAC plenum box can be calculated by the equation,

Q = V × A. Where,

  • Q is the flow rate usually measured in CFM (Cubic Feet per Minute).
  • V is the velocity measured in feet per minute.
  • A is the square feet area of the plenum box.
  • The standard velocity of an HVAC air handling unit is about 800 feet per minute.
  • Let’s say that the heat load tonnage also known as TR of our HVAC unit is 6.
  • It is a thumb rule that 1TR requires about 400 cubic feet per minute.
  • The value of Q for 6TR will become 6 × 400 = 2400.
  • So, by putting these values in our equation we get:
  • A = Q/V
  • Area = 2400/800
  • Area = 3 square feet

For Calculating Width:

As we have calculated the area, now it’s time to calculate the width of the HVAC plenum box.

  • Let’s say a plenum box has a height of 1.5 feet.
  • So by putting this value in equation Width = Area/Height, we get:
  • Width = 3/1.5
  • Width = 2 feet

For Calculating Depth:

  • Use the equation as D = 1.5 × d.
  • Here d is the diameter of the fan handler, duct, or furnace.
  • Let’s assume that d = 10 inches, so the depth of the plenum box will be:
  • D = 1.5 × 10
  • Depth = 15 inches.

Plenum box installation

Installation Process of Plenum Box:

  1. Assemble all panels and different parts of the plenum box in an efficient manner with great care.
  2. Securing the panels with screws is essential so that it can remain enclosed and doesn’t allow air to leak.
  3. Once the plenum box has been assembled, attach a starting or opening part to your plenum box.
  4. Attach the main trunk line and secure it just like all other panels.
  5. Now install a return airdrop assembly to your plenum box with proper screwing or securing.

Plenum box replacement:

Plenum box replacement is probably the worst idea you can ever have about the system. Sheet works and panels are extremely costly and honestly, a plenum box does not require replacement as a whole.

You should diagnose the box thoroughly and look for the parts that are essential to be removed. Replacing the whole box will not only cost you a lot but can waste your time or maybe affect the air conditioner’s performance especially if the plenum box was working perfectly.

Is duct and plenum box the same?

A duct is just a pipe or canal that has the job of providing a path for air or water to move from one point to another. The plenum is an enclosed space that is completely filled with air which is then mixed to balance the temperature.

How to build a plenum box HVAC

As said earlier, an HVAC plenum box can be manufactured into two different designs. There is a bit of difference in their making but most of the procedure is the same as mentioned below:

Making universal design plenum box

  1. Measure the dimensions in an appropriate manner without any error.
  2. Trim the panels into different sizes so that they can fit the measured dimensions of the system.
  3. Assemble all trimmed panels to form the structure of the plenum box.
  4. Secure these panels with screws preferably with sheet metal screws.
  5. Put and secure the top panel to make it completely enclosed.

Making custom design plenum box

  1. First, attach the three large panels to one another. Just slide the flat edges of panels into the slots in a straight line.
  2. Position the top panel just like the other three panels you have assembled.
  3. Secure all the panels, edges, and slots with the help of sheet metal screws.

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