What is Plenum in HVAC and What Makes It So Important?

Plenum in HVAC

Inarguably, your home’s ductwork is one of the most important parts of your HVAC system. If the spinal cord supports the human body, the ductwork is the backbone of your HVAC heating or cooling system.

Today’s forced-air HVAC systems use this system of ducts to facilitate the travel of hot or cold air across the system so that it may finally blow out through the unit’s fan and into the room. 

Without well-maintained ductwork, your HVAC system would not run efficiently, and without what is called Plenum, your ductwork would not be coordinated or smooth.

The Plenum, small a term as it is, is not to be overlooked when it comes to dealing with the seamless operation of your HVAC heating or cooling system- understandable, it is an integral component in the ductwork.

Questions of Plenum may arise for homeowners whenever there is an issue of repair or any replacement needed. In this article, we address different aspects concerning the role and importance of Plenum in a ducted heating or cooling system.

What is Plenum In HVAC? What does it do?

If we were given the chance to use only two words to describe the work of Plenum, it would be Air Distribution.

As you know, the ductwork in any air conditioning or heating system is concerned with the travel and transport of air so that the warm or cool air required may adequately reach the vents and blow into different rooms of the house as required.

Thus, we need something that helps to push and circulate the warm or cold air through the ducts and make the entire system work, and this is where the Plenum establishes its authority. 

Through Plenum boxes, the air is taken into the ducted interior, circulated throughout the ducts, and removed when it is necessary.

With the so-called ‘plenum boxes placed in the system, hot and cold air is variably put out, and used air is not only absorbed back in so that it doesn’t mingle with the fresh air in the house, but it is also recycled and re-circulated within the system so as to maintain consistency and reduce risks of overload. 

The plenum is located right after the air filters- and you might recognize this as another reason emphasizing the importance of cleaning or maintaining your air filters.

Dirty filters can mess with the air and stop the functioning of the plenum boxes entirely, causing the whole HVAC system to act up.

This is why it is significant that homeowners don’t abstain from gathering knowledge about internal parts like plenum, as a good plenum installation can improve the overall efficiency of your cooling/heating system and the quality of air.

Keep in mind that the plenum is not the same thing as a duct. Where a duct can mean a tube or a pipe that acts as a passageway of air, the plenum comes in boxes and generally maintains a consistent share of hot/cold air inside of it.

Types of Plenum

As you understand, the role of a plenum is variable and useful. It can help absorb, keep and remove hot/cold air. To do all this, your HVAC’s internal system makes use of two plenums in each system to maintain the balance in every air distribution cycle.

Supply Plenum

The Supply plenum comes first, and basically kickstarts the process of air circulation. Made of metal, a supply box, naturally, acts as an extension to the supply outlet, i.e, and the visible units of heating/cooling systems like Air Conditioners or furnaces. 

Any air that is brought in by the ducts is subject to go through the supply plenum. In turn, the supply plenum, also known as the intake plenum, has certain tubes or runners that emerge from the supply plenum and distribute air to the other components such as cylinders, etc.

The supply plenum follows the air filter and thus is vulnerable to easy clogs and blockages from dust and debris. Any failure on the supply plenum’s part can trigger a whole system shut-down.

Return Plenum

The second type of plenum in any HVAC system is the Return Plenum. The return Plenum is concerned with getting rid of used air in a room.

To be clearer, whenever any newly heated/cooled air is pumped into a room, the air from the previous cycle of heating/cooling is already lingering in the area. This air is pulled back into the internal system, and it is refreshed and reused to heat or cool the room. 

Like supply plenums, return plenums too have air filters that need to be dealt with at the first sign of malfunction, as a dirty filter can make the vents and ducts susceptible to the growth of mold and bacterial material.

The return plenum, thus, contributes to the redistribution of air all while ensuring that the air remains contaminant-free.

Typically, most HVAC systems house both Supply and Return Plenums to ensure that air circulation is done efficiently and effectively.

How Large Should Your Plenum Be?

One of the most important aspects when it comes to size ratio is the size of your plenum boxes. If the plenum boxes aren’t accurately sized to suit your HVAC system, they can decrease the efficiency of your air conditioning.

Factors like the size of the ducts as well as the area through which air is to be circulated, the length of its travel, are all taken into consideration. Mostly, you can leave it up to any professional HVAC technician or professional to make an estimate about your plenum box’s sizes.

Generally, when the length of the plenum duct is less than 150 feet, a 7-inch plenum is placed. You can expect a bigger plenum box for a bigger plenum duct.

When talking about designs, there are two types of plenum designs prevalent: the universal and the custom design. Plenum boxes available in universal designs are convenient as they can be sized for a required duct size and adjusted accordingly.

The Custom plenum box is manufactured with particular dimensions in mind so as to be compatible with certain ACs or furnaces.

Plenum Boxes: More Types

While the supply and the return plenum are the standards in any HVAC heating or cooling system, newer ventures have made it possible to not go the conventional way.

Diffuser Plenum Box

This Plenum box works to supply air diffusers and aims to evenly distribute cool air across the diffuser. Apart from distribution, the diffuser plenum box can also be in charge of regulating the airflow speed of a diffuser.

Diffusers generally have flexible ducts that allow air to travel at high velocities. The plenum box is structured in such a way that it receives the high-speed air in a hit and considerably slows it down before letting it out into the room.

This way, it prevents any discomfort that could be experienced by homeowners regarding their diffuser’s airflow.

Ceiling Plenum Box

As an alternative to having both supplies and return plenum boxes, some AC units use a Ceiling Plenum box. This is considered to be an easy way out of extra costs as it uses the typically freely available openings of the ceilings in many residential and commercial premises.  

Mechanical Room Return Air Plenum

These plenum types are found in Air Handling Units (AHUs). In this, rather than use a whole system of ducts, the AHUs room is used to act as a return air plenum.

A lone duct connects to the supply air outlet, while the room, with its damper and an opening for more fresh air, performs the function of a return air plenum.

Not only does this save costs as you don’t have to invest in a return plenum, but it also decreases the room size specifications. That is, with a return plenum box in the way, you’d require more space for it.

While one might incur side-effects like negative room pressure out of a system like this, it has still proven to be quite effective.

Plenum Box: Replacements, Costs, Maintenance

Any problems with the plenum box might push you to consider ideas of replacement. However, a job like this can be tedious and also ridiculously expensive.

Some consider it better to simply replace parts of the plenum box that are not working, instead of replacing the entire unit, as it can be time taking and costly.

Also, you would need to be aware of the different materials involved in the process, as well as their prices and reputability in the present market.

For instance, you can always try your hand in options like plenum duct board kits or even universal-sized plenums. Store-bought plenums may range at around $50 to $200, although prices may vary according to seasonal rates and your cost of living.

You can prevent this whole ordeal itself, by simply making sure that your plenum is handled carefully and maintained well.

While this can be hard to do, as plenums are located in the interior, you can still largely affect the state of your plenum through small actions like replacing and cleaning the air filter, and so on. 

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