In this article we will see why Plenum box is so important while connecting any unit to a duct. And also, in the end we have discussed on how to calculate the size of a plenum box in HVAC.
The Plenum Box in heating and ventilation systems are basically large air ducts that take warm and cool air from the furnace and distribute it throughout a building either through grills at floor level or ceilings. They can serve the purpose of distributing air throughout a house evenly or provide air conditioning.
In order to use a plenum box efficiently, it must be installed correctly; otherwise, it may cause damage to the structure if not properly used. Plenum box is an air duct that connects the inside plenum area of an HVAC system to the outside plenum area. A typical indoor environment has several different types of plenum boxes:
• Main plenum: The plenum box located at the top (or bottom) of the HVAC unit. It connects the conditioned space to an attic or crawl space. This is the location for air distribution, cooling coil return, blower motor, etc.
• Secondary plenum: Connects the conditioned space to the air handler. Also called the secondary plenum box.
• Return plenum: Located on the other side of the conditioning unit from the main plenum. This is where the conditioned air returns to the air handlers after passing through the filters.
• Air handling plenum: The last of the three plenums. Connects the air handler to either the outdoor environment or the interior conditioned space.
The air flow direction through a plenum box is usually from top down. This allows the hot/cold air to circulate throughout the entire space in the building. The advantage of this is that the whole room gets cooled/heated equally instead of having just 1 corner being warmer than others. If the air flows from bottom up, then only the upper half of the room would get the benefit of cooling/heating.
This is one of the most commonly overlooked aspects of plenum boxes. Plenum boxes should have a slow enough vent rate (i.e. not too fast) to allow the heated/cooled air to evenly distribute across the floor plan. But if the vent rate is way too slow, then we may end up seeing some sort of recirculation effect where warm air comes back into the plenum box via the return vents but then immediately exits out again.
Return ducts are often left open at the end of the plenum box. Ideally, they should be closed off to prevent any recirculation. Larger return duct openings will cause higher pressure loss and therefore require a larger fan to push out that same amount of cool/warm air.
If your plenum box has been around for awhile, you may notice that the ducts are starting to get dirty. This could be due to dust buildup from the air moving through the system. Not only does this cause poor airflow, but it can negatively affect the efficiency of your cooling/heating systems. The cleanliness of your ducting should be maintained regularly. At least once per year, an experienced ac technician needs to inspect your entire heating and cooling system to ensure that everything is running smoothly. They need to check if the vents are blocked and make sure that there isn't any debris lodged inside the units. If they find anything amiss, they'll need to fix the problem immediately before it starts causing damage to the equipment.
Plenum boxes are designed to prevent warm air from traveling back into the supply ducts. This prevents damage to your unit and it can cause serious health problems. If your home's AC has had any major changes you should have someone who knows what they're doing to do the work.
The Plenum box is an air supply duct that connects the fan coil unit to the furnace, typically providing air flow from the furnace to the HVAC system’s return vent. There are three different types of plenum boxes – standard, modified, and enhanced. Standard plenum boxes have a rectangular cross-section with straight sides. Modified plenum boxes have angled sides. Enhanced plenum boxes have a round cross section with an internal baffle. All plenum boxes allow for a smooth transition between the interior and exterior of the furnace, but they differ in their ability to provide airflow.
Standard plenum boxes are best suited for use where fans draw in outside air through the return vents of the system and exhaust the conditioned air back into the building. These systems can handle large volumes of air but do not distribute air evenly throughout the space.
Modified plenum boxes are useful if the HVAC system draws air in through its intake vents and exhausts it out through its return vents. They have a larger opening than standard plenum boxes and are designed to provide even distribution of airflow throughout the space. However, these systems are not ideal for ventilation applications requiring high volumes of air.
Enhanced plenum boxes are specially designed to facilitate even distribution of air throughout a room without compromising the space’s comfort level. This type of plenum box has a smaller opening than a standard plenum box and is usually used with a small fan.
Size of the Plenum Box :
The Plenum Box Size in HVAC refers to the amount of air that can flow through the plenum box into the conditioned space. When considering this number, remember that the air has to pass over the fins, then drop down to your diffuser. A good rule of thumb is to never exceed 50% of the return air volume for any given area. This allows enough room for proper airflow and helps keep pressure inside the home lower than outside.
The Plenum box is where the ventilation air comes into contact with our products. This is the area that can have the highest impact on your indoor environment. If the plenum box is too small, the air flow is restricted and not enough volume of air can move through the system. It’s best to plan out your space and make sure the airflow is adequate for your needs.
The Plenum Box Size is the amount of air that can pass through the diffuser without restriction. Usually, the larger the box size, the greater the air flow rate; however, this is not necessarily true. In some cases, the smaller diffusers require a higher air volume per minute than the bigger ones.
So if your Plenum Box Size is too small, then you may have problems with your cooling system working properly. On the other hand, if it's too large, it could cause problems with the duct work.
In order to find out what size plenum box you should use, make sure you measure it using a tape measure. There are two standard sizes for diffusers - Standard and Square. A square diffuser is 2 inches wide by 4 inches tall.
You want your Plenum Box Size to be no less than 1/4 cubic foot (0.25 cubic meters), but keep in mind that you'll need at least one inch clearance around the diffuser. This means that the width plus either the top or bottom space will add up to 1 inch. So if you're looking for a diffuser that fits into a 5-inch opening, don't just cut a piece of metal into five pieces since you might end up with a 5-inch opening instead of a 6-foot opening. Instead, figure out how many inches of clear space you'll need to accommodate the diffuser.
The amount of air that flows through the plenum box area of AC is directly proportional to the size of the plenum box. You can compute this utilizing the accompanying equation:
Volume Area x Length x Height / 0.075m^3/ft^2
Example: If the area of your plenum box is 1 ft^2 (0.097 m^2), then its volume would be calculated as follows: Volume Area X Length X Height/0.075m^3 per ft^20.909m^3
This example assumes a height of 2 feet and a width of 4 feet.
Calculating plenum box size can become confusing if you don't know how to calculate the right area. Here are a few hints to take care of you!
Plenum size area of space (cubic feet) divided by the air flow rate through that room (cubic feet per minute). If you have no idea what the airflow rate is, just refer to your heat pump's technical specs or contact your local HVAC company.
Example: You want to design a 100 cubic foot room where the air flows at 50 cfm. That means the plenum box should be 200 square inches.
If you're designing a single-speed fan ducted system, you would use the following formula;
Duct Area x Circumference x Length ~ Cubic Feet
The best way to measure the duct area is using a tape measure from the point where the incoming supply enters the return side of the ductwork until the farthest point of the return. Duct width should be measured while the ducts are hanging flush against the wall.
1. Calculation of plenum box Size - Calculate the Volume of the room at the ceiling level using the formula V Area x Height. Using the formula (7.5 ft^2) x 72 inches (1765 mm^2) 1390.7 cubic feet (39 m^3).
2. How many square feet do we need?
The number of square feet needed equals volume divided by area. In this case, divide 39 m^3 by 7.5 ft^2, 1390.7 / 7.5 19.4 sqft.
3. Square footage of the fan:
For fans that have variable speeds, measure from the centerline of the fan to the front edge of the blades. Multiply the length times width to get the blade area in square inches (square feet inches² × 36"). Then multiply the total square feet by the square root of 1.6 to calculate the volume of air moved by the fan.
Steps to calculate size of plenum box :
1. Calculating the volume of a plenum box is easy using the formula V πDxL/12 where D is the diameter and L is the length of the plenum box. The volume can then be calculated by multiplying the number of cubic feet (ft^3) by the depth of the box divided by 12.
2. If you want to measure the height of your plenum box, look at the bottom of the air ducts. You can use this measurement to determine how large the plenum boxes should be.
3. To calculate the radius of the air duct, start with the inside diameter of the air duct. Then use that value to find the area of a circle centered around the duct. This area represents the area needed for a plenum box.
4. Once you have the total area needed for the plenum boxes, divide that figure into the amount of cubic footage of the room.
5. Finally, multiply the result from step 4 by 6. That gives you the total number of plenum boxes needed.
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