There are many different sizes available for a return air grille. Using the correct return air grille size is important to ensure that the HVAC system has sufficient airflow as well as low noise. So, I created several CFM charts and a guide on how to calculate return air grille size.
To calculate return air grille size, take the CFM of the HVAC unit and divide it by 350 to get the grille area in square feet. Next, multiply the grille area by 144 to convert it into square inches. Finally, choose your preferred grille size that matches the required grille area.
Alternatively, you can use the following CFM charts to quickly find the grille size based on the CFM or airflow of the HVAC unit. Parameters used for the following charts are as below:
- Face Velocity = 500 feet per minute (fpm) or 2.5 meter per second (m/s)
- Free Area = 70%
Face velocity is the amount of air volume passing through a grille. Typically, 500 fpm is optimal and balanced between air volume and noise.
Free area is the amount of space on a grille available for the air to pass through. It’s depending on the type of grille and the manufacturer of the grille. However, many grilles have about 80% free area while some external grilles have 50% free area. Hence, I use 70% free area.
CFM Chart for HORIZONTAL Return Air Grilles
CFM Chart for SQUARE Return Air Grilles
CFM Chart for VERTICAL Return Air Grilles
Honestly, I did spent a lot of unproductive time searching for the best grille size when I was working as a junior project engineer. Why I never thought of creating a whole table for myself? Anyway, let’s see how to calculate return air grille size in the manual way.
How to Calculate Return Air Grille Size?
Return air grille size is calculated using the following formula:
Grille Area (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144
From the above formula, using 500 fpm with 70% free area, we get:
Grille Area (sq.in) = Airflow (cfm) ÷ 500 ÷ 0.7 x 144
Let’s say we have an HVAC unit with 1050 CFM of airflow, then:
Grille Area (sq.in) = 1050 ÷ 500 ÷ 0.7 x 144Grille Area = 432 sq.in
Using the CFM chart for horizontal grilles, the 18 inches by 24 inches grille has a grille area of 432 which is suitable for the HVAC unit with 1050 CFM of airflow.
If you don’t prefer to use a horizontal grille, you can use the CFM chart for vertical grilles to find the 24 inches by 18 inches grille or the 36 inches by 12 inches grille. Both grilles are suitable for the HVAC unit with 1050 CFM of airflow.
Alternatively, you can simply match the grille size to the airflow (cfm) of your HVAC unit.
If you are sizing external grilles, I suggest you use 50% free area because external grilles usually have narrow blades profile to prevent rainwater from entering.
Grille sizing is quite easy. However, it can become very lengthy if you have hundreds of it. So, I created an excel sheet that you can purchase and download to speed up the process. Check out the Design Engineer Starter Pack.
What Happen If You Use a Smaller Return Air Grille?
Using a smaller return air grille will result in a higher face velocity with the same type of grille (thus, same free area). With a higher face velocity, the grille may create audible noise. The smaller the grille, the greater the noise.
If you really need to use a smaller return air grille, I recommend you don’t exceed 800 fpm of face velocity. Alternatively, you can seek return air grilles with a larger free area. Otherwise, use more than one grille if possible.
Different grilles have different applications. If you want to learn about the different types of grille used in HVAC, feel free to check out my blog post 4 Types of Grilles in HVAC (Common Applications).
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