Return Air Grille Sizing Guide: CFM Chart & Calculation

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 the 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.

Apart from the airflow (CFM), grille sizes are depending on the face velocity and the free area of the grille. In addition, the grille should not create too much noise or causing too greater of a pressure drop.

Return Air Grille Sizes

Return air grilles are typically made in standardized sizes. However, upon request, many grille suppliers do provide custom-made sizes at a higher price.

Return air grilles are standardized based on 2″ per size increase. The smallest return air grille is usually starts at 4 inches by 4 inches. So, the next corresponding return air grille size includes 4×6, 6×6, 6×4, 8×6, 4×8 and so on. The largest return air grille is typically stops at 48 inches by 24 inches.

Here are some of the standard return air grille sizes:

• 4×4
• 6×6
• 8×8
• 10×4
• 12×6
• 14×8
• 20×10
• 24×12
• 28×14
• 32×16
• 40×20
• 48×24

Standard return air grilles typically stop at around 48 inches by 24 inches as subsequent sizes are too large for applications in residential and commercial buildings. Factories and production plants may call for larger grille sizes but they are mostly custom-made.

Return Air Grille CFM Charts

Return air grilles are typically sized based on a face velocity of 500 fpm and a free area of 70%. However, small grilles tend to have less free area and large grilles tend to have more free area due to nature of the grille construction (more on this later).

Face velocity is the amount of air volume passing through a grille. Typically, 500 fpm is an optimal balance between airflow and noise. Free area is the amount of space on a grille that is available for the air to pass through. It is about 70% on average.

However, the free area of a grille decreases along with the grille size. So, small grilles must account for the drop in the free area or else, they’ll be undersized.

Following are charts that shows the airflow capacity for different grille sizes based on a face velocity of 500 fpm with variable free area for different airflow rates:

Vertical-Oriented Return Air Grilles

Horizontal-Oriented Return Air Grilles

Square-shaped Return Air Grilles

Sizing a Return Air Grille Based AC Tonnage

HVAC units generally have about 400 CFM for every refrigeration ton. Meaning a 2.5-ton air conditioner has about 1000 CFM of airflow. So, we can also quick size the return air grille based on the tonnage.

How to Measure Return Air Grilles?

When a return air grille is due for a replacement, you must replace it with an equally-sized one. To appropriately measure a return air grille, always measure the duct opening size and look for a grille that matches it.

Generally, return air grilles are measured as follows:

Sellers and manufacturers publish their return air grilles based on the opening size. It is the “Width” and “Height” shown in the above diagram. Do not use the face width and face height of your old return air grilles to buy new return air grilles. Always use the opening size.

How to Size a Return Air Grille?

The size of a return air grille is determined by how much airflow it allows to pass without creating too much noise and pressure drop (air resistance). If you use a small return air grille, you’ll notice the HVAC system is noisier and potentially consuming more power.

Return Air Grille Calculation Formula

Most of the time, sellers don’t publish the airflow capacity of their return air grilles. This is fine in many cases as consumers can buy based on their old grille size or duct opening size. However, it troubles the first timers.

Fortunately, based on my research, most return air grilles sold in the United States have a free area of about 60-80%. Then, to ensure the noise created by the grille is acceptable, it is recommend to use a face velocity of 500 fpm when sizing a return air grille.

Although it is recommended to use a face velocity of 500 fpm when sizing a return air grille, you can use a 600-800 fpm as well but take note that the noise created by the grille is expected to be higher. Also, if you use a lower face velocity like 300-400 fpm, the grille size will be bigger.

With that, we’re able to calculate the grille size using the below formula:

Grille Area (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144

For example, say we have an HVAC unit with 1050 CFM. Using an optimal face velocity of 500 fpm and assuming the grille has a free area of 70%, the required grille size can be calculated as follows:

Grille Area (sq.in) = 1050 ÷ 500 ÷ 0.7 x 144
Grille Area = 432 sq.in

From the formula, we need a grille size that has a grille area of 432 sq.in. So, any grille sizes that have 432 sq.in of grille area or above can be used.

However, for an airflow of 1050 CFM, we may be able to use a smaller grille size like a 16 inches by 25 inches grille which only has a grille area of 400 sq.in, when we account for the actual free area of the grille.

Earlier, we calculated the grille based for 1050 CFM based on a free area of 70%. Now, what if the actual free area of the grille is 80%?

Grille Area (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144
Grille Area (sq.in) = 1050 ÷ 500 ÷ 0.8 x 144
Grille Area (sq.in) = 378 sq.in

With 80% free area, the resulted grille area is now 378 sq.in and a smaller grille size like 16×25 can be used.

Alternatively, you can also use the Grille CFM Chart I provided above to find your preferred grille size. Their airflow capacity is calculated based on variable free area. Remember I said the smaller the grille, the lesser the free area?

For instance, the 24 inches by 16 inches vertical-oriented return air grille which has a grille area of 384 sq.in is suitable for 1050 CFM. If you don’t prefer the vertical-oriented grille, you can also use a 6×30 or 14×14 grille. Both are suitable for 1050 CFM.

If you still have trouble sizing the return air grille, here is a video showing the step-by-step process:

How to Calculate the Free Area of a Grille?

The free area of a grille can be calculated by rearranging the grille area formula as follows:

Grille Free Area (%) = Airflow (cfm) x 144 ÷ Face Velocity (fpm) ÷ Grille Area (sq.in) x 100

Alternatively, you can refer to the AK factor of a grille to calculate its free area. AK factor is also known as the effective area of a grille. It can be found in the performance datasheet published by grille manufacturers.

The AK factor of a grille is expressed in terms of square inch or square feet. For example, a 10×4 grille has an AK factor of 0.14. Thus, the effective area of the grille is 0.14 sqft or 20.16 sq.in. Given that the total area of the 10×4 grille is 40 sq.in, the resulted free area in terms of percentage is 50.4%.

Since not many sellers publish their the free area and airflow capacity of their grilles but we know that smaller grilles have lesser free area, I plotted a graph showing the drop in the free area against the airflow as follows:

Grille size is directly proportional to the airflow. Hence, the above graph also represents the relationship between the grille free area and the grille area.

As you can see from the above graph, the free area of a grille varies depending on the airflow (grille size). Hence, the smaller the airflow (grille), the lesser the free area and vice versa.

Nonetheless, the free area of a grille varies depending on the manufacturer and the model of the grille. External weatherproof grilles is mostly 50% and below because their blades are packed much closer together to prevent the ingression of rainwater.

Small grilles are usually less in the free area as their blades occupy a larger percentage of their total mass. So, the free area graph is a good reference. Manufacturer’s datasheet always supersedes my recommendations.

If you’re sizing return air grilles for new construction/project, get the grille size excel calculator in my Design Engineer Starter Pack to speed up the process.

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.

Supply air diffusers are sized differently than return air grilles. If you want to learn how to size supply air diffuser, check out my blog post: HVAC Diffuser Sizing Guide: CFM Chart & Selection.

This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.

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