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.

Face velocity is the amount of air volume passing through a grille. Typically, 500 fpm (2.54 m/s) 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, small grilles tend to have less free area than large grilles.

Standard 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 by 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.

Below is a list of 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
• and more…

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

Grille CFM Chart

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.

Following are quick grille sizing charts based on a face velocity of 500 fpm with the respective free area for different airflow rates for vertical, horizontal and square grilles.

Vertical Return Air Grilles CFM Chart

Vertical return air grilles are the most common type of grille used in single-family homes with a central air conditioning system. Following are tables showing the CFM of vertical grilles:

12″ to 18″ Tall Return Air Grilles

20″ to 22″ Tall Return Air Grilles

24″ to 26″ Tall Return Air Grilles

28″ to 30″ Tall Return Air Grilles

32″ to 40″ Tall Return Air Grilles

Horizontal Return Air Grilles CFM Chart

Horizontal return air grilles are rectangular in shape. They can be installed on the ceiling, on the wall or on the floor. Following are tables showing the CFM of horizontal grilles:

Small Horizontal Return Air Grille CFM

Medium Horizontal Return Air Grille CFM

Medium-Large Horizontal Return Air Grille CFM

Large Horizontal Return Air Grille CFM

Square Return Air Grilles CFM Chart

Square-shaped return air grilles can give a more symmetrical view. They are typically installed on the ceiling and up high on the wall. Following are tables showing the CFM of square grilles:

Honestly, I did spend a lot of unproductive time searching for the best grille size when I was working as a junior project engineer. Why did I never think of creating a whole table for myself?

Anyway, let’s see how to calculate return air grille size in a manual way.

How to Calculate Return Air Grille Size?

Grilles are sized based on airflow and free area. The greater the airflow, the larger the grille. However, the free area of grilles increases exponentially as the grille size increases. So, we need to account for the variable free area.

Return Air Grille Size Calculation

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 144
Grille 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.

However, for an airflow of 1050 CFM, we can actually use a more optimal grille size. For example, the 16 inches by 25 inches grille which only has a grille area of 400 sq.in.

The reason why we can use a smaller grille for the 1050 CFM is because of the assumption about the free area of the grille.

When calculating for the 1050 CFM, we assumed that the free area of the resulted grille is 70%. However, that’s not always the case.

A typical grille with about 16 inches by 25 inches size has a free area of around 80%. So, if we recalculate the grille area based on 80% free area, the result is as follow:

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

Since the 16 inches by 25 inches grille has a grille area of 400 sq.in, it is therefore suitable for an airflow of 1050 CFM.

Now, let’s take a closer look at grille free area and compare its affect on the grille size.

Grille Free Area

As mentioned earlier, small grilles tend to have less free area than large grilles. But first of all, what is the free area of a grille?

The free area of a grille is the available space on a grille that allows air to pass through. A grille with 70% free area means that air can pass through 70% of the space of the face of the grille . Meanwhile, the remaining 30% of the space is occupied by the grille blades.

The relationship between the grille free area and the airflow (therefore, the grille area) can be plotted as follow:

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.

For instance, the free area of external weatherproof grilles is mostly 50% and below because their blades are packed much closer together to prevent the ingression of rainwater.

But, small grilles are fundamentally 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 for optimal grille sizing.

To get the grille size calculator with variable free area, purchase and download 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 grilles used in HVAC, feel free to check out my blog post 4 Types of Grilles in HVAC (Common Applications).

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

---

Digital Products by aircondlounge

These are the top selling digital products made by me and sold on aircondlounge. Check them out and see which products fit your needs.

Visit https://aircondlounge.com/shop to see all digital products sold on aircondlounge.

Recommended Air Conditioners

I reviewed and compared hundreds of air conditioners. These are my latest recommended products. Check them out if you’re looking to buy one.

Visit https://aircondlounge.com/category/buyer-guide/ to see more guides and product recommendations.