![]() ![]() But first of all, what is the free area of a grille? Grille Free AreaĪs mentioned earlier, small grilles tend to have less free area than large grilles. Now, let's take a closer look at grille free area and compare its affect on the grille size. 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. So, if we recalculate the grille area based on 80% free area, the result is as follow: However, that's not always the case.Ī typical grille with about 16 inches by 25 inches size has a free area of around 80%. When calculating for the 1050 CFM, we assumed that the free area of the resulted grille is 70%. 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. For example, the 16 inches by 25 inches grille which only has a grille area of 400 sq.in. However, for an airflow of 1050 CFM, we can actually use a more optimal grille size. Both grilles are 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. 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. Let's say we have an HVAC unit with 1050 CFM of airflow, then: Grille Area (sq.in) = Airflow (cfm) ÷ Face Velocity (fpm) ÷ Grille Free Area (%) x 144įrom the above formula, using 500 fpm with 70% free area, we get: Return air grille size is calculated using the following formula: So, we need to account for the variable free area. ![]() However, the free area of grilles increases exponentially as the grille size increases. The greater the airflow, the larger the grille. Grilles are sized based on airflow and free area. Why did I never think of creating a whole table for myself?Īnyway, let's see how to calculate return air grille size in a manual way. Honestly, I did spend a lot of unproductive time searching for the best grille size when I was working as a junior project engineer. Following are tables showing the CFM of vertical grilles: 12″ to 18″ Tall Return Air Grilles Vertical return air grilles are the most common type of grille used in single-family homes with a central air conditioning system. 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.įollowing 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. Grilles are typically sized based on a face velocity of 500 fpm and a free area of 70%. Certain places like factories and production plants may call for larger grille sizes but they are mostly custom-made. 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. The largest return air grille is typically stops at 48 inches by 24 inches.īelow is a list of standard return air grille sizes: So, the next corresponding return air grille size includes 4×6, 6×6, 6×4, 8×6, 4×8 and so on. The smallest return air grille is usually starts at 4 inches by 4 inches. Return air grilles are standardized by 2″ per size increase. However, upon request, many grille suppliers do provide custom-made sizes at a higher price. Return air grilles are typically made in standardized sizes. However, small grilles tend to have less free area than large grilles. It's depending on the type of grille and the manufacturer of the grille. Typically, 500 fpm (2.54 m/s) is optimal and balanced between air volume and noise.įree area is the amount of space on a grille available for the air to pass through. Finally, choose your preferred grille size that matches the required grille area.Īpart from the airflow (CFM), grille sizes are depending on the face velocity and the free area of the grille.įace velocity is the amount of air volume passing through a grille. Next, multiply the grille area by 144 to convert it into square inches. 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. So, I created several CFM charts and a guide on how to calculate return air grille size. Using the correct return air grille size is important to ensure that the HVAC system has sufficient airflow as well as low noise. There are many different sizes available for a return air grille.
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