Return air grilles are typically made in standardized sizes. However, upon request, many grille suppliers do provide custom-made sizes at a higher price. 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. If you’re still a bit confused, you can figure out the amount of players in the game by opening up the leaderboard. This can be done by tapping on the circle with three dots in it button or hitting tab on your keyboard. Once you have the leaderboard open, simply count the number of players that are in your server including yourself. If your server is full at the moment, then there’s 14 players in it because that’s the maximum. 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.

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: 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. 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. 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.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. To find the resulting product, find the sum of 3 individual product of multiplier and multiplicand.

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. Proper fraction button is used to change a number of the form of 9/5 to the form of 1 4/5. A proper fraction is a fraction where the numerator (top number) is less than the denominator (bottom number).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. 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. 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?

Find the product of multiplicand and most significant digit (MSD) of 3-digit multiplier, and write down the product under the earlier product but the One’s place value of product should start from the Hundred’s place value of multiplicand. Proper fraction button and Improper fraction button work as pair. When you choose the one the other is switched off.

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

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%?

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. 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 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 Find the product of multiplicand and 2nd least significant digit of 3-digit multiplier, and write down the product under the earlier product but the One’s place value of product should start from the Ten’s place value of multiplicand.