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Home Home Improvement How to Size Return Air Grill?

How to Size Return Air Grill?

The furnace filters are usually located within the HVAC unit or the return air ducts. As the filter traps filth from the passing air, it needs to be changed every 30-90 days. And sometimes you might have to replace or fix the filter in return grill yourself. It might be challenging if your heater is hard to access. Hence, getting into hidden areas of the home to replace the filter might not be practicable. So, a proper-sized air filter grill is a necessity for optimum functioning. For this, you must know how to size return air grill. Read this article if you want to learn.

How to Size the Grill?

Step 1

Unlock your furnace cabinet to access the service plate. Examine the plate to know how many British Thermal Units (BTUs) your furnace brings out.

Step 2

You need to divide the BTUs by 50 to determine how many Cubic Feet Per Minute (CFM) your heater blows. For instance, if your heater has 50,000 BTUs, it would bring out 1000 CFM.

Step 3

Compute the area in square inches you require for your filter grille. For optimum functioning, plot on 2.5 CFM per sq. inch of filter grill zone. So, if your heater brings out 1000 CFM, you would require 400 sq. inches of filter grille zone.

Step 4

Then multiply the lengths and breadths of the standard filter grill dimensions to compute each zone area (in square inches). Once you obtain the one that matches or surpasses the above-computed figure, you can keep it your ideal filter grille dimension.  

A few common sizes include 20 x 14, 25 x 16, and 20 x 20. Therefore, you can find out that the square inches calculate 280, 400, and 400 correspondingly using these examples. If you consider the above example, the heater demands 400 sq. inches’ area for the grill so that you can choose a 25 x 16 or 20 x20 grill.

What You Need to Be Careful of?

So, it might be pretty straightforward to order the perfect-sized vent. But for this, you need to be mindful of some basic rules.

You should know that the air registers and grills are ordered considering the size of the actual duct opening in the wall, ground or roof that the vent might fix into or cover. However, you might not require the total size of the current vent cover.

When ordering grills and filters, you should be aware that they are directional. So, you can purchase a horizontal (long direction) or vertical (small direction) grill.

So, if you order a wall return air grill or filter grille, you must know that these are steering. Hence, you can order a horizontal grille (blades in the long direction). But you should consider the duct opening size (height x breadth) before purchasing. It is crucial if you have to install the grill on the wall. Hence, it is usually not significant for a roof installation, but you might prefer it due to beautifying reasons. 

If you purchase a supply register for your wall, ground, or roof, the direction is not imperative. However, all wall and roof registers are listed (with the big hole size followed by the small size). Hence, sellers may enlist the smaller size first and then the bigger one for ground registers and return grills.

If you are unsure, you may contact Heating and Air Conditioning Service in Buford. 

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Air return grill

Which are the Important Components?

Supply Register

It is the opening through which the air blows out into the indoor space. It usually has a damper on the backside.


It is a device on the backside of the supply register that allows you to open or close the airflow. The damper is usually in the form of a handle or lever.

Return Air Grill

It is a grill through which air is returned from the surroundings into the duct. Then it is pulled back into the wall. Hence, it can be a non-operable flat cover or a hinged face that will disclose or fluctuate down to place the filter into the back filter rack.

Duct System Cannot Self-Balance

The nearest thing to self-balancing the duct system will be an appropriate return system in every room that features a supply vent. However, this is not very feasible and is not practically possible. Installed ducts do not guarantee ideal performance and need to be regulated individually after fixing. For this, Air Duct Cleaning in Buford may use apt testing, corresponding methods, and correct instruments.

Which Are the Important Terms?

Cubic Feet Per Foot (CFM)

If the typical size of the sidewall grill is 4 inches, you have 250 CFM/ft. that will direct you to all remaining parameters. You know that the available enlisted dimensions are 6,8, and 10 inches.  

Enlisted Height

As you have three options: 6,8, and 10 inches, your decision will depend on the chosen depth of the duct.

Friction Losses

Now, you have CFM/ft. and the enlisted height, you can quickly identify that the friction losses will be 0.01 Wg. You may accept the value or reject it. For this, you should review your blower performance in contrast to the external static pressure in the air conditioner catalogue.

Face Velocity

You can compute the face velocity as per the SMACNA manual.


You will see that the sidewall trough ranges from 21 to 32 feet. You may choose it as per your indoor space requirements.

Roof Height

In this scenario, the roof height needs to be a minimum of 15 feet.

NC (Duct Leakage Test)

25 NC is ideal for housing applications

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