How To Size Return Air Grill

What Is A Return Air Grille

How to Size my Return Air Conditioning Grills Correctly?

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A Return Air Grill Is An Essential Part Of Any HVAC System. A return air grill connects to ductwork that allows air to return to any cooling or heating system. The openings that connect to ducts and other spaces for the returning air are normally covered with grillwork.

Similarly, what is the difference between supply and return air? The biggest difference between supply and return vents is the direction in which the air flows. In a supply vent, the air flows out of the ductwork. In a return vent, the air flows into the ductwork. A second difference is the size of the vents.

Just so, what is a return air?

A return air vent is the part of your HVAC system that allows that warm air to flow back to your air conditioner. Not only will that air be cooled again, but it will also be cleaned by your air filter or air cleaner.

How do you increase return air flow?

3 Simple Ways to Improve Your Furnace’s Airflow for More Comfort

  • Unblock and clean your vents. Here’s a super easy way to increase air flow.
  • Change your filters and clean your return grill. A dirty air filter impedes air movement and makes it hard for your furnace to work efficiently.
  • Seal leaky ductwork. Snap on some goggles and a dust mask, it’s time to check out your air ducts.
  • 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 . 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 dont 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.

    What’s The Best Location For Return Air Ducts

    Supply air from furnaces and air conditioners should be on outside walls. The return air vent openings need to be on the opposite side of the room so the conditioned air is pulled across the room.

    If the supply ducts are in the floor, then the return air should be located up high. This pulls the air across your body.

    If the supply ducts are high or in the ceiling, then the return-air ducts or grills should be low on a wall.

    FREE Call From Tim Carter About Your Ducts – .

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    Do Bedrooms Need Return Air

    Does Every Room Need Air Return Grilles? While it is a myth that air return grilles are required in each and every room in the house, it is definitely necessary to have more than one of these grilles installed at strategic places in the house. The most important place to have these would be the bedroom.

    Sidewall / Ceiling Vents & Return Air Grilles:

    hvac system ceiling eggcrate return air grille diffusers ...
    • First, determine whether you need a return air grille or a register for your sidewall/ceiling area. Registers have dampers to open and close air flow, whereas returns do not.
    • Measure the hole opening in your wall or ceiling and place your order based on that size. For example, if the hole opening in your wall or ceiling is 12″ wide x 10″ high, place your order for a 12″ x 10″.
    • IMPORTANT FOR WALL VENTS: Always measure the width of the hole opening first . Then measure the height of the hole opening . Otherwise the fins of the grille or register will be the wrong direction.
    • The overall faceplate dimensions of the register or return will be larger in order to mount in place. The overall dimensions should also be listed on the product page of each item.

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    How To Calculate Return Air Grille Size

    Return air grille size is calculated using the following formula:

    Grille Area = Airflow ÷ Face Velocity ÷ Grille Free Area x 144

    From the above formula, using 500 fpm with 70% free area, we get:

    Grille Area = Airflow ÷ 500 ÷ 0.7 x 144

    Lets say we have an HVAC unit with 1050 CFM of airflow, then:

    Grille Area = 1050 ÷ 500 ÷ 0.7 x 144Grille Area = 432

    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 dont 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.

    Alternatively, you can simply match the grille size to the airflow of your HVAC unit.

    If you are sizing external grilles, I suggest you use 50% free area because external grilles usually have narrow blades profile to prevent rainwater from entering.

    Some Things To Consider:

    What do the terms grille and register mean? A grille is a vent that does NOT have a damper. A register on the other hand is a vent that does have a damper.

    What is the difference between wall and floor grilles/registers?Floor grilles/registers can support the weight of a person while wall ones cannot.

    When sizing grilles remember:

    • Supply or exhaust application? It matters for some grilles.
    • Dayus grilles are sized by the hole they fit into. We call the hole size the List Size. They are typically slightly undercut to make sure they fit the hole.
    • Filter grilles are openable, should you need access or want to install a filter.

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    My Recommended Return Air Grilles

    While making the CFM charts for different grille sizes, I found some good return air grilles that you might be interested in buying. These return air grilles received good feedback from many customers and they are made of durable steel with a decent amount of free area.

    Here are my recommended return air grilles, each of them is available in multi different sizes. So, follow the link to see other available grille sizes:

    Add More Return Air Inlets & Ducting

    Proper Selection & Sizing of Return Air Grilles

    Adding additional return air inlets and ducts to increase airflow to the air handler is an effective way to improve air conditioning or or warm air heating system performance, provided that the system is in fact running “air starved”.

    There are several easy and amateur ways to check for an air conditioning or warm air heating system that is not getting enough return air.

  • Visual inspection for inadequate return air: if there is only a single air return inlet, where is it located? Is the return isolated from some rooms in the building if the doors to those rooms are closed?Is the return air inlet size smaller than the cross section of the air inlet end of the air handler or blower assembly? A mismatch in return air inlet grille or duct size will reduce the system’s effectiveness.
  • Visual inspection for prior attempts to “improve” return air such as holes cut into an existing return air duct, or worse, openings cut to admit more “makeup” air into the air handler from an un-conditioned space such as an attic or crawl area.These are discussed

    Also keep in mind that a properly-working air handler or blower assembly will always be running with negative air pressure in the blower compartment – otherwise it wouldn’t be moving any air through the duct system. So a certain amount of “pull” of air rushing into the blower that also wants to re-close the blower compartment door is normal.

    Details about this switch are at BLOWER DOOR SWITCHES.

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    The Basics Of Air Grilles

    When it comes to air grilles, there is not a whole lot of differentiation between productsthough we proudly offer the Shoemaker brand as one of our highest quality manufacturers. There are a few things to know before deciding upon your return air grille.

  • Measure the Opening: Every home is different, meaning there is no standard size grille to fit all. With this in mind, you need to measure the opening in which youre installing the grille so you get the right piece. Even if large grilles are more expensive, you cannot tamper with the size of the opening where you will be installing your air grille. These openings were designed with your home ventilation in mind, so do notunless prompted by an expertchange the opening.
  • There are a few different features from which to choose when deciding on your air grille. In most homes, the filter is placed directly by the furnace, but some homes are not equipped for that kind of setup and you may need a return air filter grille.
  • If the air duct has not been formed yet, then you have a much bigger task on your hands than this article can help you with. Contact us for more information about duct installation, or choosing your air grille features.
  • Quick Simple Check For Air Flow At Return Registers

    A simple test for air movement at the return air inlet is illustrated in our sketch.

    Just hold a tissue or piece of toilet paper near the inlet grille face. If air is moving into the grille the tissue will be pulled against the opening.Sketch at left courtesyCarson Dunlop Associates.

    For true air flow measurements that provide quantitative results such as air flow measured in CFM,

    In that article at HVAC AIR FLOW RATE CFM, TYPICAL Specifications for HVAC equipment we describe typical HVAC air flow rates measured in CFM.

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    Blower Fan Too Slow For Cooling Season

    An HVAC system that is simply not capable of moving enough cubic feet of air per minute will not be able to adequately cool or warm the occupied space.

    Higher air speeds are needed during the cooling season than during the heating season.

    Carson Dunlop Associates‘ sketch points out that the desirable rate of cool air flow in an air conditioning system is around 400 to 450 cubic feet per minute per ton of cooling capacity.

    Example: a two-ton air conditioning system will move or about 900 cfm.

    Details about measuring the air flow rate in HVAC systems, air handlers, and duct work are

    Size And Orientation Of Return Grilles And Supply Registers

    Return Air Grille Cfm Chart

    Proper sizing and orientation of grilles, registers, and diffusers may seem like such a simple thing, but it’s an area where confusion and mistakes are commonly made.

    First, let’s define some terms.


    A return draws air into a return duct system with negative pressure compared to the space, usually via a fixed grille .


    The supply vents, registers, or diffusers blow air into the conditioned area with positive pressure and are responsible for distributing and mixing the air.


    A vent is a generic word for a designed opening or cover that air passes in or out of. When in doubt, just say vent.


    A grille is a fixed vent type that contains no damper or adjustable louvers. Grilles can be used for supply but are most commonly used in return applications. The grille shown above is a steel-stamped return grille.


    A register is a vent that contains an internal adjustment damper and often externally adjustable louvers. Registers have the same inlet neck and outlet face size. Air will move straight through registers and grilles. Registers are the most common type of supply vent. The register shown above is a common aluminum, adjustable, curved blade, one-way 10×6 ceiling register.


    Sidewall Straight Blade vs. Curved Blade Ceiling

    Sidewall registers and grilles have straight louvers to force the air straight into the space with no turning at all at the face. Curved blades direct the flow at an angle and are generally used for ceiling applications.

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    My House Is Hot Or Cold Is This A Return

    Yes, if your house is either hot or cold, I’ll bet you have inferior or lacking return air ducts.

    If you live in a home that has rooms that are either too hot in hot weather or are cold in cold weather you could have a return-air duct issue.

    Return air ducts are necessary for just about every room. However, there are two rooms that I never place return air ducts – kitchens and baths. Let’s try to keep the garlic odors and hairspray in these rooms if you don’t mind!

    The reason for return air is simple. Your furnace is simply a recirculating pump. Instead of pumping water, it is pumping air.

    We need to get the air back to the pump. Furthermore, without a return air duct in each major room, the supply duct air has to “push” all of the air in the room out of the way as it makes its way towards you. The return air ducts help pull the air as it is being pushed.

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    to get FREE BIDS in hours from local HVAC companies to have new return air ducts installed in your home.

    Adding Return Air At The Air Handler

    When the cooling ability of an air conditioning system is inadequate, particularly when the volume of air being delivered in the building seems too low, we often see evidence of an attempt to boost heating or cooling air delivery in this “stopgap” manner.

    We find extra return air openings having been cut in the return plenum right at the air handler unit at a combination air conditioning and hot air heating furnace or at an attic or basement air conditioning-only air handler.

    Indeed this boosts the air coming out of the system if the air handler was “air starved” due to insufficient return ducts in the first place. An example of this poor practice is shown in the photograph.

    But this is a very inefficient way to operate the system since a significant portion of the air volume is moving only “one way” from an attic or basement into the cooling unit and out to a remote living area.

    This is an expensive way to run an air conditioning system: keep taking “new” air, cool it, and blow it where it’s wanted. Proper design re circulates air from the occupied space which permits it to be cooled and filtered.

    Flue gases: may be drawn into the duct system if these “improvement” openings are cut too close to heating equipment, particularly gas-fired furnaces, boilers, and water heaters. We say more about this at UNSAFE OPENINGS below .

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    Return Air Grill Sizing

    Kinds of Grills

    There are three different ways air is brought into the closed systems present in your home.

    Ventgrills are coverings for vent openings where air is forced into or out of a room. This air is then circulated into either the heating or cooling unit.

    Heat registers are vent covers that cover the hole in the wall usually near the floor or in the wall where the duct goes into the room. It may have adjustable dampers that will control air flow.

    An air return is a vent grill that is often located in the hallway or the ceiling. It extracts air from the room and sends it back through the closed air system.

    Return Air Grill

    Air that is in your HVAC closed system is either coming into the system or going out. The return vents are connected to the return ducts which means the air is moving from the room to the inside of the system. When the air handler is functioning, it will suck the air from the rooms, into the return ducts and back to the heating and cooling system.

    A closed HVAC system will be pushing air into a space causing an increase in air pressure in that room. The air must have a place to either vent itself or go back through the system. The return vents and ducts serve to equalize this pressure and allow for the continued circulation of air. The cover is usually adjustable and will allow the air to be either increased or decreased based on personal preference.

    Air Flow Measurement

    Symptoms of an Incorrectly Sized Return Grill

    Sizing of the Return Grill

    How To Measure For Replacement Registers And Grilles

    Return Air Filter Grille Filters – Find The Right Size!

    Measure the opening in your floor, wall or ceiling. The width and length of your opening should match the duct opening sizes.

    1. Note if the register or grille you are replacing is for a wall, ceiling or floor.

    2. Remove screws holding the vent in place. If the vent is held in place by its own weight or tension, pull the vent forward and out of the opening.

    3. Measure the size of the air duct opening. Do not measure the old register or vent. Write down the measurements as Width and Length because this is how register and grille packaging is labeled.

    4. Make note of the size of the existing hole, you will want to ensure the register or grille you choose will cover the opening. Look for units with wide borders to compensate for large openings.

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    What Happens If Return Air Is Undersized

    If the return duct is too small, there are an inadequate number of return grilles or they are undersized, it cant return enough air to recirculate back into the system. This poor airflow not only makes your system run longer than necessary to provide comfort, but increases your energy bill as well.

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  • |#1

    Rich, In general, the size of the grille is the same as the size of the return duct. An 8×14 grille needs an 8×14 duct.

    To make sure that your return air ducts are adequately sized, though, you need to do some airflow calculations. Return airflow is equal to supply airflow.

    If you know the airflow of your system, you can consult the chart below.

    Note that some homes have only one return air grille, but better systems have multiple return air grilles.

  • |#2

    From your description of your air infiltration I think your fault or faults are in the supply ducts.

    I suspect that the duct work is located in unconditioned spaces like the attic or a crawl space and they are very leaky so when the heat or AC blower is running the air leaks into the unconditioned spaces escapes to the outdoors and you have noticed the air entering your home thru every gap to replace the air that leaked out of the supply ducts.

    If at all possible get your duct work into the condition space by eater moving the ducts below the ceiling or moving the thermal boundary insulation against the roof and sealing the attic vents.

    Sealing the duct work is better than doing nothing but not by much.


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