Photograph of a return air register for commercial office spaceInadequate Return Air, HVAC

  • INCREASING RETURN AIR - CONTENTS: How to increase air conditioning or heating return air flow to improve system performance Return air adequacy on heating and air conditioning duct systems. How to Check or Detect Air Flow at the Return Register Inlets.
  • POST a QUESTION or READ FAQs about how to improve heating or cooling air supply by improving the return air into the HVAC system

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HVAC return air improvement guide:

How to increase HVAC system return air to increase heating or cool air output by improving the flow of return air to the air handler. This article describes problems with return air inlet size, location, and ductwork.

Inadequate return air seriously limits both air flow rates and also the degree to which building air is cooled (or heated) by the HVAC system. The photograph above shows a return air inlet grille for a commercial office space after the air conditioning return register and ducts were increased in size as part of improvements in the building cooling system.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How to Increase Return Air Flow or Supply in Heating or Air Conditioning Duct Systems

Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipmentHere you will find a list of "Improvements" to "fix" inadequate return air ducts & airflow for air conditioners and furnaces addressing the blower and supply duct system.

Add More Return Air Inlets & Ducting

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.

  1. 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 (length x width) 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.
  2. 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 at ADDING RETURN AIR at the air handler.
  3. Temporarily or momentarily opening an air handler cover: if by opening the cover on a blower assembly or air handler unit you feel a dramatic increase in the airflow coming out of the building's air supply registers, then the system is probably return[-air starved. We have opened a cover just a few inches and released it to hear it slam with tremendous force against the blower cabinet when the system lacked adequate return air.

    Watch out
    : See our safety warnings just below.
  4. Have the HVAC system examined by a professional: really this is the best approach once you've eliminated very obvious mistakes like those listed


Watch out: don't leave the cover off of an air handler - it's potentially very dangerous, as we explain at ADDING RETURN AIR at the air handler. 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.

Watch out: it may be necessary to temporarily tape or bypass a blower door compartment interlock switch to try this subjective test. Don't leave the door interlock switch bypassed or taped - doing so is dangerous. Details about this switch are at BLOWER DOOR SWITCHES.

How to Check or Detect Air Flow at the Return Register Inlets

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

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 courtesy Carson Dunlop Associates.

For true air flow measurements that provide quantitative results such as air flow measured in CFM, see AIR FLOW MEASUREMENT CFM.

In that article at Typical Manufacturer's Air Flow Rate CFM Specifications for HVAC equipment we describe typical HVAC air flow rates measured in CFM.

Adding Return Air at the Air Handler - Sometimes a Dangerous Idea

Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipment

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.

Watch out: Worse than inefficient, the approach of taking return air from a basement or crawl space utility area where gas or oil fired heating equipment is located can be dangerous, in particular if by the location of the "new" return air opening draws flue gases from a nearby draft hood or barometric damper, or if the heating equipment is located in a small enclosed space where drawing return air can interfere with the provision of adequate combustion air for the heating equipment.

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 (see link at left).

Improve Return Air Flow by Fixing These Return Air Flow Defects

Air conditioner air flow rate notes Carson Dunlop Associates

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 (left) points out that the (typical) desirable rate of cool air flow in an air conditioning system is around 400 to 450 cubic feet per minute.

The illustration also points out that if air flow is too slow across the cooling coil, that component may become ice or frost-blocked.


What slows down the air speed in an air conditioning or warm air heating system?

Here we provide a list of causes of inadequate air flow, including conditions that slow the speed of movement of air through the duct system as well as other HVAC duct system defects. For our complete list of HVAC duct system inspection, diagnosis, and repair topics

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

Measurement of Air Conditioner or Heating Duct System Air Flow in Buildings

How do we measure air flow in CFM (cubic feet per minute) in an air conditioner or furnace

How is CFM measured? - Anon.


Air flow rates for HVAC systems are expressed as a volume of air being delivered at some rate, typically cubic feet per minute (CFM) or m/sec (meters per second), ft/sec (feet per second), or ft/min (feet per minute).

A nice clear technical answer of how we measure flow rate is provided by Flow Kinetics:

Flow rate is measured by calculating an average velocity for the conduit of interest, and then, multiplying this velocity by the cross sectional area of the duct at the measurement location. The velocity value may estimated using a single reading, or a survey across the duct at a station.[12]

Here's a simplistic example: If I held up a one-foot square sensor in front of an air source (say an air supply register) and the sensor measured air velocity at 12 inches per minute, I'd be measuring 1 CFM of airflow. (One cubic foot = 12 x 12 x 12 inches). Or if we measured an air velocity at an air supply register of one foot per minute and we knew that the duct work was a 12-inch square duct, we'd figure we were seeing one cubic foot per minute of air supply at that location.

Actually here are more than one answer to your question about how airflow is measured in an HVAC system because there is a range of air flow measurement instruments on the market.

The measuring devices vary in price, accuracy, and in operating principle, and there are also of course multiple sources of CFM data: manufacturers specifications, theoretical numbers, and actual measurements. We are most interested in the last category.

For details about how to measure HVAC system air flow see AIR FLOW MEASUREMENT CFM where we also give typical air flow rates for heating and cooling systems.

References for Technical Help in HVAC Return Air & Duct Efficiency Improvements


Continue reading at UNDERSIZED RETURN DUCTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Or see AIR FLOW MEASUREMENT CFM - how to measure HVAC system air flow








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