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Photograph of commercial air conditioning system ceiling plenum with debris Balancing Heating & Air Conditioning Air Flow

Heating & air conditioning HVAC ductwork air flow balancing:

This building air supply ductwork diagnosis and repair article describes proper (and improper) balancing of heating & air Conditioning duct air flow in residential buildings and in commercial office space including high-rise buildings.

Our page top photo shows how individual office occupants who are too hot or too cold can foul up carefully balanced air distribution in a building. Just push over a section of suspended ceiling.

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Balance Air Ducts for Proper Warm or Cool Air Delivery

Flex duct in a horrible routing of excess lengths (C) Daniel FriedmanHVAC duct air flow balance means that we have adjusted the flow through the air duct system such that we get the desired quantity of cool or warm air in all of a building's occupied spaces.

If the duct air flow system is out of balance you will find that when heating, some rooms are not warm enough while others are too cool. While in cooling or air conditioning mode you'll find similarly that some rooms are not cool enough while others are too warm.

Shown in our photo: rather excessive loops of flex duct in an attic.

When ductwork runs like a crazy twisting worm all through an attic its unnecessary length and extra sinuous bends and crips can seriously block air flow to some building areas. Generally we want to keep ductwork as short and direct as possible.

[Click to enlarge any image]

Article Contents:

Look for indications that the system has not been balanced for optimum air flow:

You may want your service person to review air flow in different rooms, to add manual balancing dampers in the duct system (and show you where they are located and mark on the duct just where the balancing damper levers are normally set.

Air conditioning cool air balancing in offices & commercial spaces or in homes with long HVAC duct runs

Location of Air Supply and Air Return Registers Causing Un-Balanced or Poor HVAC Air Flow

Photograph of commercial air conditioning duct system registers

The photographs above show both supply and return ceiling registers in a commercial office space. In this case they are located too close together. The second photograph (above right) shows that a ceiling tile has been removed from a suspended ceiling over an office which uses the ceiling cavity as a common return air plenum.

Photograph of lifted ceiling tile subverting air conditioning system air flow

While this may produce more airflow in the office where this suspended ceiling tile was removed, it has unbalanced the entire air conditioning system for the rest of the offices in the building.

Many commercial spaces and offices are cooled (and sometimes heated as well) by an air handler which delivers supply air to each office area by flex duct connected to ceiling registers.

Return air is passed back to the air handler through the large common space above a suspended ceiling over the occupied space.

Special vents either around the supply registers or placed separately are intended to pass return air to the common ceiling space, and their location and adjustment are important to provide balanced airflow in the work space.

Photograph of a return air register for commercial office space

Single Return Air Inlet & Un-Balanced Cooling or Heating Airflow

Use of a single return air location invites un-balanced air flow delivery into various rooms in a building, especially if room doors are left shut and there is no provision for air movement out of closed rooms to the central return register.

In addition, distance to the central return air register, obstructions, or circuitous routing through hallways or even among different floors in a building all argue that a single central return air register can contribute to an air-starved air handler or uneven air delivery to the conditioned spaces.

See RETURN AIR REGISTERS & DUCTS

and

UNDERSIZED RETURN DUCTS for details.

RETURN AIR, INCREASE may also be helpful.

Poor Heating or Cooling Air Flow & Debris & Contaminants in Ceilings Used as Return Plenums Risk Entrainment in Building HVAC Airflow

Photograph of commercial air conditioning system ceiling plenum with debris

This photograph shows a common ceiling plenum over a commercial office space in a Manhattan high rise building. Notice the considerable amount of debris atop the ceiling tiles?

All of the office conditioned air moves through this area, risking additional levels of irritating or harmful particles, particularly if the debris contains asbestos fragments or lead paint chips. (In this particular case tests showed that this was not the case.)

Here are some common concerns or defects in commercial installations that use this design:

Openings that Subvert Proper HVAC Duct Airflow

These photos show a common ceiling return air plenum over commercial offices. In the first or left hand photo, notice that rectangular opening in the distance?

Photograph of commercial air conditioning system ceiling plenum with debris

The second photo (below) is a close-up showing a big surprise: the ceiling area used as return air plenum for an office suite is wide open to the rest of the building ceilings on the same floor.

Photograph of commercial air conditioning system ceiling plenum with debris

Openings had been left open between the office space and the top floor hallway and also between the hallway and other office spaces in the building, so that particles, leak-related mold, or other irritating particles developing over any office will be shared among all of the tenants on this building floor.

Watch out: unanticipated or improper openings between building areas may also be a fire hazard, contributing to the rapid spread of fire from one building area to another.

See FIRE & SMOKE DAMPERS, AUTOMATIC.

Also see UNSAFE DUCT OPENINGS.

Missing or incomplete ceiling partitioning

in large commercial buildings may mix air (and odors or contaminants) from multiple offices or building use areas, redistributing these un-wanted odors or particles to other building areas.

One of our clients who maintained a law office in a strip mall complained of chemical odors which were traced to a beauty parlor located at the extreme other end of the building.

Photograph of commercial air conditioning system subverted by open window

Open windows,

especially in a tall office building such as shown by this photograph, cause a tremendous up-draft through the building, moving particles, gases, or other potential IAQ concerns up through the building.

In this instance (above left) the office occupants on the 18th floor of this Manhattan office opened their windows and also their office entry door to try to cool off their offices because they were unable to turn off the building heating radiators.

See AIR MOVEMENT in BUILDINGS for details.

Other Causes of & Cures for Un-Balanced or Poor Air Flow Through Commecial or Long Residential HVAC Duct Runs

In addition to the HVAC air duct flow balancing problems & recommendations discussed above, consider these possible causes of inadequate cool air or warm air deliver to some building areas:

Research on HVAC Air Duct Balance & Design Improvements

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-05-14 - by (mod) -

Doug

Thank you for perceptive question and of course you get one attaboy pat on the back for figuring out what was going on while there is still time to do something about duct routing.

I expect there are duct airflow balancing problems in the situation you describe although they occur in all buildings regardless. You may want to increase the size of the more distant duct and include balancing dampers in both so that once the ductwork has been installed you can fine-tune the airflow.

Remember that you need plenty of return air as well not just Supply Air, and that the source of the return air effects the operating efficiency of the system.

On 2020-05-13 by Doug

I just removed a non-weight-bearing wall, and realized that the wall cavities were serving as Return chases for two upstairs bedrooms. There is an unfinished basement below, and it was easy to see that this was the case.

The easiest solution is to branch two new return lines from the only remaining upstairs bedroom (whose return is still intact after the wall removal, and run them through the attic to the walls of the two bedrooms in question.

My questions are: Is there a maximum distance from the return plenum that a duct can run? And will those three rooms feeding the one cavity to the basement cause a flow problem? Will air be pulled at a greater rate from the first return grate, and less through the ones at the end of the line?

On 2017-10-08 - by (mod) -

I speculate that your contractor is taking a compromise-position since optimally cool air is delivered near the ceiling while warm air would be delivered near the floor.

On 2017-10-08 by ottotoottoototto

My contractor is installing wall registers for the forced air heating/ac supply line. He's not putting them just below the ceiling but 3 1/2 feet below (10 ft ceilings). Is that okay or should I ask him to raise them to just below the ceiling?

On 2014-11-18 - by (mod) -

Dan

In heating mode I don't think so. Imagine if you blocked ALL of the air leaving the supply plenum. The blower would spin, no air would move through the furnace, the heat exchanger would get hot, and the fan limit switch would shut off the system.

On the other hand, if *return* air is blocked or inadequate indeed the system efficiency and effectiveness are reduced.

On 2014-11-17 by Dan

Home heating and cooling question....Closing off registers, in the basement and bedrooms, wouldn't this be a strain on the heating and cooling systems, and wouldn't it impact the running efficiency of the system?

On 2014-09-08 - by (mod) -

Anon

Yes. Try partly closing supply dampers to the too-hot or too-cold (heating or air conditioning) zones. Also check for obstructed ductwork and for incomplete or inadequate air returns.

On 2014-09-08 by Anonymous

I have three zones, two on the main floor and one in the basement. All my dampers are normally open. Most of the time the basement AC/heating is ON trying to cool (summer) or heat (winter), while the main floor, gets very cold (summer) and very hot (winter). I have tree thermostats that are programmed to control this imbalance of temperature. Is there something else I can do to balance the temperature considering I have already programmed the thermostats and have closed/opened vents/registers accordingly? Does the normally-open vs normally closed dampers affect this?

On 2014-09-08 by rseville

I have three zones, two on the main floor and on in the basement. All my dampers are normally open. The basement AC/heating is most of the time ON trying to cool (summer) or heat (winter), while the main floor, gets very cold (summer) and very hot (winter).

I have tree thermostats and try to control this imbalance of temperature. Is there something else I can do to balance this imbalance of temperature considering I have already programmed the thermostats are programmed and closed/opened vents/registers? Does the normally-open vs normally closed dampers affect this?

On 2014-06-14 - by (mod) -

Varghese

Interesting question about air flow.

If the air flow volume (really we could say its speed across the cooling coil) is too great for the design of the cooling system then the rate of removal of heat from the air and moisture from the air will be slowed. A result may be that the system has to run longer to satisfy the thermostat.

However air flow that is too slow is also a problem, not only resulting in inadequate cooling but possibly also cooling coil icing and blockage.

On 2014-06-14 by Varghese

if the air volume flow is greater than the required value, will this affect the cooling of the system positively or negatively.

On 2014-03-10 - by (mod) -

Sorry, I just can't say with so little information. It is a bit unusual to connect return air from two different systems - but I suspect there was a reason since extra trouble and cost would be involved. Perhaps the return air to one or both of the air handlers was inadequate. Without a more thoughtful look at the site and its duct work, air handlers, room layout, air movement, and other stuff I haven't thought of, it's just speculation.

A start might be to actually look at the duct layout, the air handler sizes, capacity, location, duct sizes, and in particular the return air inlet locations and duct sizes. It'd be easy to shut off one of the return air sections (say the downstairs) and then see if the upstairs air handler is air starved.

Just what a shared return does to indoor air heating or cooling adequacy is something I can't pose with just the data in your note. Kudos to the inspector for recognizing something a bit out of the ordinary.

On 2014-03-10 by A Carruth

Just had a home inspected. 3800sq ft with 2 3.5 tone A/C units, one upstair and one dwonstairs. There was a large diameter duct connecting the two return plenums and inspector said he had never seen that arrangement before. Is this an indication of a problem?


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