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

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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.

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.

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 Photograph of lifted ceiling tile subverting air conditioning system air flow

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.

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.



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? The second photo 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 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.



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


Continue reading at DUCT ROUTING & SUPPORT or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see DUCT AIRFLOW BALANCING FAQs - questions & answers posted originally at this page



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BALANCING AIR DUCT FLOW at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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