- BALANCING AIR DUCT FLOW - CONTENTS. Balancing Air Ducts for Proper Warm or Cool Air Delivery, Heating & Air Conditioning Air Flow Balancing Advice
- POST a QUESTION or READ FAQs about balancing air delivery or air supply in duct systems for heating and air conditioning
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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.
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Balancing Air Ducts for Proper Warm or Cool Air Delivery
Look for indications that the system has not been balanced for optimum air flow:
- Uneven temperatures throughout the air conditioned space, room to room, or within rooms, when the system is operating in either heating or cooling mode All registers are open but airflow varies widely from register to register In cooling mode, uneven condensate production (maybe) Also see ZONE DAMPER CONTROLS where we explain the use of manual or automatic duct dampers for adjusting and balancing building airflow through its HVAC duct system
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
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.
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.
Poor Heating or Cooling Air Flow & Debris & Contaminants in Ceilings Used as Return Plenums Risk Entrainment in Building HVAC Airflow
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:
- Missing ceiling tiles: People in individual offices subvert the system by seeking more or less cool air by removing suspended ceiling tiles.
These un-planned openings make it impossible for the system to be properly balanced. Common return air plenum Use of the ceiling space as a giant return plenum above the work area means that any debris, mold (perhaps from roof leaks), fiberglass, asbestos, or other problematic particles that are in this space are readily circulated through the building air
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.
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.
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.
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:
- HVAC Duct layout problems. Un-balanced HVAC air flow or cool air or warm air delivery due to differences in HVAC duct length, diameter, bends, restrictions may fail to properly balance air flow across a long flat building, between building floors, or where ducts have to make tortous passage from one building area to annother;
See DUCT ROUTING & SUPPORT Vertical air flow problems. in a tall building, even a residential property, if a single air handler is used for both heating and cooling, a basement located air handler will have an easier time pushing warm air up into higher floors of the home than it will pushing cool air up into the same spaces during the cooling season.
(Warm air rises through a building by convection while heavier cool air tends to fall). Increased fan speed for cooling operation or booster fans may help. To avoid this problem some HVAC designs use two air handlers, placing the second unit in the attic or ceiling above the uppermost floor. HVAC Duct leaks. leaky ducts or even open duct connections, especially un-discovered in hidden areas, can completely subvert balanced cool or warm air flow distribution in a building even if the original duct layout and design were well done.
See LEAKY DUCT CONNECTIONS Stuck or not-working HVAC Duct Dampes, Booster Fans: check to make sure that duct balancing dampers have not been manually left in the wrong position (shut, open when should be partly closed, etc) and that automatic devices such as duct balancing dampers and duct booster fans and their controls are in fact operating Inadequate return air or tight rooms. if a building uses centralized return air inlets to the HVAC system then remote building areas may lack adequate air flow because of inadequate return air flow out of that building area back to the system air handler; for example closing the door to a room that has no in-room return duct inlets may substantially reduce the flow of conditioned air into that room.
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