How to Increase Air Conditioning or Heating Return Air Flow InspectAPedia® -
How to increase air conditioning or heating return air flow to improve system performance
Defects in return air ducts & registers for heating or cooling systems
Return air adequacy on heating and air conditioning duct systems
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This chapter of "How to Inspect the Central Air Conditioning or Cooling System" describes
how to increase air conditioning or heating return air flow to improve heating or air conditioning system
performance.
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. Also see RETURN AIR REGISTERS & DUCTS and the suggestions listed at RETURN DUCT AIR LEAKS. At SUPPLY DUCTS & REGISTERS we discuss how to increase the supply of warm or cool air in buildings by addressing the blower and supply duct system.
The master document, of which this is a chapter, describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and
home inspectors of common cooling system defects.
INCREASING RETURN AIR - List of "Improvements" to "fix" inadequate return air ducts & airflow for air conditioners and furnaces
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.
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 mis-match 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 at Adding return air at the air handler.
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.
Have the HVAC system examined by a professional: really this is the best approach once you've eliminated very obvious mistakes like those listed at RETURN DUCT AIR LEAKS.
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.
Adding Return Air at the Air Handler - Sometimes a Bad Idea
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).
Find & Repair HVAC Return Air Duct Leaks & Obstructions
Air filter or other item that has been sucked into the duct system will block air flow and can risk a fire if drawn into the blower assembly fan, Dirty Air Filter Problems are perhaps the most common cause of unsatisfactory airflow in an HVAC system.
Blower Fan: dirty blades on a squirrel cage blower assembly fan significantly reduce the blower fan's ability to move air into the HVAC system from the return-air side as well as reducing its ability to push conditioned air into the occupied space. DIRTY A/C BLOWERS
Fire damper that has become stuck in the closed or partly-closed position interferes with proper airflow through the system FIRE DAMPERS in DUCTWORK
Flex duct defects: Collapsed sections of flex duct block or stop airflow in either supply or return air systems DUCT ROUTING & SUPPORT (see our photo, above-left).
Leaks in the supply air duct system are a very common HVAC duct defect that results in poor heating or cooling air flow. LEAKY DUCT CONNECTIONS.
Also see these defective duct materials that may lose insulation and also may develop leaks:
Return air inlets: Return air inlet grilles that are obstructed with dirt, debris, or furniture or that are improperly located or are just too small mean that because the heating or cooling system is "starved for air", the supply air flow into occupied spaces will also be reduced. RETURN AIR REGISTERS & DUCTS
Zone dampers that are stuck partly closed obstruct air supply into that building area, or if stuck "open" when the zone damper should be closed, airflow to other building areas will be reduced. ZONE DAMPER CONTROLS
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Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects"
section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and
home inspection educator.
Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
Air Diffusion Council, 1901 N. Roselle Road, Suite 800, Schaumburg, Illinois 60195, Tel: (847) 706-6750, Fax: (847) 706-6751 - info@flexibleduct.org - www.flexibleduct.org/ - "The ADC has produced the 4th Edition of the Flexible Duct Performance & Installation Standards (a 28-page manual) for use and reference by designers, architects, engineers, contractors, installers and users for evaluating, selecting, specifying and properly installing flexible duct in heating and air conditioning systems. Features covered in depth include: descriptions of typical styles, characteristics and requirements, testing, listing, reporting, certifying, packaging and product marking. Guidelines for proper installation are treated and illustrated in depth, featuring connections, splices and proper support methods for flexible duct. A single and uniform method of making end connections and splices is graphically presented for both non-metallic and metallic with plain ends." The printed manual is available in English only. Downloadable PDF is available in English and Spanish.
Engineering toolbox properties of water - http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html and email: editor.engineeringtoolbox@gmail.com web search 09/16/2010
Owens Corning Duct Solutions - www.owenscorning.com/ductsolutions/ - provides current HVAC ductwork and duct insulating product descriptions and a dealer locator. Owens Corning Insulating Systems, LLC, One Owens Corning Parkway, Toledo, OH 43659 1-800-GET-PINK™
"Flexible Duct Media Fiberglas™ Insulation, Product Data Sheet", Owens Corning - see owenscorning.com/quietzone/pdfs/QZFlexible_DataSheet.pdf "Owens Corning Flexible Duct Media Insulation is a lightweight, flexible, resilient thermal and acoustical insulation made of
inorganic glass fibers bonded with a thermosetting resin."
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
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