Under-sized Heating or Cooling Return Air Ducts

Under-sized or incomplete return air ducts for heating or cooling systems
Return air adequacy
Air Conditioning, Heat Pump, (or Heating) Duct Defects
Questions & answers abouttoo-small or blocked or undersized air conditioning, heating, and heat pump ductwork return air registers and duct connections
References

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Undersized return air duct problem diagnosis & cure: this HVAC ductwork article describes the problem undersized return air openings - a problem that can be observed during inspection of the defects in return air ducts for heating or cooling systems (HVAC). We also discuss duct troubles such as such as missing air conditioning cool air supply or return air registers, undersized air conditioning duct openings, improper cooling duct routing, cooling (or heating) air duct corrosion, leaky air duct connections, defective heating or cooling ductwork materials. The photograph above shows a small HVAC return air duct located on an interior building wall. Whether or not this return air duct is adequate depends on how many such returns were installed in the building and whether or not the return air capacity is balanced with the air handler and air supply ducting.

UNDERSIZED OPENINGS - Under-sized Return Air Openings/Ducts

Photograph of an inadequate, under-sized single return air duct for a home.

AIR CONDITIONING & HEAT PUMP SYSTEMS, 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. Asbestos in or on HVAC ducts is a possible hazard for which we provide links to a separate document - see "Asbestos HVAC Ducts" at below left.

Under-sized single central air return: when we see a comparatively small single air return in a building, such as a 12X18" grille in a ceiling or wall, we suspect that insufficient return air ducts are present. This concern is elaborated next.

The photograph shows an under-sized single air return for a duct system serving an entire two story home. We increased the size of the return duct to match the air handler and observed a doubling of the air flow in the system from this single improvement!

Central return air openings are very likely to be undersized when we see that the opening measurements are much smaller than the opening for air provided right at the blower fan.

At another example installation, the blower fan or return air plenum opening at the furnace was 20"x18" while the return air opening at the door to the utility room was 13"x13" (reduced further in effective size by a louvered screen).

When we see amateur attempts to add supply or return air to a system, such as observation of unanticipated openings in building walls, rough-sawn door undercuts, additional openings cut in the return duct or plenum near the air handler, we suspect system design problems. It is possible that previous occupants have found that the warm air supply was not adequate.

Should you confirm problems with inadequate heat delivery in some areas you'll want to consult with an HVAC engineer to make accurate measurements.

Door undercuts, central cool air return: If room doors are shut and not under-cut to permit air flow out of the room, the system's ability to cool the room may be reduced, sometimes significantly.

Try running the cooling system for 10 minutes with all room doors shut. Then open a room door slightly and see what happens!

When a building uses a central return air grille and doors are not undercut heating some rooms may be difficult - system cannot push cold air from the cooled or heated room back to the furnace.

While modest amounts of new duct work (to take return air from the living area) are usually not costly, extensive re-ducting of air flow may involve significant expense

Frequently Asked Questions (FAQs) about about air conditioning, heating, and heat pump ductwork return air registers and duct connections

Question: is our heat pump return register too small? What about noisy heat pump operation?

We recently had installed an American Standard 2 ton 14 seer heat pump in an 1,100 sq ft basement. The basement is now finished but I continually questioned the installer on the size of the return.

The unit is multi-staged and when it ramps up the return is very loud. He installed a 14 by 20 inch return because it fit neatly between the studs. He used 8 ft of round flexible ducting to the air handler.

He also suggested I use the cheap thin filters for lower sound levels and better air flow. There is also a place for a 20 by 20 inch filter at the air handler which he said could be used instead of a filter at the wall return.

Could you please let me know how large the return should be and any pointers on filter use. - D.K.

Reply: Return duct eyeballing and guesstimating

A competent onsite inspection by an HVAC expert usually finds additional clues that help accurately diagnose a problem or could certainly make a more accurate statement about the needed return air inlet and duct layout. And I don't want to get too far into speculation by second guessing your installer since I don't know enough. That said, here are some things to consider:

The size of the filter at the air handler gives a clue about the anticipated size of the air return that equipment manufacturer expected to have.
Your 14 x 20 return air inlet = 280 sq.in. of air return;
Your filter size at the actual air handler is 20 x 20 = 400 sq in of air return
That means that at the air handler the design expected 120 more square inches than what you've got - or a 30% reduction in return air intake area.
A very crude test can tell you if the system seems to be return-air-starved: if we briefly open a cover or case at the air handler by a few inches to admit free-flowing air into the air handler and then let the cover go and it "SLAMS" back with great force into place we figure the system is running with high negative pressure and may be air starved. (A door safety interlock may prevent this test on your system - don't mess with it or you risk having a finger cut off by a moving blower fan - let an expert look into this problem]
Using a thinner filter may slightly improve air flow but in my OPINION that sounds a bit like a band aid and a bit inadequate if the system return air is 30% less than design.
Probably an HVAC engineer expert in this would give a more accurate accounting but it sounds as if the return may be a bit small. - longer distances of flex duct also can reduce air flow - installers try to keep the duct distance from return air inlet to air handler as short and as straight as possible for best airflow If the system is return-air-starved its performance will suffer and operating costs will increase.

Noise: I'm not sure what noise you are describing; if it's airflow it could be related to the return and return duct

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Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

Thanks to AMark 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.
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, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.

Building inspection education & report writing systems from Carson, Dunlop & Associates Ltd

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
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.
Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
"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]
Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
Asbestos HVAC Ducts and Flues field identification photos and guide
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation

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