UNDERSIZED RETURN DUCTS - CONTENTS: how to diagnose & fix under-sized or incomplete return air ducts for heating or cooling systems. Return air adequacy - or inadequacy traced to bad return duct sizing
POST a QUESTION or READ FAQs about too-small or blocked or undersized air conditioning, heating, and heat pump ductwork return air registers and duct connections
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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
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
Continue reading at AIR FLOW IMPROVEMENT, HVAC or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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.
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
Question: return opening bigger than return ducts
(May 30, 2015) Duct size said:
I have a new system where the duct entering the return air plenum opening at the furnace is 16" x 25" but it is fed by a duct between the joists that is only 16" x 12". Would it be a good idea to just increase the duct size so it extends below the joists, to say, 16" x 25" for example"
Possibly. First I'd check the system for inadequate return air since it's possible that the return inlet was simply larger than needed.
Question: improve air conditioning by increased fan speed, cleaning blower fan, clean air filter, remove duct obstructions before changing duct size
(June 15, 2015) Victoria G said:
I recently purchased a house (60 years old) with central air. 3 ton unit. The home heating was fine this winter but now that we are using the central AC the house never goes down more than 2 degrees. It just doesn't cool the house like it should. It appears from researching this it is 1.) insufficient air returns AND insufficient duct work to deliver the cool air. I am reading central air requires larger duct work to deliver the cool air than heat does ? Is that true ?
It may be possible to improve the performance of your central air conditioning system by increasing the fan speed, changing to a variable speed fan, cleaning the blower fan assembly to improve its air movement ability, making sure that the air filter is clean, that no ducts are crimped or disconnected and similar measures.
Before even considering the costly change of duct size in a building I'd want to investigate not only those basic air flow improvements but I'd want the system examined by an expert who could tell us if the air conditioner is working properly: what is the temperature drop across the cooling coil? is there an improper refrigerant charge? is the thermostat properly set and located? etc.
(July 9, 2015) GRF said:
I have a central air system in my house blower unit in the attic evaporator coil and compressor outside . it is a three ton cooling capacity two stage system made by Bryant. my issue is the main part of the house cools very good but our family room on the far end of the house is consistently 5-8 degrees warmer . the room has cathedral ceilings. the incoming cool air vents are up high and the single return vent is at same level in between the to incoming vents. my question is if I increased the size of the return air to the system will i get better cooling in this room?
Please see your question and our reply at inspectapedia.com/aircond/Return-Ducts-Registers.php
(Aug 5, 2015) Pdaz said:
3ton 1200 cfm blower,2 12x28 return registers,what size return ducts should I use? Returns are approx. 15/18' long. Supply is 16" flexible 4' long to sheet metal plenum 40' foot long,start collars off of plenum are 3 6",1 9",4 7",2 8" with respective flex from each to registers.
Is there a question here?
Question: can under-sized ducts cause HVAC air conditioner or heater to short cycle?
(Dec 22, 2015) Emre said:
Is it possible for an undersized duct system to cause a heater or air conditioner to short cycle
Emre: in my view, yes, possibly so. If back-pressure causes overheating in the supply plenum a fan limit might shut down the system.
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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 - firstname.lastname@example.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
"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]
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates 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.
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.
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