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

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-06-17 by danjoefriedman (mod) - ok to reduce then expand HVAC duct?

I can't say, Mike. Try reviewing the symptoms of inadequate return air given in this article. You will see that any restriction in duct size will cut airflow; shorter restrictions have less effect than longer ones.
Let me know the result.

On 2020-06-16 by Mikedard

Replaced 24 year old HVAC and enlarged HVAC return duct from 14" to 16", and then they reduced one end back to 14", is that ok?

On 2017-08-22 by (mod) - high pitch whistling noise from the Return Air Plenum

TS am I right that you asked and we replied to this in email? The common causes are an air leak or an air flow constriction in the duct system.

I suggested trying to track down the actual noise source, perhaps using foil tape on suspect openings or to change edges and angles in the air flow. Just making the return inlet larger would make perfect sense if there was an air volume or flow mismatch, but if the noise is from air moving over an obstruction the size change may not help.

On 2017-08-18 by tsthumpr

We just had our A/C & Furnace replaced with a new unit that was very expensive and since the installation we are experiencing a high pitch whistling noise from the Return Air Plenum, the installers used the same plenum 12"x20" and bent a bunch of sheet metal to adapt to a larger return air grille 17"h x 30" w.

The company has sent their warranty tech out twice to no avail, they keep trying to make the return air plenum bigger by adding bent sheet metal. Everything I have read says the return air plenum needs to be much larger. I am looking for answers and getting very frustrated. Answers anyone?
3.5 ton Lennox A/C and Lennox variable speed Furnace (fan).

On 2017-08-17 by Tim

We just had our A/C & Furnace replaced with a new unit that was very expensive and since the installation we are experiencing a high pitch whistling noise from the Return Air Plenum, the installers used the same plenum 12"x20" and bent a bunch of sheet metal to adapt to a larger return air grille 17"h x 30" w. The company has sent their warranty tech out twice to no avail, they keep trying to make the return air plenum bigger by adding bent sheet metal. Everything I have read says the return air plenum needs to be much larger. I am looking for answers and getting very frustrated. Answers anyone?
3.5 ton Lennox A/C and Lennox variable speed Furnace (fan).

On 2017-01-17 by Donald

It feels like we are in a wind tunnel when the air is going up the cold air return vent. What causes this. It is in the bedroom

On 2016-10-25 - by (mod) -

Curt

Please see an expanded reply to your question now found at

BANGING NOISE at AIR HANDLERS or in DUCT SYSTEMS

Also see

INCREASING RETURN AIR

and see SOUND CONTROL for AIR DUCTS, HVAC

On 2016-10-25 - by (mod) -

Curt,

I've seen and heard metal panel bangs, clunks, and thunks that can be a startling heating system noise easily traced to just what you observed. When the fan starts up the system is at least initially a bit air-starved creating a vacuum on the return side of the system. A powerful fan can also cause the same noise by bulging out rather than in sheet metal components on the supply side of the heating or cooling system.

If the system is truly air-starved then it makes a lot of sense to add more return air inlets since not only will you be curing the start-up or shut-down BANG noise in the duct and air handler system but you'll also improve the effectiveness and efficiency of the system.

If the problem is a powerful fan at start-up but no air-starved concerns, then I would add reinforcement U-channel to stop flexing sheet metal noises.

If the system is not air-starved, a fan control that ramps up the fan speed improves the situation but that's not a normal control.

Running the fan always on the lower speed defeats the purpose of having a multiple speed or variable speed blower fan in the first place: different speeds for varying needs.

When you have to leave the a blower compartment door open then it's a pretty good indicator that the system needs more return air. Another similar test I make is to wait until the blower has been on for 5 minutes or more, then open the air handler blower compartment door about 6" and let go. If the door SLAMS shut with a powerful BANG then the system is probably air starved.

On 2016-10-25 by curt

I've installed a new 2 stage variable speed furnace in my basement, the air return is above it in one central
register on the main floor about 30 x 12. I have found that when the furnace starts it sucks in the metal air return panels in the basement that lead into the back of the furnace. I added another heating vent to the system into
the basement, but have not increaased the air return yet. I have also turned the down speed of the fan to the lowest it will go, so now it ramps up slower. those 2 items have helped a lot. however I need as well to leave the front of the furnace door open for additional air, otherwise it still sucks in the metal return. I was told to put bracking inside the return, which is an option, another option I thought would be to add another return to the other side of the furnce with another opening and filter. A furnace man came out and said that would work, but wanted some others ideas and opinions. Thank you in advance, email to cws56501@yahoo.com

On 2017-08-22 by (mod) - when to make the return air inlet larger

TS am I right that you asked and we replied to this in email?

I suggested trying to track down the actual noise source, perhaps using foil tape on suspect openings or to change edges and angles in the air flow.

Just making the return inlet larger would make perfect sense if there was an air volume or flow mismatch, but if the noise is from air moving over an obstruction the size change may not help.

On 2017-08-18 by tsthumpr

We just had our A/C & Furnace replaced with a new unit that was very expensive and since the installation we are experiencing a high pitch whistling noise from the Return Air Plenum, the installers used the same plenum 12"x20" and bent a bunch of sheet metal to adapt to a larger return air grille 17"h x 30" w.

The company has sent their warranty tech out twice to no avail, they keep trying to make the return air plenum bigger by adding bent sheet metal. Everything I have read says the return air plenum needs to be much larger. I am looking for answers and getting very frustrated. Answers anyone?

3.5 ton Lennox A/C and Lennox variable speed Furnace (fan).

On 2017-01-17 by Donald

It feels like we are in a wind tunnel when the air is going up the cold air return vent. What causes this. It is in the bedroom

On 2016-10-25 by (mod) - metal panel bangs, clunks, and thunks can be a startling heating system noise

Curt

Please see an expanded reply to your question now found

at BANGING NOISE at AIR HANDLERS or in DUCT SYSTEMS

Also see INCREASING RETURN AIR

and see SOUND CONTROL for AIR DUCTS, HVAC

On 2016-10-25 by (mod) -

Curt,

I've seen and heard metal panel bangs, clunks, and thunks that can be a startling heating system noise easily traced to just what you observed. When the fan starts up the system is at least initially a bit air-starved creating a vacuum on the return side of the system. A powerful fan can also cause the same noise by bulging out rather than in sheet metal components on the supply side of the heating or cooling system.

If the system is truly air-starved then it makes a lot of sense to add more return air inlets since not only will you be curing the start-up or shut-down BANG noise in the duct and air handler system but you'll also improve the effectiveness and efficiency of the system.

If the problem is a powerful fan at start-up but no air-starved concerns, then I would add reinforcement U-channel to stop flexing sheet metal noises.

If the system is not air-starved, a fan control that ramps up the fan speed improves the situation but that's not a normal control.

Running the fan always on the lower speed defeats the purpose of having a multiple speed or variable speed blower fan in the first place: different speeds for varying needs.

When you have to leave the a blower compartment door open then it's a pretty good indicator that the system needs more return air. Another similar test I make is to wait until the blower has been on for 5 minutes or more, then open the air handler blower compartment door about 6" and let go. If the door SLAMS shut with a powerful BANG then the system is probably air starved.

On 2016-10-25 by curt

I've installed a new 2 stage variable speed furnace in my basement, the air return is above it in one central
register on the main floor about 30 x 12. I have found that when the furnace starts it sucks in the metal air return panels in the basement that lead into the back of the furnace

. I added another heating vent to the system into the basement, but have not increaased the air return yet.

I have also turned the down speed of the fan to the lowest it will go, so now it ramps up slower. those 2 items have helped a lot. however I need as well to leave the front of the furnace door open for additional air, otherwise it still sucks in the metal return. I was told to put bracking inside the return, which is an option, another option I thought would be to add another return to the other side of the furnce with another opening and filter.

A furnace man came out and said that would work, but wanted some others ideas and opinions. Thank you in advance, email to cws56501@yahoo.com

On 2014-10-20 by joblo

If a Florida room has all block and casement windows as walls that do not properly seal should the room be air conditioned? It also has French doors that are not undercut and no return ductwork as this room used to be a porch/lanai.

On 2014-10-16 by Cynthia Romeo

I have a retangular air return grate and the duct work that connects to the heater is round, is this oK

On 2014-09-12 by (mod) - Dual Pak HVAC unit 7 years old (Rheem) stopped supplying cold air.

Gerry

Good going sticking to your guns in the first place. If a cooling system isn't dropping the temp below 78 something's wrong somewhere.

Low refrigerant (which means there's a leak) can cause coil icing that would then block air flow and prevent cooling.

It is certainly true that if a cooling coil is leaking and is corroded and fragile the HVAC tech is likely to advise that it's not repairable. Tossing in the TEV may be a bit like shotgunning the repair diagnosis though it is often a less expensive approach to replace several doubtful parts than to run up a bill with longer diagnosis.

Smashed AC duct is important to fix and hopefully is a minor addition.

Keep us posted.

On 2014-09-11 by Gerry

My Dual Pak HVAC unit 7 years old (Rheem) stopped supplying cold air. I have coverage on my HVAC system with AHS. The first company came out and insisted that since the outside air temp was 100 degrees and i was getting 78 degree supply air it was working fine. I disagreed with him and told him.

He then went up and said there was some insulation blocking the air duct and that was the problem. We still were getting a lower volume of air (tEmp 78 degrees. He went up again and after an hour said my TXV was not working ( he was basing this on the fact that the pressures didnt equalize at the compresser.

This tech finally just went out to his truck and just left. AHS sent out another company and this teck said the shrader valve was leaking and the unit needed 3 lbs of freon 22 and he also said the compressor needed a hard start put on it along with a main contactor replaced. After he did all this the supply air temperature dropprd to 57 degrees.He said it should work fine now. I still didnt feel the air flow the unit has had since i had it installed 7 years ago. We used the unit for 12 hours and it ran constantly and by the late afternoon the temperature got up to 89 degrees in the rooms.

I called the company back and the same tech came and checked the unit and said there was a leak somewhere and he couldnt find it so he finally left without saying anything to my wife. I called him and finally got thru and he said he was giving up and he called AHS and they would be sending out another company.

This company came out and the technican said the unit was low on freon and there was a leak somewhere. He finally located the leak in the EVAP coil and said it couldnt be repaired so they would have to order one. He also said he he thought the TXV wasnt working correctly so it would also need replaced. The last thing he said was that my return air duct appeared to be smashed a little lile someone may have stepped on it. He said they would have to replac the return air duct work also. He left and said someone would call

On 2014-08-03 by richard formula for area of duct return grilles necessary for heating

Is there a formula for area of duct return grilles necessary for heating only in a two story house. Return grills needed on both floors?

Thanks

On 2014-07-09 by (mod) -

In my opinion, probably yes - as it's a restriction.

I agree that a too-slow air flow can contribute to coil icing.

Remember to check with great care the blower fan itself - a small amount of dirt on the squirrel cage blower fan blades may not look like much but I've found that dirt there can cut air flow by 40% or more.

On 2014-07-09 by dcr

If a return duct has an elbow and connects to air handler at a smaller size than the wall intake opening, will the air flow be compromised?
We are trying to solve a problem with freezing coils and have ruled out low refrigerant, dirty coils, dirty filter, blocked ducts and non functioning blower, valve (tXV) metering refrigerant was replaced.
Thanks.

On 2014-06-18 by (mod) -

400 sq.in is more than 160. If the filter describes the air handler return plenum size there's the answer. Probably not.

On 2014-06-18 by Reg

Is 160sq.in.of return vent adequate in a heat pump unit with a 20x20 air return filter.

On 2014-05-10 by (mod) -

Greg the efficiency of a furnace is indeed impacted by a constriction in the air delivery and return system, since to satisfy the thermostat while pushing air through a restriction means the system has to run longer. I'm not sure it's a trivial calculation however as in my experience there are usually multiple, overlapping efficiency reductions - so removing one does not necessarily assure that the boost in performance is exactly what you might hope or calculate.

Of all restrictions, however, a return air constraint is particularly ugly as it constrains *all* of the air movement through the air handler and on to the supply duct system. So a 33% reduction in return air is significant. But with just that single number I can't compute the actual real-world effect. An interesting technical article on the complexity of calculating the effects of changes in the air handling system is available from the US DOE: See

• Burdick, Arlan, "Advanced Strategy Guideline: Air Distribution Basics and Duct Design", U.S. DOE (2011) at Available electronically at http://www.osti.gov/bridge Available for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831 - 0062 phone: 865.576.8401 fax: 865.576.5728 email: mailto:reports@adonis.osti.gov Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: orders@ntis.fedworld.gov online ordering: http://www.ntis.gov/ordering.htm
• Take a look at RETURN AIR, INCREASE

On 2014-05-08 by Greg

The air return duct is 12"x 19", 228 sqin, about 2/3 the area of the factory recommended air filter 14"x 25", 350 sqin. How much is the under sized return air duct reducing the efficiency of my new 95% furnace?

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

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"

Reply:

Possibly. First I'd check the system for inadequate return air since it's possible that the return inlet was simply larger than needed.

See UNDERSIZED RETURN DUCTS

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 ?

Reply:

Victoria

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.

Question:

(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?

Reply:

Please see your question and our reply at inspectapedia.com/aircond/Return-Ducts-Registers.php

Question:

(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.

Reply:

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

Reply:

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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Continue reading at AIR FLOW IMPROVEMENT, HVAC or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

HVAC Return Air Improvement Articles

• AIR FLOW IMPROVEMENT, HVAC
  • AIR FLOW TOO WEAK
  • AIR LEAKS in RETURN DUCTS
  • LEAKY DUCT CONNECTIONS
  • REGISTER & DUCT LOCATION
  • RETURN AIR, INCREASE
  • RETURN AIR REGISTERS & DUCTS - home
  • UNDERSIZED RETURN DUCTS

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UNDERSIZED RETURN DUCTS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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