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Blocked air return duct (C) D Friedman M BurtonReturn Air Improvement FAQs
Q&A on how to get more return air in an HVAC system

HVAC return air improvement guide:

How to increase HVAC system return air to increase heating or cool air output by improving the flow of return air to the air handler.

This article describes problems with return air inlet size, location, and ductwork.

Inadequate return air seriously limits both air flow rates and also the degree to which building air is cooled (or heated) by the HVAC system. 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.

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How to Increase Return Air Flow or Supply in Heating or Air Conditioning Duct Systems

Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipmentHere you

On 2018-06-20 by (mod) - why is my air handler pulling air into the basement?

Carl

I emphasize that I can only guess as I know almost nothing about your situation, but perhaps there is inadequate return air ducting or a too-small return air register for the system that's running. I'd start there.

On 2018-06-20 by Carl

I have duel fuel system with 2 zones. The basement system is in off position with the return sealed off, however my unit is pulling air into basement...can't see any obvious breeches in return plentym. Any suggestions as to how I can find what is creating this issue?

On 2018-06-08 by (mod) - Does the flue affect air conditioning flow?

An exhaust flue should not affect air conditioning in a building.

Watch out: however, as we illustrate in the photograph just above, a return air inlet opening too close to an oil or gas fired heating furnace can be dangerous, risking both interference with adequate combustion air for the gas or oil burner (causing improper combustion and production of dangerous carbon monoxide), and the drawing of exhaust from a draft hood or barometric damper into the building's heating duct system.

See UNSAFE DUCT OPENINGS

and also see BACKDRAFTING HEATING EQUIPMENT and also COMBUSTION AIR for TIGHT BUILDINGS for additional details.

On 2018-06-08 by dillard

Does the flue affect air conditioning flow?

On 2018-05-30 by (mod) - ok to use angled duct to accomodate the addition of an air cleaner at the air handler

Richard,

Thank you for an interesting question and one that I haven't considered in detail.

I think what you read elsewhere is probably correct and I would be grateful for any authoritative reference or citation that you can provide.

For the most unambiguous answer if we had the brand and model of your heating system we could or you could simply call the manufacturer directly to ask them their opinion.

I'll do some further research to see what I can find.

On 2018-05-30 by Richard

Replacing existing Lennox HVAC system with Lennox Signature SLP98 and XC25 system plus adding Lennox Pureair Cleaner.

Because new air cleaner is 8" wide vs existing 1" wide filter, the vertical return air duct will need to be modified. Installer wants to modify return duct with an angled section of duct to accomodate size adjustment.

Is there any design considerations that need to be followed? I read at least a 4' straight section needs to enter blower entry point. Is this correct?

On 2018-03-09 by (mod) - What percentage of a furnace filter is plugged before we need to change it out?

Blocked air return duct (C) D Friedman M BurtonAnon

It's a perfectly fair question to ask, but not one for which we can give a simple percent answer, in part because we don't have a percentage-blocked measuring method for your specific filter, air handler, duct system. In my experience if an air filter surface looks coated with debris I change it out.

I tried researching this question without more quantitative results. Sorry, I'm still looking.

On 2018-03-08 by Anonymous

What percentage of a furnace filter is plugged before we need to change it out? I realize visually we can’t tell but just curious to know and that air flow is so important for the unit to run efficiently !

On 2017-11-08 by Cas legend

What is the recommendation for the distance from the back of the fcu to the duct that feeds it,my guess is approximately 5-600mm

On 2017-09-16 by (mod) - According to HERS the airflow is now 1532 and the minimum should be 1500. Is this ok?

Sherry

From your note the air flow you see is above the minimum so that shouldn't be an issue.

If, however your opinion is that you don't have adequate cooling in your home then something needs to be investigated and perhaps repaired.

For example someone might measure air flow at a location that is not affected by a crimped, blocked, loose, or leaky duct.

On 2017-09-16 by Sherry

I have paid $8000 to get a brand new ac unit including the docking.

According to HERS the airflow is now 1532 and the minimum should be 1500. Is this ok or should I ask the company to fix something?

On 2017-08-26 by (mod) - Is the return air plenum too small?

Tim:

Use the page bottom CONTACT link to find my email to send me some sharp photos of the installation.

On 2017-08-26 by Tim

A /C unit probably does have a more powerful fan, but if you could see what they have done to make the existing plenum fit the new Grille you would understand why I believe the plenum size needs to be increased. I wish there was a way to send you pictures and a video (audio) of the noise.

I am no expert but other A/C installation companies I've had come look at the problem believe it is an installation issue including too small a plenum. Let me know via my email if there is a way I can send you pictures and videos of my noisy unit

They will be coming out for the third time Tuesday, their installation supervisor said the installers should have done it properly the first time, I am very annoyed with the whole issue.
I think there's a way to email me your contact info.
Thanks, Tim

On 2017-08-21 by (mod) - - Is the return air plenum too small?

Indeed it sounds as if there's an air noise, perhaps disclosed if the new fan is more-powerful. I'm not sure that making the plenum or return air inlet is the fix - but of course I cannot possibly be smarter than your onsite people who know the equipment and installation.

I'd ask them if there might be an air leak or an obstruction in the air path that's causing the noise. I have seen techs address a problem like this by experimenting with small bits of aluminum tape to either close a suspected opening or change its shape.

On 2017-08-21 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).


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