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Photograph of added return air cut at basement AHU also may draw flue gases from nearby gas fired equipmentHVAC Return Air FAQs

  • RETURN AIR FAQs - CONTENTS: questions & answers on air conditioning or heating return air flow to improve system performance Return air adequacy on heating and air conditioning duct systems.
  • POST a QUESTION or READ FAQs about how to improve heating or cooling air supply by improving the return air into the HVAC system
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HVAC return air questions & answers:

Frequently asked questions (and answers) about HVAC system return air; how to improve heating or cool air output by improving the flow of return air to the air handler.

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



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Return Air Flow in Heating or Air Conditioning Ducts: FAQs

Photograph of a return air register for commercial office space

These questions & answers were posted originally at INCREASING RETURN AIR

On 2017-08-26 by (mod) - how to send us photos or documents for comment

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 - find the source of air noise in HVAC ducts, issues with air plenum size

Our new 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) - track down air noise in HVAC return air duct or air handler or plenum system

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).

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 2017-07-27 by (mod) - not having enough return ai

John,

It sounds like you've diagnosed the first problem, not having enough return air. If you checked the return ducts and everything is connected and tight end not leaking then you may need to add additional return air

Watch out though, when you get the return and Supply Air balanced, if the air conditioning system is in fact oversized you may find that it cools but it does not de humidify.

Question: What is the industry standard CFM for a residential HVAC system?

(June 20, 2016) Anonymous said:
I just had my 20 yr old Hvac system replaced
I live in a 4 story urban town home Hvac unit on bottom floor no Ac gets to the 4 floor
What are or is the industry standard for Cfm in a residential setting supply side & return side

Reply: 400 CFM per ton of cooling capacity

Measured across the cooling coil, typical A/C air flow is about 400 CFM per ton of cooling capacity.

Question: short circuits in air flow - supply air flows right into the return air duct

(May 19, 2014) fgh said:

Should the return vent and the vent that the air flows out of be located side by side. It seems that the return is sucking up the cool air and the air-conditioner doesn't cut off on a hot day.

[Click to enlarge any image]. Shown in photo: return air inlet grille in an office area air conditioning system.

Reply:

Fight
I agree that what you describe is a poor design. It's what is called a

Short circuit

In the airflow and it will limit the effectiveness of the system and increase operating costs.

Question: Debris-clogged evaporator coil reduces air flow across the coil

(June 29, 2014) john said:

can a dirty (or old) residential a/c evaporator coil effect and/or reduce the forced air flow(coming from the vents)thus effecting the units overall cooling efficiency, even if the evaporator coil is not freezing over and the vent air temperature is adequate?

Reply:

John

Certainly yes. Debris-clogged evaporator coil reduces air flow across the coil and also reduces the transfer of heat from the air to the coil even if the coil is not iced or frosted over

Question: ratio between air return grille size (opening) and filter size?

(Aug 15, 2014) Anonymous said:

Is there a ratio between air return grille size (opening) and filter size? If I have "clear air" (all doors are open to all rooms being supplied by the a/c handler) provided the return and filter are the same number of square inches, is that sufficient to ensure my system is not air starved? Thank you, Ed

Reply:

Anon, usually the air filter length and with dimensions are matched to the size of the return air grille itself.

It is the total return air capacity, determined by the number, location, and size of return air inlets that must be matched to the air handler's blower capacity and supply duct design.

There are some crude rules of thumb and an elementary test that can give a good guess at return air adequacy:

If the return air inlet square inches is smaller than the cross section of the blower or air handler's return air inlet in cross section then there is probably not enough return air.

Also we propose a crude return air test: with the blower fan running (and if there is no door safety interlock switch in play) if we open the door and release it and it SLAMS shut with vigor I figure that the blower compartment was under high vacuum and there is probably not enough return air. (Admittedly this is crude).

Keep in mind that even if good quantity of return air inlet by location, size, and ducting are all provided, if we are considering a system with central return air and room doors are shut (or not under-cut), the ability of the air system to force conditioned air INTO the rooms will be limited.

Question: water in return air compartment

(Sept 6, 2014) lee said:

had new a coil installed,getting water in return air compartment

Reply:

Lee this is baffling but I suspect that there is so much moisture in your return air that there is condensation in the return air plenum OR *more likely* depending on the type and position of your system's air handler, there may simply be leakage from the condensate handling system (or on occasion from a poorly-located or leaky humidifier on combined heating/cooling systems).

Question: Thinking of changing filter size to improve air flow

(July 25, 2015) Allen Marshall, Mobile, Al said:
I have a 4 ton, 2 speed compressor with Variable speed inside fan. It has 2 16x25x1 pleated paper, merv 4. even with filter changes I think the air flow is restricted. I am Looking at modifiying the wall filter housing to accomodate a 16x25x4.

Reply:

A larger air filter at the air return inlet makes sense up to a size equal to the cross-sectional area of the duct inlet. Larger than that? I'd look first at the air flow rates and pressure drop across the filter you're using.

Question: upstairs bedrooms get too hot

(Dec 9, 2015) Ronnie said:
My warm air heating unit works quite well, however, in order to heat the downstairs open plan living areas, it means that the upstairs bedrooms become extremely hot. All the air vent throughout the house have a slider lever but none of the levers are now functional. If I replace the air vents with slider levers that do work, will this enable me to channel the heating for example, to the downstairs parts of the house by closing the air vents in the upstairs rooms? Any advice

Reply:

Ronnie:

Yes you can try closing down the upstairs supply registers. But a better approach would be to install a manual (or automatic) zone damper at the beginning of the ductwork that sends warm air upstairs.

Also take a look at the location of return air venting. If all of the return air vents are upstairs (say in a ceiling at the top of the stairs) that, too, tends to move warm air upstairs.

Question: common causes of reduction in air flow in a heating or cooling system

(Dec 19, 2015) Debbie said:
I have a heater that is 18 years old and has not been heating well. Repairman said that my return air flow is not large enough which makes no sense for this to happen all of the sudden. Plus, I was told that if I replace the heater I would also have to replace my AC unit (which is not that old) due to change in refrigerants used now? Wondering if this is true as it would be a very costly repair.

Reply:

I agree Debbie - if the problem were one of design (ducts too small) the problem would have always been there.

There COULD be inadequate return air - that's very common and easy to demonstrate - but if your system air flow has CHANGED then I suspect

- a dirty air filter
- a collapsed or blocked air duct
- dirt on the squirrel cage blower fan assembly
- a slow or damaged blower assembly such as a slipping belt, loose pulley, or even low voltage or a bad motor.

Question: HVAC air duct fills under pressure and then bounces around

(Dec 30, 2015) Rolando Bartolome said:
Our church has an adjoining A frame building. Below the ridge there is this fabric duct which fills with air and bounces off at the sealed end. This duct runs almost as long as the building. The effect of the duct filling then bouncing the air is like blowing into a paper bag with the sound much louder. I've never seen such duct before (obviously there are no branches from it). I believe the design is not cost effective. I'd appreciate any help that would shed light on this design. Thank you.

Reply:

This sounds to me as if there is an air flow restriction; and duct bouncing around sounds like excessive length of free, unsupported flex duct; bouncing ductwork also invites duct damage, leaks, higher HVAC costs.

Use the page bottom CONTACT link to send some pictures and we can comment further, Rokand. I'll be glad to do what I can.

Question: Adding a return duct in the basement

I want to add return air ducts in the basement to improve my home's air conditioning and heating. It's cooler down there.

Reply:

A significant number of residential AC systems I've examined had insufficient return air in the first place, so adding more return air usually isn't a problem, it's an improvement; you can always adjust the balance between the return duct openings later.

Watch out: avoid taking return air where it may cause unsafe operation of heating equipment such as causing backdrafting and fatal carbon monoxide poisoning;

See UNSAFE DUCT OPENINGS

Comment: always ok to add more return air

A significant number of residential AC systems I've examined had insufficient return air in the first place, so adding more return air usually isn't a problem, it's an improvement; you can always adjust the balance between the return duct openings later.

But watch out to avoid taking return air where it may cause unsafe operation of heating equipment such as causing backdrafting and fatal carbon monoxide poisoning;

Reply:

Thanks

Question:

(Jan 27, 2016) eileen said:
had a new ruud gas furnace installed in October of this year in my condo. recently kept short cycling-heating contractor installed new thermostat (had to wait two days and listen to short cycling after I was told the thermostat had to be acclimated to new location and temp?). Continued to short cycle after new stat. the result is that contractor cut a hole in the plenum and installed a register to release the hot air. I do not believe this is very safe as I have to leave the door to the closet open to release the hot air and the rooms in the back of the condo are not as warm as should be.

I am a senior citizen and have had nothing but problems with this and prior furnace for the 10 years I have lived here. cannot afford to have this taken out and replaced. I have a picture that I have taken of this "design." Please give me your thoughts on how this was cherry picked and what can be done to fix.

Reply:

x

Question: Definition of AC technician & AC mechanic

(Feb 10, 2016) tarkeshwarparjapati said:
What is ac teknisiyan &ac makeenik

Reply:

Tarkesh

In typical language, an A/C or HVAC technician is typically someone who installs, services, or repairs the air conditioning compressor, air handler, or heat pump.

An A/C mechanic is a term often used to refer to someone who installs HVAC ductwork or possibly who installs equipment. It's likely that these two work areas overlap.

Question: return air inlet opening size adequacy

(Apr 3, 2016) Bill said:
Will a 20 by 30 return for a 2and half ton unit work for a 1 floor cottage 1350 sq ft?

Reply:

Bill it's the right question to ask but not one I can guess-at by a one-line e-text. Factors such as duct run length, duct sizing, blower size, fan type, fan speed, and dimensions of the air handler as well as more technical considerations that an HVAC engineer would consider all need to be evaluated for a technically correct design. I suspect that most installers use a few simple rules of thumb, starting with matching the air return dimensions to the air inlet of the air handler.

See UNDERSIZED RETURN DUCTS

Question: Effect of a soccer ball jamed in the ductwork

(May 24, 2016) Kathy D said:
What kind of problems will it cause to the heating and air conditioning units, to have a soccer ball logged in our main return vent? And is there any danger involved?

Reply: here are possible effects & costs of blocked or obstructed heating and air conditioning ductwork

  1. Blocked air flow means:
  2. Increased heating & cooling costs
  3. Possible duct collapse and damage
  4. Possible fire if the obstruction is drawn into the blower fan
  5. Inadequate building heating and cooling, including, in freezing weather, risk of frozen water pipes, burst pipes, building flooding, costly mold or structural damage

Question: chilled water air handler with set point too low

(June 23, 2016) anonymouse said:
in a chilled water air handler system with DX back up. If the set point is set too low and the compressors kick on, will the compressors kick off on low pressure due to the evap coil being cold in the first place and not removing heat ?

Reply:

Sorry anon that's not a system for which I know operating details; if you have the brand and model of the unit perhaps one of us could find the operating manual or the manufacturer's tech support contact info.

Question: air conditioner doesn't remove humidity & takes too longto cool

(July 26, 2016) Jmendoza said:
NEED HELP. SUGGESTIONS. I installed a new carrier AC 2.5T for 1140 sq foot home. My problem is that it's not removing humidity and it takes forever too cool. House is a split level. Pressures and sub cooling are on point. Only got two returns one upstairs and one downstairs next to the furnace. I found I had too much air going across the coil.

I slowed down the blower. I did many tweaks to the system where I finally got the system to remove humidity. I opened up my blower motor compartment. Closed the door where the furnace is at. It's in the laundry room by the way. It's a small room. I blocked 3/4 of the return in the lower level of the house.

This is the only way the system will remove humidity and cool quickly. My question is would adding another return in the upstairs level help remove humidity better. I feel like it's not sucking any return air from the upstairs level and I feel like 90% of the return air is coming from the downstairs level which is always cooler than upstairs and less humid. Any suggestions on how to fix this?

Reply:

JM

Typically an over-sized unit will cool down the space too quickly and thus fail to dehumidify. When a unit won't cool AND is not dehumidifying I suspect a different problem, including a duct problem just as you're thinking.

As you're on-site you can best think about the air distribution: if return air is not moving hot air from the occupied space back to the A/C air handler then yes the system will not be nearly as effective.

Even a crude subjective measure (holding a tissue near an air return or air supply) can give a feel for how air is moving.

But before adding returns or supplies, review the whole duct system looking for snafus such as leaks, disconnected ducts, crimped or squashed ducts, or a dirty filter.

Question: clearance between top of air handler to ceiling for ductwork?

(Sept 17, 2016) SL said:
How many inches is recommended between the top of air handler to ceiling for the duct? I only have 56 inches clearance and having a tough time finding a 4ton 16 seer split system ac.

Reply:

SL if your installer can't fit a supply plenum and duct connection in nearly six feet of free space over the air handler, then something is rotten in the state of affairs - call someone who knows something.

Question: do I need a low or high return?

x (Nov 12, 2016) Theresa Phillips said:
I'm having different views from different hvac guys I have compares about 10 and still need to know if I need a low or high return ? I have 1206 sq 12 registers that will be in ceiling I have hardwood floors with 2 1/2 tin unit please help me

Reply:

For air conditioning, placing the return air up high means you're pulling warm air from where it tends to collect (up near the ceiling) - a more efficient cooling air return inlet location than on the floor.

Question: ok for return air to go in at the ceiling?

(Nov 12, 2016) kat said:
in a manufactured home is it adequate for the return air to go in the ceiling. We had a metal roof put on and now we are having an incredible amount of dust. I have to dust every other day. Do we need make one of those vents at the eve of our home?

Reply:

Kat it sounds to me as if we're mixing up some questions. It's common for homes to put a central return air inlet in a ceiling. (Though that works better for cooling than for heating).

But if you're seeing excessive indoor dust levels we need to start by finding the source of the dust. Perhaps having a forensic lab analyze a settled dust sample would help point to a source. Search InspectApedia.com for TAPE SAMPLE FOR DUST to see details.

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Continue reading at RETURN AIR REGISTERS & DUCTS - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AIR LEAKS in RETURN DUCTS

Or see AIR FLOW IMPROVEMENT, HVAC

Or see AIR FLOW MEASUREMENT CFM - how to measure HVAC system air flow

Or see INCREASING RETURN AIR

Or see LEAKY DUCT CONNECTIONS

Or see REGISTER & DUCT LOCATION

Or see UNDERSIZED RETURN DUCTS

Or see SLAB DUCTWORK

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

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