Return Air Duct Size

[Lisavano]

Hello, I live in a 1936 house meaning that A/C was an afterthought. Anyway, the air handler is in a closet that connects through to linen closet where the return duct or grill is located (the grill replaced the bottom drawer). This was the design when I bought the house. I followed it when I bought a Trane in 2011. The return duct has never been completely sealed and so it blew dust all through the drawers of the linen closet. For the first time ever I got my ducts cleaned and I asked the guy about the dusty drawer problem. He said he could seal it up. It sounded like a great idea. Since then I feel I have not had as good of A/C. I have always wondered if the return duct grill is too small. How do you determine the correct size??? I called trane and they said they couldn't figure it out for me. I also had the original contractors come out and I didn't understand what he was saying and he was very impatient with me. I paid 100 for not understanding; he basically said how could it be installed wrong. It was very different from what I found on the internet when I searched how to size your return a/c duct/grill.

I did make certain that air was coming out of the other ducts since they were cleaned and maybe something got knocked loose but that doesn't seem to be the case. Although some of the foam stuff peeled off them. Is that important??

Sorry so many questions. I just want to make sure before I have someone else come out and look that I have all the information /?'s that I might need.

Thanks

 

[LXCam]

You need to know the tonnage of the unit, cfm output of the fan and what size duct is the return including length of run and bends if constructed with hard bends which it sound like you have. You can even look the required size of the return grill online with that info, search transfer grill opening calculator. And it sounds as though the dust collecting in the drawers is from the suction leak of the return drawing in the dust around those areas. It won't blow out dust as its under negative pressure. I know that's not much help, but it's a start until you can find the other information.

 

[gregtwojeeps]

Since it worked satisfactorily for you pre-contractor work...start with the simple stuff and work upwards. Does it have a filter ? Where is it located? Is it in decent condition ? Is the return air opening/tunnel.... back to the furnace at least 20" x 25 " thereabouts ?

 

[zmaxmotorsports]

Pictures?

 

[75gmck25]

Since you live in an area where A/C is used frequently, you might want to find a contractor who can make a significant upgrade to the return air part of your system.

Using one central return air duct of the right size will work (I think that is what you have), but it means that all interior doors have to be open to let the return air get back to the single return grill. If you install a plenum in the attic above the indoor A/C unit you can have one large return in the middle of the house, and multiple smaller return ducts connected to any number of other rooms out of that plenum. That means that when a bedroom door is closed you can have both a supply and return duct in the same room, which helps airflow quite a bit.

Bruce

 

[pseudorealityx]

1) Provide the model number of the unit in the closet.
2) Measure the dimensions (width and height) of the return duct between the unit and return grille.
3) Measure the return grille itself.
4) Give us the size of the filter. It should be printed on the side of the filter.

 

[Lisavano]

the model no is Trane 4TEE3C04A1000AA
the return grill is 24 in horizontal and !2 in high; there is no filter behind this and it goes straight back to air handler (no twist or turns)
the space from the grill to the air handler appears to be 54 in long and about 13-14 high, not much higher than the return grill.
the only filter is in the air handler which is 20 x 20 x 1

the guy who told me the return grill size was correct said the outdoor unit was 3 tons and the air handlers was 4.5 tons. He said based on his calculations the return grill needed to only be 288 sq inches. Whereas a utube video came up with 900 inches (maybe that was counting other vents in other rooms)

I do not have attic space, believe it or not. Also not sure what CFM is??? or how to figure out some of the other ? that I didn't answer. Trane did send me an installation manual yesterday I will see if I can find that in there. I will also try to get a lantern and take some pictures. Thanks so much so far.

 

[gregtwojeeps]

Without getting to mechanical now, the unit's return air filter opening is 20"x20" which is 400 sq. in. of RA opening ....which is the required return duct opening for the air handling unit. The tech states there is a 24" x 12" opening in the "tunnel" and that is adequate per his specs, yet it is only 288 sq. in. .. And we are assuming this is the ONLY return air duct ( 24" x 12 " tunnel) back to the air handling unit (blower) ??

This is the first "sign" of lack of return air, as the RA tunnel has a 288 sq.in. opening and the air handler wants a 400 sq. in. of RA. And as 75GMK says, you have to be sure that the closing of doors anywhere ....will not restrict the air that is blowing out of the various room's supply air registers, will keep RA from getting back to the return air duct(tunnel).

 

[Lisavano]

I have been shutting off my rooms doors, as I have been cleaning and painting. So you are saying that shutting off room doors could be the culpruit because that air can't get back to the return grill. I think that is what you are saying.

 

[Lisavano]

Should I change the linen closet to make the return grill space equal to the 400 inches?? Also, I opened all the doors and it does same to make a difference. I have had 2 techs at the house and not 1 mentioned that I needed to keep the doors open.

 

[sands35]

Air needs a way to get back to the main unit, or it won't move at all (called dead-heading when an air circulation system can't circulate). Either leave doors open or cut off about ~1" off the bottom of the doors.

Any decent AC installer should be able to calculate the size required for the return air duct. The air velocity needs to be high enough to keep dust suspended, but low enough that you don't get a lot of wind noise.

There are techs, then there are techs. People that know how to repair AC units don't necessarily know how to design and install them in a house. (which is also different than retrofitting an existing house with an AC unit).

Also, most air handling units allow the blower to be set at multiple RPMs and hence multiple CFM flow rate. The only way to tell is to find the manual and look at how it is setup.

I would suspect that the size of the return duct into the air handler is sized more to match the size of the sheet metal on the unit rather than what is needed for a properly sized air return.

 

[DC73]

Should I change the linen closet to make the return grill space equal to the 400 inches?? Also, I opened all the doors and it does same to make a difference. I have had 2 techs at the house and not 1 mentioned that I needed to keep the doors open.

It does sound like your return air duct is now too small.

As far as a proper return air path, if you like to leave the doors closed, you could consider something like this:

http://www.tamtech.com/3-Pack-Perfect-Balance-In-Door-Return-Air-Pathway_p_106.html

I used these in my house and they work well. Installation does require cutting a space out of the bottom of the door. There are other options including through-wall return air vents.

Good luck.

DC

 

[TNToy]

CFM stands for Cubic Feet Per Minute and is how airflow is measured (anywhere that doesn't utilize the metric system.)

When you have a room sealed off, it's like blowing into a balloon with your lungs - even the initial bit of pressure quickly makes it much, much harder to exhale. Air handlers are designed to move large amounts of air at very low pressures - not to force air into sealed containers like a room with a closed door.

Modern houses have return air vents in all of the rooms to make it more like what happens when you breathe into one end of a paper towel roll - air easily exits the other end as it enters, so no pressure builds up.

Next time your A/C is running, hold your hand against the bottom of the door in one of your rooms where the gap is rather small. You'll probably be surprised how much air is trying to flow through that tiny gap.

Is there any way to add a second vent out one of the other sides of the return air duct? Come out the side of it into a hallway next to the closet or a bedroom, perhaps?

Increasing the size of the return air vent and installing the grilles shown above in your doors would go a LONG way toward letting the house stay cool.

Since your are painting in one or more rooms and trying to prevent the spread of fumes, in the meantime I suggest closing the vent in that room and leaving all of the other doors open so that the rest of the house still has heat and A/C.

 

[Lisavano]

thanks so much everyone for your information. I am definitely going to try to resize the return air vent. In the meantime, opening the doors has helped tremendously. I can't thank everyone enough; as I do not think another technician would have not even mentioned the closed door problem.

The easiest solution for me will be to take out another drawer of the linen closet and use that space and get a bigger grill. My final question is do I have to worry that the return grill might be too big??? That would make the return air space (unless I had them seal some off) now roughly 24 x 24.

 

[gregtwojeeps]

Don't worry about getting too much RA space as the actual amount of air returning to the air handler is dictated by the unit's filter frame size. In other words you could unhook the RA duct from the air handler and the blower will only pull as much air in as the furnace's blower squirrel cage fan needs.... plus only as much air as the metal frame opening of the filter rack allows it to do. ...

When a RA duct is sized really too small, one can literally hear "whistles" from the air trying to be sucked through cracks or crevices in the ducts/ RA grilles by the fan blower trying to **** in its needed air...

We all will repeat now. Pay attention to the way the RA gets back from the rooms where it is blown out in to IE: though doorways, through the wall registers etc. . and be sure the RA has a way to get back to the RA duct/tunnel regardless of doors being opened or closed.

If the RA cannot get back from all the rooms it is being discharged out in to, back in to the RA duct... making a RA duct or tunnel in your linen closet as big as an Angus bull....will not help. Good luck. JMO

 

[Lisavano]

thanks, got it!!!

 

[Falcon67]

Sounds like an unmatched system too - 3 ton external doesn't use a "4.5T" air handler. We had that issue here. The contractor putting in our new unit this year was blunt when I asked why the old inside unit was a 3.5T and the outside was a 3T - "Your builder got a deal on something."

Something else you should look into - sealing up your duct work. We had a similar issue on dust in the air return plenum because it was sucking air from the wall cavities, under the floor plates, etc. We also had a lot of loss inside the air grills, both at the flex duct connection to the grill and at the grill-to-drywall connection. Sealing up all that really helped our system efficiency. The duct seal comes in a tub and is cheap - it's just a messy DIY job. Sealing a duct with the goo is like icing a cake using a rubber glove and your fingers. The aim is to make sure that the system is only taking in conditioned air from the house and to try and make sure the conditioned air goes back into the house. And not the attic, between 2x4s in the walls, etc.

 

[Lisavano]

OH, I took one of the drawers out, the drawer closet to the return grill is stuck. But this is what I found. It looks like there needs to be dry wall filled in; I believe Falcon this is what you are saying. Should there be a filter behind the return grill as well????

 

[Ohmthis]

Lisa,welcome to GJ, a couple things. It's hard to tell from the picture, but is the cavity that is open being used to pull air from another area? It's common to use the space between studs to make a "duct" or pathway for the return air to move. You can also post the model number of your outside unit and we can tell what size the outside is. On piston orifice (metering device for the freon) and R22 systems it was quite common to "upsize" the indoor coil a half a ton to help with efficency. Hence why Falcons old system would have a 3.5 ton indoor coil and a 3 ton outside unit. Lastly, sorry for your bad luck with service techs. Any worth their own salt know, or know where to find the calculations for return sizing.

 

[gregtwojeeps]

OH, I took one of the drawers out, the drawer closet to the return grill is stuck. But this is what I found. It looks like there needs to be dry wall filled in; I believe Falcon this is what you are saying. Should there be a filter behind the return grill as well????

Wow, I see the pic now and as said above, it appears the HVAC installer is using your hollow stud cavities as a RA channel. Which is done often in ducting jobs BUT... they control what RA is brought in to the stud/joists spaces carefully by the use of panning ( flat sheet metal pieces attached across the studs/joists) in conjunction with the stud spaces ....

Seeing your home is vintage like me, it could have openings in that knee wall where that un-conditioned air is being sucked in to the RA "tunnel". You want your return air being sucked back to the air handler to be as much of the conditioned air that blew out, as possible ( Keep the heated return air recycling and the cold return air recycling) to the air handler (furnace) .

Any air being pulled in from the attic space in to the RA, would raise the temperature of the return air to the air handle (blower) in the cooling season and lower it in the heating season .. NOT what you want to happen. So with a flashlight, this needs to be checked out....

And yes ....you could put a washable green disposable filter (cut to fit) in that wall register that would help your main filter from pulling all that wall dirt in to it
( looks like cellulose insulation from somewhere)


The washables are not all that efficient so it should not hurt your air returning volume that noticeably. BUT I would wait on doing this UNTIL you or your tech found your RA problems. Do not want to slow the RA down now, when if you are right and the RA is troubled already...

With the washables I did the same thing at our tri-level home so our Golden's dog shedding of hair would not get sucked back in to the duct system return wall registers ....that we had just paid $450.00 to get cleaned. They really helped trapping the dog hair and did not seem to affect the air distribution. I used a hinged return air grill on mine with knurled knobs on the two holding screws, so my wife could get the grill door dropped down easier to vacuum them off. JMO

 

[James-W]

Would it be possible to put a cold air return in every room?

 

[TNToy]

Anything is possible, with a large enough bank account.

 

[CNGsaves]

Think of air handler/squirrel fan needing to BREATHE . . . . . the return air ducting is only way it gets air. If constricted, then the supply air pushed out is less at the registers in room. Same thing with the closed doors in rooms. . . it prevents air from circulating back to the RA vent and repeat process.

With a slew of details, an HVAC guru here on GJ could help you:

a) square feet of each room with conditioned air
b) size of register(s) in each of these rooms
c) size of pipes supplying air to those registers (also length of pipe)
d) size and length of main duct leaving air handler
e) does return air only have that one path thru 24"x12" grill in linen closet ??

Guru can crunch these numbers and suggest tweaks to both RA and Supply Air.

+1 that most HVAC companies in general & techs just ignore these important air flow (CFM) issues and just bandaid the problem.

 

[91 zeee]

going back to the original question.... yes, your return grille is too small.
I cant see enough of what else is going on, is it possible to use a louvered door in that closet to open up the return path?