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Old 07-22-2020, 01:07 PM   #16
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Re: Can Central AC return duct sizing be too big?


Use the inside dimensions for your grille measurements ie: 36x14. Ideally you would build a new box for the new grille and tap the 24x14 into it. You could possibly modify the existing box in situ. Either way you may have some framing issues above the drywall to contend with. Do you have decent access above the ceiling? If you do, adding an additional return would give you a better result. The 24x14 duct back to the unit is a bit smaller than I would use but I don't think it's a problem at 900 FPM (velocity). How old is the package unit?
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Old 07-22-2020, 09:20 PM   #17
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by 57TinkerMan View Post
Use the inside dimensions for your grille measurements ie: 36x14. Ideally you would build a new box for the new grille and tap the 24x14 into it. You could possibly modify the existing box in situ. Either way you may have some framing issues above the drywall to contend with. Do you have decent access above the ceiling? If you do, adding an additional return would give you a better result. The 24x14 duct back to the unit is a bit smaller than I would use but I don't think it's a problem at 900 FPM (velocity). How old is the package unit?
Good question. I will have to remove the sheetrock tomorrow and see what it looks like under there. would it be ok to add an additional 36x14 a couple of feet from there? or where space permits? that would give me 36x28 in effective area, and maybe if i go with less restrictive filters and I can get to where I need to be. That said, just changing one filter is nice too.
Also for building the new box, do you know if there are any places I can get a "bolt" on flange where I have to do minimal cutting/modifications?
the package unit was installed back in 2007. so not a young pup. I have replaced several parts on it already, including a blower and a relay. I wonder if this small return accelerated the original blowers demise.
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Old 07-22-2020, 10:28 PM   #18
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Re: Can Central AC return duct sizing be too big?


Hard to answer your question regarding an additional return without knowing how much room you have to work with. It might be possible to add a grille/box in the ceiling behind the existing grille and tap into the vertical return duct with flex duct. I suspect your existing grille is only 14" wide due to the framing above. Before you start removing sheet rock, drill a small hole in the ceiling and stick a straightened coat hanger or measuring tape into it and see how far it goes before you hit the roof. Let us know what you find. I'm not clear on your terminology regarding a bolt on flange, can you clarify that a bit. I doubt your motor problem was due to the return.
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Old 07-23-2020, 10:52 AM   #19
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Re: Can Central AC return duct sizing be too big?


Just noticed this...

Can Central AC return duct sizing be too big?-unf.png
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Old 07-23-2020, 12:43 PM   #20
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Re: Can Central AC return duct sizing be too big?


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Originally Posted by 57TinkerMan View Post
Just noticed this...

Attachment 608295
Great question. I have no idea, and I will look into it. Good catch though. I saw it there, but didnt think anything of that being a "return". thanks!
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Old 07-24-2020, 07:16 AM   #21
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Re: Can Central AC return duct sizing be too big?


Quote:
Originally Posted by 57TinkerMan View Post
Use the inside dimensions for your grille measurements ie: 36x14. Ideally you would build a new box for the new grille and tap the 24x14 into it. You could possibly modify the existing box in situ. Either way you may have some framing issues above the drywall to contend with. Do you have decent access above the ceiling? If you do, adding an additional return would give you a better result. The 24x14 duct back to the unit is a bit smaller than I would use but I don't think it's a problem at 900 FPM (velocity). How old is the package unit?
I had a chance to measure the velocity last night with my new anemometer and without the filter I was getting ~620 fpm and with the filter attached (couldnt get perfect seal with the anemometer behind it) I was getting ~830 fpm. that said, i can't see it getting lower than 620 fpm with the larger duct, and I dont want to switch out all of the 24x14 ducting at this time. do you think that a 36x30 equivalent filter area is still needed, or do you think I can do something closer to a 36 x 24?
As for the sheetrock around there, I still havent ripped it up to see around there, so I'll share that info once I have a chance. hoping to get to it later tonight. thanks!
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Old 07-24-2020, 10:39 AM   #22
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Re: Can Central AC return duct sizing be too big?


Yes a 36x24 filter grille will help reduce the noise level as long as you don't install super restrictive air filters in it. Is the noise level at the return acceptable to you if you have the grille face installed but without filter?

It's more important to know the velocity in the duct itself. not sure what type of meter you have but you need to measure the velocities in the duct at several points (think horizontal plane and 6" squares). You can calculate the "actual" (close enough) CFM and select the correct grille from there.

It would be interesting to know what the existing condition at the grille face is. If you feel like it, measure the velocities at the grille face, follow the (in bold) directions below. Use a grid pattern and calculate the average.

*Tested without filters. Typical disposable 1-inch capacity is 2 cfm per square inchof gross filter area. Recommended velocity is 300-400 fpm. Velocities higher than500 fpm will decrease filter performance. Increase flow resistance, and possiblyblow off agglomerates of collected dirt. Velocity measured 1 from face."
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Old 07-24-2020, 11:27 AM   #23
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Re: Can Central AC return duct sizing be too big?


I should add that yes, I know that it's a 5 ton unit with a nominal 2000 CFM airflow. For filter grille selection, using the anemometer is not technically necessary because there is only 1 return. Regardless, it's interesting (to me anyway) to know the actual flow in the duct as it is borderline (IMO) too small and the unit fan speed may have been lowered by the previous owner to reduce noise at the grille.
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Old 07-26-2020, 03:18 PM   #24
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Re: Can Central AC return duct sizing be too big?


Quote:
Originally Posted by 57TinkerMan View Post
I should add that yes, I know that it's a 5 ton unit with a nominal 2000 CFM airflow. For filter grille selection, using the anemometer is not technically necessary because there is only 1 return. Regardless, it's interesting (to me anyway) to know the actual flow in the duct as it is borderline (IMO) too small and the unit fan speed may have been lowered by the previous owner to reduce noise at the grille.
Interestingly enough, the additional "return" was run to the wall near my old walk in closet, but the inlet was completed sealed off. so perhaps it was a return at some point and then they decided to cut it off, for some reason... very strange.
I ripped apart the sheetrock around the return and as you expected, it is basically taking up the entire width/length of the space in between the joists. So I would need to add another return regardless since I cannot physically fit anymore without cutting off the joist.
One other thing I noticed was I swapped the filter out with a MERV 8 (currently MERV 13) and that immediately brought down the dB to ~50dB, so a 5 dB difference. Im curious if I get the 2nd return functioning again if I might be around the mid 40s level. If I am, then I might be ok with just keeping the return the way it is.
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Old 07-26-2020, 04:22 PM   #25
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Re: Can Central AC return duct sizing be too big?


Quote:
Originally Posted by ilumo View Post
Im curious if I get the 2nd return functioning again if I might be around the mid 40s level. If I am, then I might be ok with just keeping the return the way it is.

I doubt you would see much improvement and it would also be unfiltered so you would have to address that. What is the dB level with grille in place and no filter installed? In your case a MERV 8 is still quite restrictive with the CFM you have. A cheapo filter and wider spacing on the grille face louvers (mentioned earlier) may be all you need. How much clearance is between the roof framing and ceiling framing?
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Old 07-26-2020, 04:55 PM   #26
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Re: Can Central AC return duct sizing be too big?


Quote:
Originally Posted by 57TinkerMan View Post
I doubt you would see much improvement and it would also be unfiltered so you would have to address that. What is the dB level with grille in place and no filter installed? In your case a MERV 8 is still quite restrictive with the CFM you have. A cheapo filter and wider spacing on the grille face louvers (mentioned earlier) may be all you need. How much clearance is between the roof framing and ceiling framing?
dB level with just the grill is about 44dB. which is quite acceptable. as for the clearance it's about 7.5 inches.
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Old 07-26-2020, 05:45 PM   #27
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Re: Can Central AC return duct sizing be too big?


I'd probably try the filter grille with the 3/4" spacing and MERV 4 (cheapo) filters. I think what they are offering (fixed bar vs. stamped face) looks better than what you have, although from your dB measurements not a huge improvement in sound reduction. They do make filter grilles for 2" filters too, if you feel the need. I didn't see any 2" grilles with 3/4" spacing, but I didn't look that hard. I don't think using a high MERV rated filter will work in any configuration of your existing situation without adding additional returns.

3/4" spacing, 1" filter:

https://hvacquick.com/viewbasket.php

1/2" spacing, 2" filter:

https://www.plumbersstock.com/shoema...mped-face.html
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Old 08-12-2020, 10:14 AM   #28
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Re: Can Central AC return duct sizing be too big?


Quote:
Originally Posted by 57TinkerMan View Post
I'd probably try the filter grille with the 3/4" spacing and MERV 4 (cheapo) filters. I think what they are offering (fixed bar vs. stamped face) looks better than what you have, although from your dB measurements not a huge improvement in sound reduction. They do make filter grilles for 2" filters too, if you feel the need. I didn't see any 2" grilles with 3/4" spacing, but I didn't look that hard. I don't think using a high MERV rated filter will work in any configuration of your existing situation without adding additional returns.

3/4" spacing, 1" filter:

https://hvacquick.com/viewbasket.php

1/2" spacing, 2" filter:

https://www.plumbersstock.com/shoema...mped-face.html
Hey Tinker, thanks for all the help. So I think I am going to stick with the 14x36x1 with some MERV 8 filters and will try using the adiditonal 6" flex duct pipe as a secondary return.
That said, do you know how I can convert this 6" flex pipe into a return "housing" that can fit a 1" filter of some sort? I can't seem to find any adapters that can house a 6" duct. the closest one I found was for a 12" duct. appreciate it.
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Old 08-12-2020, 05:37 PM   #29
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Re: Can Central AC return duct sizing be too big?


Is the 6" return from the bedroom in the wall or ceiling?
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Old 08-12-2020, 05:48 PM   #30
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Re: Can Central AC return duct sizing be too big?


Quote:
Originally Posted by ilumo View Post
I had a chance to measure the velocity last night with my new anemometer and without the filter I was getting ~620 fpm and with the filter attached (couldnt get perfect seal with the anemometer behind it) I was getting ~830 fpm. that said, i can't see it getting lower than 620 fpm with the larger duct, and I dont want to switch out all of the 24x14 ducting at this time. do you think that a 36x30 equivalent filter area is still needed, or do you think I can do something closer to a 36 x 24?
As for the sheetrock around there, I still havent ripped it up to see around there, so I'll share that info once I have a chance. hoping to get to it later tonight. thanks!

You won't get a higher velocity from an air filter being in. You got a bogus reading.


The small pipe may be a fresh air intake.
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