Return too small

[roccol]

My return is 8 x 26 rigid metal(unlined) duct(20 feet long 2 90 degree bends) for a 3 ton variable speed system. My central return grille is 8 x 30(quality HArt & Cooley Floor return). The return is is very audible when the blower is in high gear. This is of course more noticeable when the AC is running. You can hear whistling in high speed and the sound of air rushing(no rattling though).

Removing the grille make no difference.

Is my opening simply too small?

or

Do I need a additional return?
If I need a additional return, how large? Would 8 x 8 be suffice?
How far should it be from the central return?

Thanks.

[docholiday]

10X26 would be better. If you needed 8" then it needs to be 34-35"

1200 cfm would need a return grill of 24X18. ID say you would need another return close to the same size (at least 175 square inches which would be 8x24).

You also need to check your supplies, if you load up the return you may end up with higher supply velocities and static.

[beenthere]

Could be that the blower is set too high.
Might be noise from the ells.
8x8 may or may not be enough to make a difference, depending on what is causing the noise.
You need someone to find out what is causing the noise.

[roccol]

The noise is from air rushing through the return. I have lowered the blower speeds, it doesn't make much of a difference.

[beenthere]

If you lowered the blower speed, then the air wasn't rushing through the return.

What air filter are you using.

[roccol]

If you lowered the blower speed, then the air wasn't rushing through the return.

What air filter are you using.

I am using a large April Air Media Filter. I have even taken the filter out and it does not make much of a difference. Duct installer blames the furnace and the floor return opening. Basically anything but his work. I never had any issue like this on 40 year old duct work with multiple returns which definitely had a smaller leaky return overall.

[dash]

Assuming the floor return is a stock #265 from Hart and Cooley,it can 675 cfm cfms ,as they are very "open" grilles.

The 8X26 duct around 1200 cfms @ 950 fpm which should be noisey.

So I'd add another return duct and grille of the same size,to be safe.Metal duct is noisier then returns thru floor joists,flex ,or duct board,larger ducts will help.

Sound also transmits from a furnace blower so two 90° turns between the furnace and the return grille will reduce transmission of blower noise.

[beenthere]

Turbulent noise from ells.

Another return tied into same duct between the ells and current grille probabaly won't help.
Need to slow the air flow before the ells.( a lot )
Another return about the same size would help, if it can be tied in that the 26 x 8 isn't pulling all the air.

[beenthere]

The 8X26 duct around 100 cfms @ 700 fpm which should be noisey.

Hope thats a typo. :)

[roccol]

Hmm. Another 26 x 8 return would be physically impossible.

What is an "ell"?

"Another return about the same size would help, if it can be tied in that the 26 x 8 isn't pulling all the air."

This statement is confusing to me. Sorry.

[dash]

The 8X26 duct around 100 cfms @ 700 fpm which should be noisey.

Hope thats a typo. :)


Surely a mistake,thanks.

[dash]

Hmm. Another 26 x 8 return would be physically impossible.

What is an "ell"?

"Another return about the same size would help, if it can be tied in that the 26 x 8 isn't pulling all the air."

This statement is confusing to me. Sorry.


ell is an elbow,the 90° turns you mentioned.

"So the 8X26 isn't pulling all the air" means not to take the additional return duct off the 8X26 ,as all the air will still flow thru it at some point.I don't agree 100% on this but it would be safer to do it that way.

Hard to tell without being there and testing the ducts.

[roccol]

So you both agree my current return trunk is too small, correct? Regardless of my return opening. I will need another separate return trunk and opening. How far away should it be from the 1st central return?

[dash]

So you both agree my current return trunk is too small, correct? Regardless of my return opening. I will need another separate return trunk and opening. How far away should it be from the 1st central return?

Sounds that way to me.Location is not all that important.Without knowing your home,I'd say put it in the Master BR,but functionally it could be next to the other one.

[docholiday]

As I said, you dont have to add another trunk, just replace the one you have with a 10x26 and you can still take off anywhere. I also agree with adding vanes to the elbows.

[beenthere]

As the above postings. Make the existing return bigger, Or put another one beside it or some where else. Just don't use the same duct.

[roccol]

Sorry Doc, I must have missed your post.
It is not possible to make the existing return much bigger. I can make the existing duct maybe 10" wider, but that is about it. It looks like I am going to have to run serveral additional returns. I am not sure how this is going to be accomplished. If anyone in is Northern NJ, let me know. The duct installer refuses to acknowledge that the return is too small, so it is kindless of pointless trying to work this out with him.