View Full Version : Air return velocity/noise
07-21-2007, 01:05 PM
Hello, I recently replaced my furnace/ac with a Trane 3.5 ton unit and there
is some return air flow noise, I think from air velocity, that I would like to lessen.
I believe everything is sized OK -
1 - 3.5 ton unit (about 1575 cfm ?)
2 - left/right supply branches 20x8 (about 1700 cfm (855x2)?)
3 - one main return trunk serving two floors 24x12 (about 1450 cfm? )
4 - Filter 16x25 (pleated - cheap)
5 - Blower on medium
6 - Cools both floors ok
My questions are -
- how large should the opening behind the two return air grills be? The grills themselves are 22x22, but the cutout is only 12x12.
- is lining the duct worthwhile?
- is there better sound absorbing grills
- other solutions ???
Thanks in advance - G
07-21-2007, 01:25 PM
12"x12" is good for 700 cfm x 2 =1400 CFM. Should be adequate return air flow.
07-21-2007, 02:06 PM
those 22 x 22 are they stamped louver or bar type grills? if you take them off does the noise go away?
07-21-2007, 02:49 PM
Anything over 3 ton I move to a 20x25 filter and make sure the ductwork can handle it. I fabricate a 3 inch base to make the opening of the furnace large enough to accomadate.
07-21-2007, 04:52 PM
Thanks all for the help!
Forgot to add that the supplies seem to work fine, and are quiet.
The return grills are stamped louvered - pretty cheap looking. It didn't seem to make difference in sound when the grill was off, but don't think that they are helping at all either. Should I replace with better ones? Does the main return trunk size sound sufficient? If so I would then guess the lower air box (plenum?) that holds the filter is causing some turbulence. Would if be a good idea to make that return plenum larger, to hold a 20 x 25 filter? What other options are there to make the ducts 'handle it'? Lastly, what about duct sound proofing on the lower return? Does that stuff work?
Thanks again !
07-21-2007, 05:01 PM
I've reduced noisy grilles by taking needle nose pliers and opening them up...but if you have to do that, you are living on the fringe of what your system is capable. I'd say do away with pleated and change the filter more often...no more cost in the long run and the system breathes more effeciently with your undersized opening to your furnace.
07-21-2007, 09:51 PM
it is common with equipment changes. you are used to the eq. noise. check all the above postings,all good ideas.
07-21-2007, 09:57 PM
bar type would be an upgrade over stamped. going to a cheap filter would be good just change it every month in a/c season. you could try and line the inside if you think there are harmonics making the noise, but if the return duct is short and close to the blower tht may be what you here. there is another product called sound flex that could be cut "in-line" to help as well made by casco, im sure there are other brands as well.
07-23-2007, 07:32 PM
Thanks again all.
One final question, for my own knowledge. Should the supply and return duct sizes (sq. inch - cfm) be the same as the nominal furnace/ac blower output, and equal to each other? I guess to a newbie I would think the supply should match the blower and be big enough to move air where you want it. But the return could be way over sized, where the bigger size the less the air flow velocity in the duct. Lastly do you normally measure the static pressure, or is there a simple rule of thumb to use.
Appreciate the help !
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