Noisy return

[Armis]

Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.
Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on . 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

[second opinion]

Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.
Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on . 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

Could you post a picture of furnace with duct work attached?

[Armis]

[Brent Ridley]

You should barely hear the system operating with properly designed ductwork.

Assuming the blower motor and wheel are operating correctly, this system needs some ductwork modification.

Call a pro, get a full ductwork assessment complete with CFM, FPM, and static pressure readings.

With these readings, any decent contractor will know what needs to be done.

[udarrell]

Have two returns for 2.5 ton furnace , one 8x20 , other 10 inch round. They both go to return plenum with 24x20 filter box.

Its very loud noise coming from 12x12 return grill (10" return duct) then furnace is on a 10" round is about 25' long with four 90ties. Take off is 7x14.
Any ideas ?
I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

Is there more than one Return air grille?

If there is more than one RA grille; what percentage goes through the noisy 12X12? That is only one sq.ft., of open air area without deducting the area the grille blades block.

If that were the only RA grille the velocity through that grille would be well over 1000-fpm; that would be over twice the velocity needed, as 400 to 500-fpm limit is required to reduce airflow noise; I'd prefer grille sizing so the velocity through the grille area(s) would not go above 450-fpm.

[Armis]

for 10" duct one 12x12 grille , for 8x20 duct - 20x16 grill

[George2]

for 10" duct one 12x12 grille , for 8x20 duct - 20x16 grill

The way the 10" flex is kinked, the 12"x12" grille (288cfm) is probably enough and not worth the bother to fix.

The 20"x16" grille (640cfm) should be enlarged if possible. The furnace is probably moving at least 1,200cfm.

[udarrell]

for 10" duct one 12x12 grille, for 8x20 duct - 20x16 grill

Hart & Cooley Engineering Data: 12x12 has only .62-Ak or sf of free air area, & will only flow 249-cfm @400-fpm, @500-fpm it flows 311-cfm;
@450-fpm * .62 is 279-cfm.

The 20x16 has only 1.37-Ak, @ 450-fpm *1.37= 616.5-cfm; 500-fpm * 1.37 is 685-cfm.

The system needs to move 1000-cfm @500-fpm or less through the RA grilles for a 2.5-ton A/C.

311-cfm + 685-cfm = 996-cfm @500-fpm velocity through those grilles. Grilles will work if enough room behind them so there is no restriction.

According to manual D initial velocity "through a cheap media filter" should be 300-fpm.

When you figure using .62% for reduced free air filter area; the free air area (Ak) is 2.06 * 300-fpm = will only flow 620-cfm; therefore it could also use a larger filter area, however...

When you divide 1000-cfm by 2.06-Ak the result is 485-fpm which is better than a lot of filter areas; so you might let it pass.

If the other duct feeds into the 10" flex duct moving 1000-cfm is way too small; optimally it needs a 16" flex duct to get the velocity down to a little over 700-fpm. It's late & I didn't verify that situation...

[Armis]

The way the 10" flex is kinked, the 12"x12" grille (288cfm) is probably enough and not worth the bother to fix.

The 20"x16" grille (640cfm) should be enlarged if possible. The furnace is probably moving at least 1,200cfm.

Had to clarify before, 10" is solid sheet metal , not flex

[udarrell]

Had to clarify before, 10" is solid sheet metal , not flex

With a sheet metal RA duct, I'd still go with the 16" RA duct, the friction rate between flex & metal goes from 0.06 for flex to 0.02 for metal @1000-cfm of airflow volume. Either way @1000-cfm of airflow the velocity is 717-fpm.

However, if it is flowing 1200-cfm as George2 believes, then, with (2) 90's, over 25' the friction loss is 0.03"; over 100' it would be 0.08" friction loss & either way the fpm velocity is 860. The standard is to not go over 800-fpm velocity on a Return Air duct.

[mechanical'al]

noisein the return usually means one thing its starved

[Armis]

I took filter panel off - same thing. i took furnace cover to expose fan - same thing. Something is wrong with this duct?
Thanks

Starving from what? Air ? Most of it is coming from other areas.
Only thing to make him quiet is shut him off