loud return duct

Paul S_3

hey everyone i installed a new 100,000 btu 95% warm air furnace new duct

work supply and return....everything operates fine, except for the

return duct is very loud...furnace is in basement and return duct goes

straight up 10 feet to first floor into a return box....it is NOT a AIR

FLOW ISSUE, temp rise is fine and if i take the blower door off to allow

maxinum air to go through system i still have the noisy return. the

noise just sounds like the blower wheel & motor running....is there a

way to reduce the noise with running the return duct a certain way or

is there such thing as noise reduction ductwork for example if i get

ductwork thats insulated inside it??? any help would be greatly

appreciated   thanks   paul s

nicholas bonham-carter

Silence is golden

Most of us here do not have experience with forced air problems.

Try reporting this in the heating section.

Is this a new furnace? Sounds like a noisy blower. Can you switch out with a new furnace, and see if the noise goes away?--NBC

Paul S_3

new

its new installed it yesterday......

unclejohn

Whats your CFM

And whats the size of your duct. Is it a floor or wall return.

unclejohn

Whats your CFM

And whats the size of your duct. Is it a floor or wall return.

Paul S_3

1600 cfm

1600 cfm.....and return duct is 10 by 18 with another duct 10 by 10 on opposite side.....its not an air flow issue with a good temp rise..its a wall return ....the sound sounds like the blower motor is echoing through duct work.......return is same size as previous system same btu same 4 ton drive.....and like i said if i take the blower door off and run it which mimics a much larger return i still here the noise..........

Paul S_3

?

could that return be too small

unclejohn

I'd say

Its too small.Your velocity in FPM is almost 1500.Also the newer hi eff. furnaces run a very hi heating fan speed. That could be almost twice your old furnace. You could try and put some 90* in the return. We use to run 3 90* in shot supply runs in attic systems. The farther you get from the fan the quiter it will be.Can you install the furnace on a sheet metal box and use a bottom return.

Paul S_3

ok

you mean run the return with a few 90's ,,,not to have it go straight up and no i would not be able to use a bottom return box

Jim Godbout

noise

Is your new furnace variable speed, if it is dial down fan speed

if not either increase duct size and return air.

duct should be lined as well to reduce noise especially on such a short run

get your installer back to make changes if you are not happy with final product

good luck

Techman

Noisy return

I'm going to say the return is way to small.it should be more like 8x20 or 8x18 depending. I agree w/unclejohn that 3-4 90* bends are required to stop the noise, and what size is the supply duct ?Returns are supposed to be larger than supplys.What size is the return box? Yes you can get a "lined duct" but with the increased velocity of an undersized duct the liner falls apart after several years and the liner material gets into the air. Is there A/C involved?

billtwocase

furnace

Is this a hiboy, lowboy, counterflow? Sounds like duct size and configuration, and blower speed. Is the blower motor ECM?

Paul S_3

not ecm

it does have a/c & it does not have a ecm motor....its not a airflow problem there is another return duct 10 by 10 on the other side....if it was a air flow problem wouldnt i have a high temp rise and i ran the system with the blower door off and i still get the noise......and also changed fan speeds from low to high and loud noise is still heard.... im going to go a head with the 3 90 bends to reduce noise......and if its small what is the correct size the return duct should be for a 4 ton drive 100,000 btu furnace 95%

Techman

duct size

With 4t of ac you need 20x12 or 16x16 or 10x25  or so .What kind of air filter? A 10x10 is about 500cfm and a 10x18 is about 1100cfm.

clammy

Bottom return

When ever i do hot air and  i am replacing anything over a 3.5 ton drive i use  the bottom return instead of the side cut out return .Alot of the hvac supply house now  sell bottom return duct kits it makes life a bit easier over doing cut out on the left and right of the unit . This was some thing that we always did on 4 ton and up units and this was from 20 years ago .The return duct is defenitly to small 20 x 12 would be fine i woulsd also look at the type of registers you have i always spend the extra  money and get wide bar registers instead of the cheaper stamped face ,the wide bar are a little less restrictive plus they look alot better in my eyes and with hot air you are stuck looking at registers for ever unless you switch to a wet source of heat.peace and good luck clammy

Paul S_3

thanks

thank you guys much appreciated