Heating Help

loud return duct

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

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

new

its new installed it yesterday......

Whats your CFM

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

Whats your CFM

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

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..........

?

could that return be too small

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.

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

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

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?

furnace

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

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%

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.

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

thanks

thank you guys much appreciated