View Full Version : Noisy Duct
02-19-2007, 10:43 AM
I have a FHA furnace serving a 2,400 square foot 2 story house. The furthest bedroom on the second floor was a bit cold and when I checked it was the last take-off on the main trunk line in the basement. To get the room a bit more air, I installed an inline Fantech fan. The problem that I am having is that the ductwork is now noisy. The configuration is as follows. Sheet metal trunk line in basement with a 7" diameter take-off to the second floor. I installed the fan on the 7" line in the basement. the 7" line transitions in the basement to a 3"x12" oval duct thru the first floor wall, when it reaches the second floor, it goes across the floor joists (first floor ceiling, second floor - not attic) with a 7" flex duct to the diffuser. I am hearing a lot of noise in the wall were the 3"x12" duct is. Using my ductolator, I come up with a velocity of about 950 FPM which I know is a lot. Presently I have a reostat installed on the fan and have it running about 1/2 capacity. When I turn the fan down the noise goes down but the room gets cold.
What are my options? Is the majority of the noise due to the flex duct or a combination? Any help would be appreciated.
1. Inspect duct work for leakage. Tape if necessary. You can lose 30% of your air from leaking ducts.
2. If no return air from room, find a way to draw air back to furnace. (especially important for AC)
3. Inspect for insulation. Look at windows, electrical cover plates. Areas of the wall missing insulation. Put your hand on the wall and see if you can feel cold spots.
4. Remove flex and replace with regular duct. (Flex will decease air flow, especially in 90 degree angles).
5. Remove 7" and replace with 8" duct.
6. Insulate duct work, especially if it's close to outside wall.
7. Have customer where ear plugs when inside room.
02-19-2007, 11:33 AM
Thanks for the reply.
1) Already done, found a few leaks, repaired with duct mastic.
2) There is a return in the room.
3) Used IR temp meter and walls, ceilings, windows, etc are in good shape.
4) I will try to replace flex. There is about 15' of this in the ceiling of the first floor. As the flex is not smooth on the inside, will this help reduce noise significantly?
5) will do
6) All duct work is insulate.
Any other suggestions. is the 950 FPM way to much for a 3"x12" oval duct or just a bit high? would you expect this to be the problem of the noise or more the flex?
I am a homeowner in S.Texas and have like you had ducts become noisy from pushing air too fast. In my case it was also flex duct, the speed increase came from a variable speed air handler. My air speed was not as great as your 950 fpm, but "only" about 750 fpm.
Seems to me the answer is you need larger duct capacity, either changing to a larger duct size or running a 2nd duct to your room. An 8-inch duct instead of 7-inch would increase capacity over 30%. Hard metal duct has higher airspeed limits, but I suspect that is not a realistic option for you.
My tech keeps urging me to change from cheap plastic supply grilles to a moderately expensive metal grille with movable louvers. Something like this:
The exact one I used was: item #C2SMW12X06.
I have trouble believing that is the source of my noise, but you might try it out. Far easier and cheaper than the other ideas, the metal supply grille will flow more air due to less resistance.
Best of luck -- Pstu
If the noise is in the duct in the wall ,lyou likely ned a larger duct there to reduce the noise.May have some "poor " connections that add to the noise in the wall.
Changing flex to metal,will increase air flow at the same fan setting,don't see how it will reduce noise from a duct in the wall.
02-19-2007, 11:59 AM
Thanks for the input. I do have metal low restrictive floor registers.
My big questions is the 3x12 duct the culprit or the flex. I can get to the flex much easier to replace than the 3x12 which I would have to cut a wall open to change out.
Other help please.
If the noise is coming from the 3X12 in the wall,I don't see changing the flex making any difference.
Tell us more about the noise.
A 3X12 oval is equivalent size of a 6.4 Round Metal.
02-19-2007, 05:07 PM
The best way I can explain the noise is it sounds like a vibration or humming sound. At the fan itself which is in the basement, there is minimal noise and no vibration. The only thing I can think of is that since there is so much restriction (small duct and flex) is that the ductwork is being pushed out against the sheetrock and what I am hearing is the sound of th air in the duct trying to overcome the static resulting from the small supply.
Sounds about right?
Thanks for the help.
02-19-2007, 05:20 PM
Does removing the grill quiet it down much?
02-19-2007, 05:42 PM
how about this for a theory... whoever did the rough possibly left a piece of the duct out of the s-cleat and didnt realize until he was done and then closed it with a piece of foil tape? And your left with vibration and humming of the tape and duct not attached properly.
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