Author Topic: soundproofing a loud furnace in attic  (Read 9240 times)

Rick P

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soundproofing a loud furnace in attic
« on: November 22, 2005, 11:19:13 PM »
We have a furnace unit in the attic above our master bedroom/hallway.  We hear the pre-ignition cycle and a whining sound.  Our HVAC guy says the whining sound is the furnace motor and there's nothing he can do about it.  The humming sound is most prominent directly underneath the unit in our bedroom, but since it is located near the edge of the room, it can actually be heard from 4 or 5 different rooms.

The furnace is bolted to two upside down metal V's which are bolted to plywood platforms.  The platforms are bolted to the ceiling joists.  Presumably there is insulation between the wooden platform and the drywall in the ceiling.

What's the best way to try to sound proof this -- from the top or the bottom?  Do we soundproof the entire attic or add material to the ceiling of all of the affected rooms?  If we do it in the attic, do we need to soundproof the entire attic or just directly underneath the unit?

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Re: soundproofing a loud furnace in attic
« Reply #1 on: March 09, 2007, 04:14:59 AM »
I have a similar problem...we have loud furnace & duct noises, but they are in the basement below our master bedroom.  Here's their info:

Ducts/ducting and air vents.

Ducting needs to be covered with closed cell sound absorbing foam (not open cell foam), for sound and thermal insulation.  Metal ducting needs to be physically isolated from the fan shroud or aircon unit with a flexible coupling.  Otherwise is will re-radiate sound vibration along it's length.  Fiberglass or flexible ducting usually won't have this problem, but will lose thermally if not insulated.

Liquid soundproofing can be painted/sprayed on or in the metal ducting for even more sound vibration reduction.

Vents are a problem if the design causes an air noise.  To see if this is a problem, temporarily remove one to see if the sound level drops appreciably.

Watch for vents to rooms that have a single common feed up through the wall- (or down through the floor to the ceiling below),  if you can peer though the vent into the other side, it means sounds in one room will transfer to the other room.

"We painted the ducting inside with Super Soundproofing Liquid, then covered it with the mat.  We still had a bit of a problem, because the sound was following the ducting, so we made a hinged door the fits inside the duct.  It is covered with the mat and is hinged closed so as to swing open when air passes through the duct.  Otherwise it's closed at all times, effectively blocking any sound through the ducting. That took care of the problem. Ducting made of fiberglass instead of metal would have perhaps prevented the problem in the first place".

I would do all this first, and then the 2nd step would be to add MLV to the attic floor, or to the ceilings of the rooms but then you'd also have to add ceiling tiles or drywall to finish it.