How to fix a noisy gas furnace?
November 1, 2011 2:19 AM   Subscribe

Safe, effective sound dampening options for a noisy gas furnace?

We have our gas furnace next to our living room downstairs. It's getting colder and the furnace fires up every 15-30 minutes, heats up some air for another few minutes and then shuts off.

During the air-heating step, the furnace is loud. Very loud. To the extent that relaxing to a movie or just chilling out on the couch has now become a bit of problem.

I'd like to figure out what I can do to safely dampen sound from or around the area where the furnace is installed.

It should be effective, but safety is an equal criterium here: The furnace needs to draw fresh air and exhaust carbon monoxide the way it does now (or the way I assume it does), and I also don't want any dampening material to overheat and catch fire.

Is this something where I would have to spend thousands of dollars to hire an HVAC specialist, and so I will not be able to do this, or are there dampening materials that work for this application, which are reasonably priced and are safe to install?
posted by Blazecock Pileon to Home & Garden (12 answers total) 1 user marked this as a favorite
What does it sound like? Can you tell where the noise is coming from? Is it a roaring flame, squealing fan motor bearings, rushing air in the exhaust / intake pipes, or maybe vibration in the whole unit?
posted by jon1270 at 4:04 AM on November 1, 2011

I assume the furnace is in a closet, closed-off from the living space? I would bet that there's no insulation of any sort in the walls surrounding the furnace. It's possible to install sound-deadening insulation in the walls. To do it correctly, though, you will need to strip the drywall off, then lay-in the sound-deadening bats, then hang new drywall.
posted by Thorzdad at 5:06 AM on November 1, 2011

I had a similar problem with my new gas furnace. It was so loud I couldn't hear myself think. I called the company that sold it to me and installed, it and they sent someone out who removed a baffle and duct taped a furnace filter over some of the vents. According to the service guy, this was recommended by the manufacturer to cut down on the noise.

It's still loud, too loud for my true and complete happiness, but it's a lot better.
posted by Dolley at 5:57 AM on November 1, 2011

"installed it," of course.
posted by Dolley at 6:03 AM on November 1, 2011

Is the door solid? I've noticed that the doors for those rooms are no different from the standard flimsy closet doors you get in a house. Changing out the door would likely help. Also, adding weatherstripping around the seams of the door can help a lot.

(If the furnace is drawing its makeup air through louvers in the door, you should consider changing that. If there is a way to get fresh air into the closet from the outdoors, the furnace would operate more efficiently, and you can get rid of the louvers. This could be cheap if the furnace has outside wall access.)

Another thing is that they make a bellows kind of thing that HVAC installers use to isolate the furnace from the rest of the ductwork. The furnace will still make its noise, but it won't be so solidly attached to the ducts, so they won't amplify it too much.

If this is a fairly new occurrence, you should figure out what is causing it. If you can hear the flame roaring, you'll need to have somebody adjust it. The gas pressure is too high. If you can hear the motor running, maybe the rubber isolators on the motor have hardened up. (Once the motor gets up to speed, the sound of the air wooshing should be louder than the hum of the motor.) If the blower is clanking or thumping, your squirrelcage has gone out of balance and needs to be fixed or replaced.

It also might be possible to upgrade the furnace to a variable-speed motor. This would be a more expensive option, but it is purported to save energy, so it might pay back.

Check the dampers in the ducts and on the wall registers. If the majority of them are closed, your furnace is going to have to work harder and thus noisier. Rebalancing the system can turn into a panoras box, but will probably lead to quieter and more efficient operation.

Finally, the filter. If you are using one of those fancy HEPA-like filters, they don't flow air as well and cause the furnace to work harder. Make sure you change it often, at least. Depending on the setup, you might be able to get an HVAC person to install a larger filter opening so you can use one with greater surface area. It always seems to me that the filters on furnaces aren't sized correctly for optimum flow.
posted by gjc at 6:05 AM on November 1, 2011

If there is a way to get fresh air into the closet from the outdoors, the furnace would operate more efficiently,

It would very unusual to have a forced air furnace drawing all its air directly from the outdoors, though some HVAC systems include fresh air intakes and/or heat recovery ventilators. But please don't unilaterally make big changes (eg ducting the intake directly from the outdoors) without talking to the furnace manufacturer and a local HVAC person.

I don't think you have given us enough information to give genuinely useful answers, but my guess matches much of what has been said above: get the furnace checked out, add sound insulation, see if there are any easy ways to change where the furnace draws from, etc.
posted by Forktine at 6:30 AM on November 1, 2011

The one at my sister's place sounded like a jet getting ready for takeoff. If that's the case, it could be too big for the house or just a poorly designed model and maybe it's time for a replacement.

We're getting an energy efficiency upgrade on our place and had a few sales guys come by. I'm told the newer furnaces have a 2 stage (or variable) fan. The newer models also draw all their air from the outside (through a pipe).
posted by bonobothegreat at 6:54 AM on November 1, 2011

Oh, I think I misread, and gjc is talking about drawing the combustion air from outside. That may well make sense, though again I'd want to check with an hvac person first.
posted by Forktine at 7:43 AM on November 1, 2011

Can you tell where the noise is coming from? Is it a roaring flame, squealing fan motor bearings, rushing air in the exhaust / intake pipes, or maybe vibration in the whole unit?

As described in the question, the noise is from the air-heating step. This includes the flame and fan—mostly the fan. It's not squealing, it's more of an engine turbine sound.

My question was really: What can I do to kill the sound other than calling an HVAC company? It's a fairly modern-looking furnace and was serviced two years ago.
posted by Blazecock Pileon at 12:48 PM on November 1, 2011

The sound it makes is pretty much normal. Your best bet is going to be sound insulation in the surrounding enclosure (if there is one). If the furnace is free-standing in a room, you could look into enclosing the unit in a closet that is adequately soundproofed.

That said, without the unit being located far-away from the living space (like in a basement) you are probably always going to hear it somewhat.
posted by Thorzdad at 2:29 PM on November 1, 2011

I don't mean offense, and I appreciate answers, but if you're going to answer, please read the question, first. Thanks.
posted by Blazecock Pileon at 2:55 PM on November 1, 2011

Unfaced fiberglass batts if not within a couple inches of the body of the furnace are safe to install in a furnace room even when not covered by drywall. If your furnace room walls aren't finished (IE: bare studs) then fiberglass batts installed in the stud bays will help reduce some of the high frequency noise. Best would be to install the insulation and then cover with gyproc even if you don't mud afterwards. Two layers of gyproc would be even better.

This would be a minor renovation project but depending on your layout and how handy you are a doubling up on the gyproc on the living room side of the furnace room could help reduce the noise fairly cheaply. For best results use a resilient channel between to the two layers. Obviously you'd need to mud and paint in this case.

If your furnace isn't depending on leakage around the furnace room door then regular exterior weatherstripping around the door will significantly reduce the amount of noise that escapes via that route. Obviously this won't have an effect if the door is louvred or there are wall louvres.

Some furnaces have switches to select between two or three different fan speeds; selecting the low setting will reduce the fan noise. The trade off is the longest runs of ducting may have noticeably reduced heat outputs.

A high quality pleated filter like the ones sold for people with allergies in place of a cheap spun fiberglass filter can reduce the noise carried in the intake ducting. Not a lot usually but sometimes noticeably. Cheap and easy to try at any rate.
posted by Mitheral at 2:44 AM on November 28, 2011

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