How to reduce low frequency noise from a gas furnace?
 

I had a new gas furnace instilled at my house. When the burners come on it makes a very loud rumble, similar to a drum-roll on a timpani. The installing company claims this as normal, and refuses to do anything other than sell me another furnace. While I may buy a different furnace – the 90+ % efficient furnaces are reported as much quieter than most 80% efficient furnaces, I definitely won’t be buying it from the same vendor. I want to investigate to see if the noise can be dampened before spending several thousand more to replace the furnace again.

I'm looking for feedback on how reduce or eliminate the noise. The source of the noise is the burners. The noise passes through ducting and comes out of baseboard heat vents. I guess the frequency is somewhere between about 20-70 Hz. The noise can be heard at all the vents at the downstairs area, but not at all upstairs. Note that the furnace is mounted under the downstairs area, in a crawl space. Based on this, one possible solution is to change the installed ducting to use a kind that has sound absorption material. Another possible solution, is to modify the plenum to provide better sound absorption - or something along those lines.

It occurred to me to use the plenum or the ducting to create a muffler of sorts, but I don’t know how mufflers are made, or if concept is applicable to a furnace.

Any suggestions on how to eliminate the noise without loosing a lot of air flow?

TIA

You could check and see if any of your ducts are loose and secure them or use a product like this:
http://www.owenscorning.com/comminsul/products.asp?product=289

Good luck.:)

You have a pulse furnace - my home has two!

Here are some hints:

You could build a box around it out of insulation and cardboard - that should help.

Isolate the furnace from the house structure - if it is up in the attic or bolted to a floor other than the basement slab, insist that the installer isolate it properly - he should have known better!

Make sure the intake and exhaust lines are not tightly attached to the house structure - they vibrate and transmit noise too.

Finally, you will still hear the noise no matter what you do. I got used to it pretty quickly and don't mind it at all now.

Yeah...I thought it was a characteristic sound of gas furnaces...I remember the same in homes that I grew up in.

Here in Texas, the units are in the attic, and with the insulated foil ductwork, the noise is greatly reduced.

I'm not sure you can do much with solid ductwork...noise transmittal is guaranteed.

I think you can replace a small section of the duct near the furnace with a flexible piece. Some HVAC guy should chime in with the proper name

Not a HVAC guy but this sounds like a good idea

Buy some rolls of car audio sound deadening sheets, like Dynamat, and cover your furnace ductwork. Clean the ductwork first and check it for resonance. Cover the spots that vibrate the most.

I like that idea about the sound deadening. Dynamat is pretty expensive, I used a similar product that was much cheaper (in my truck)

http://www.b-quiet.com/

http://www.hvac-talk.com/vbb/forumdisplay.php?f=1

Very knowledgeable folks there...

This or something similar might be a big part of the solution. I was looking at how mufflers work at this site. http://auto.howstuffworks.com/muffler3.htm It appears that one of the tools they use in muffler design to set up resonant chambers which serve in part to cancel the noise. They also use different metals to absorb some of the noise. Mufflers run under much higher pressure than duct work and I don’t think I can get use of a resonant chamber, but can do something to absorb the low frequency sounds.

I'm not sure i know what a pulse furnace is. Can you elaborate?

They say mine has "inshot" burners. These are attributed as the cause of the noise.

All excellent comments! Part of the problem is attributed to the furnace vibrating while it’s running. The furnace is suspended from the ceiling of the crawl space. This was not a problem for the old one, but when you are in the room over the new one, you can tell it is under you feet. One of the plans is to decouple the furnace from the building and support it from the floor.

It seems we are not getting used to this. As it is now, it wakes us up nearly every time the timpani drum roll occurs. This has been on-going for nearly 3 months, since we had the furnace replaced. Only due to the seasonal warming which lets the furnace run less has been the only relief

They say the 90% efficient ones have a completely encapsulated burner assy. Also because of other elements of the design, the intake and exhaust are vented by way of pvc. Anyway, they say they are much quieter in operation than the 80% ones. I wish the morons who sold me mine would have mentioned this. Evidently they didn’t feel it important enough to mention. Anyway, our older furnace used a burner that looked more like something found in bbq grills. From the perspective of inside the house it was quiet in operation.

One of the points of debate is whether it’s wise to use flex ducting. The pro side is that it will help to decouple the furnace from the duct work, and help eliminate transmitted noise. The con side is that, accordingly, rodents can easily eat their way through the ducting.

There is a kind of ducting which is rigid, but which has dense insulation lining on the inside. It is pricy enough that it will be less cost to just buy a 90% efficient furnace.

I’m thinking of replacing the plenum with a bigger one, and using many layers of noise absorbing material as a kind of baffle. The idea is similar to that used in glass pack mufflers. I don’t know if rigid noise absorbing material is going to have better results than soft material. I haven’t found anything that specifies the sound dampening qualities of the material.

Checking the ducting for vibration is a great suggestion. The current ducting has a coat of fiberglass insulation on the outside. I don't know that additional treatment on the outside will help reduce noise. (I’m sure it will help reduce heat loss!) But I definitely want to be sure none of the noise is the result of a sympathetic vibration resulting from the burners.

The real problem is when the burners first off and before the fan engages. After the fan engages you can’t hear the drum roll any longer.