Reducing Noise from an Oil-Fired Furnace

I recently was notified by a neighbor that noise from my furnace was causing them some concern. I typically operate my furnace 2 to 6 hours per week in individual 2 to 3 hour sessions ordinarily during daylight hours. (I have had a few early evening pours.) Their house is located about 180 feet from my outdoor furnace work spot (borrowed space on a friend's land) which is located in a rural area where moderately expensive homes are scattered on 2 to 5 acre plots. Theirs is the nearest home and the only one in direct line of sight. Modifying operation times to midday was not a sufficient concession for them.

I set up my furnace the next day for a test run and noted 73 dB of sound ten feet on "my" side of the property line using a free noise meter app on my iPhone. As many probably know about 65dB is a normal spoken voice.

Soooo, exactly how does one muffle an oil-fired furnace? I have read nothing on the subject as I have not come across information about such an endeavor and a search of the web failed to turn up information relating to oil-fired furnaces. I am aware of one general article that contained much useful general information and would recommend it.
https://www.isvr.co.uk/bcffp/forge.htm

After much daydreaming and not inconsiderable consternation (I was not at all confident that I could successfully effect a significant noise reduction at a cost that made continuing this hobby viable.) I cooked up a muffling system that consisted of the following elements:

1) Noise reduction from the chimney which by careful observation seemed the primary noise source.
2) Sealing the joint between the furnace body and the furnace lid. mine was quite loose with a gap in one segment of nearly 1/2 inch. I had not previously been worried about that as it seemed to function as an "auxiliary chimney."

Placing the furnace in some sort of enclosure would have required considerable expense and was ruled out. Placing a barrier between the furnace and the neighbor's house would have been expensive and not likely to work well---see the paper cited above.

I first tried putting a 3 foot long 7" diameter stove pipe on the chimney using a piece of kaowool as a gasket and also stuffing the gap with kaowool. This did reduce the noise by 6 or so dB (not too bad for such a minimal modification) at the property line.

Encouraged I tried a 4 foot section of 10" pipe on the chimney with further noise reduction. Finally, I stuck the 7" pipe onto the 10" pipe with dramatic noise reduction. I used 3 guy wires and turnbuckles to secure the muffler and lined both pipes with 1" of kaowool (3000 degree rated) to prevent the pipes from heating white hot and then collapsing and to make them stable when opening and closing the lid. this arrangement has worked out well and has not noticeably reduced furnace performance. Attached is a picture of the furnace in operation with the muffler in place.

I am not sure how much interest there may be in muffling a furnace. If there is interest, I am more than willing to provide additional details as may be requested. I will say that trying to use the typical fiberglass wood stove rope-like gasket material did not work well. I did not have high expectation for this and did find that it will hold up pretty well if it makes a full seal. But if a gap forms due to metal warpage related to heating, then hot gases pass over the gasket and it quickly melts. Kaowool holds up OK in the short run. It may deteriorate after multiple melts but rigidizer seems to toughen it somewhat---we shall see. I do feel that it was easier than expected (still took quite a bit of work and about 2 weeks of fussing) to get significant noise reduction these changes and I was surprised they worked as well as they do.

Denis

 

Great work. I hope it makes your neighbor happy and keeps you in business!
R

 

Seriously? If it's not past 10pm, I would have 2 words for my neighbor and the 2nd word would be "off"
Where are you at? Just crank up some Metallica and drown out the noise of the furnace. I had neighbors with 3 yapping little dogs that would bark from 7am-10pm all day. I told them I would install a power hammer and take up blacksmithing as a full time hobby. Before the dogs left, I would fly low, slow and LOUD over my house around 3am. (I live on final, it is what it is)
The dogs have since disappeared. F-ing renters.

 

That's valuable information about muffling the roar of the furnace, makes me wonder what other designs would work too.

 

Yes, that is the first inclination in situations like this. However, permanently alienating neighbors can have unpleasant and unexpected consequences. Not to mention the county noise ordinance which prohibits "loud or unusual noise." Is that vague or what? And positively guarantees 1 year of legal proceedings if push came to shove. I think most people would not have objected to the level of noise generated. But, the objection was raised....

Indeed, I wonder what other simpler and more effective designs might be possible. What I did came purely from intuition (a fancy term for farmboy engineering) and some good luck. For instance would just 5 feet of 7" pipe work as well. How about 7'? Would that work better? What about an elbow on the top pointing sound away from the property line? Would a 2" section of 15" diameter pipe work well as all of what I tried? These are tantalizing questions that I probably will not answer as this seems to be "well enough."

Hopefully we'll hear more from others who have done or will do some work along this line. I suspect I am not the only one with noise sensitive neighbors.

Denis

 

Glad you found a solution. Looks like it probably cost less than building a baffle wall around your set up.
When we lose power & I run my generator (a large job-site model I have to baffle the noise. 4 movable plywood walls. Serious p.i.t.a. but it keeps the geriatric hysterics to a minimum.

 

My furnace is making a bass rumble I can tune deeper by sliding the nozzle/combustor in the tuyere away from the furnace that I would like to filter. I was wondering if a series of giant washers inside a steel tube with different random spacings (like a silencer) would work.

 

A baffle wall was my first thought. But, I would need to be quite large as it would otherwise interfere with pouring and related activity. In addition, the noise generated by my furnace is a relatively low frequency and therefore long wavelength compared to IC engine noises. Long wavelength sound tends to not be effectively inhibited by line-of-sight berms, walls, etc., Not saying it would not work, but seemed likely not to and would be very inconvenient for several reasons.

Denis

 

Would that be illegal? ;-)

Sounds like a worthy experiment and not too tough to do.

I had a similar thought about rings in the exhaust pipe. Simplicity, not elegance, was the deciding factor for what I did.

Denis

 

A quieter blower solution for me has been to use a 20v battery-powered leaf blower. I power it with 12v DC from a 12v power supply. The battery-powered blowers are much quieter than gas blowers and shop vacs. I intend to put mine in a box today as that should really quiet it even though its contribution to noise is relatively small. Since I reduced the sound from the furnace I notice the blower more and enclosing it is simple.

Denis

 

Sorry about the trouble. Unfortunately even if your able to muffle the noise, just the discernible sound of it will probably set them off from here on out. And heaven forbid an odor or plume of smoke heads their way.
My neighbor on one side is about 300 feet away across an open lot which is unobstructed and mowed. On some days I can hear their conversations across the yard as if they're standing 20 ft away, and other days I can't hear a thing. Im sure it depends on other environmental noise and possibly atmospheric conditions but I'm not sure.
My operation is behind my barn and out of the direct line of sight (or direct sound waves) from either of my neighbor's houses, so I'm sure that helps. My other neighbor wouldn't say shit if he had a mouthful of it anyway. I've never left my furnace running so I really have no idea how loud it is over there. I just lost a very old low hanging apple tree to heavy snow load last year alongside my work area that I think provided quite a bit of additional noise blockage as well.
One action that I felt was necessary was to put my blower (an old Kirby vacuum) inside the barn and run 2-1/2" PVC through the wall as well as the router speed controller that I use to run it. It's a screamer and that noise outside would definitely raise eyebrows. Even a castle blower or some other blower which may seem reasonable up close to the action may be an audible irritation a couple hundred feet away I suppose.
I've been considering putting up a couple of sections of stockade fence where the tree used to be to further minimize the noise and block the view of my sometimes unsightly work/storage area from the road and neighborhood, both out of courtesy to my neighborhood and for general privacy.
I use the hole in my lid for monitoring, loading and preheating so the chimney would be a significant issue, plus the sight of it might draw other unwanted attention not to mention making the lid a pain to open.
You mentioned that physical barriers weren't really an option, but they're the controls I use, such that they are.

Pete

 

I am figuring any back-feeding noise from the tuyere will be contained by the blower box. The box will have a long inlet pipe—-a known compressor muffler dodge. But, you raise a good question about backfeeding noise.

I’ll post info on results when available.

Along this line, small leaks in an enclosure can allow disproportionate amounts of sound leakage as I was warned and have observed. Plugging them makes a difference.

Denis

 

Interesting conversation, and sorry about your neighbor issue.

I guess I have to consider myself lucky. The few times I have fired up my furnace so far have found me right out in my driveway. Typically my immediate neighbors end up coming over to "check it out" at some point. The guys anyways ... most of the gals just shake their heads.

 

Are they they the type of neighbors you could just "butter up" with a bottle of wine and box of chocolates and let them know what it is, that it doesn't last long but it's a good indication you're out and about, keeps you out of other mischief....and your outside presence is good for neighborhood security and all that.

 

Sucks about the picky neighbor. Strange they don't see reason during the day. It's not like your doing it at bad times or constantly or for very long.

Maybe a concrete cinder block wall with some padding on one side will help reduce noise.

I cast metal for 5 years in the same spot in the same way and one day at 8 pm some divircee guy apparently with his kids over his house walks up out of the dark and says his kids were afraid to sleep because they heard a "monster" outside. His house was very far away (maybe 500 to 600 feet away) and up the side of a hill with many obstacles in the way so I really don't know how those kids heard it. Or if they were just in a new place for the first time and hearing other things and I happened to be the brightest beacon at night on his stroll. Only time I ever got asked to be quiet. Although we do have new neighbors right next door so we will see what fuss they make when I next cast.

I think a lot of the nuisance noise the furnace makes is transmitted through the ground as a low frequency sort of sound which can travel a long way. Perhaps putting the furnace on a bed of gravel will reduce the sound that reaches your neighbors? Older people generally lose high pitch hearing and so the lower frequency rumnling sounds are probably a bigger irritant to them than the high pitched sound of the blower.

 

Once your ears or head are tuned into a particular noise most people become hyper sensitive and alert to it. Human nature that probably dates back to wielding clubs and dragging the women around by their hair!! A small amount of reduction in dB might help, if nothing else they can see you've made an effort. What Jimmy said is a good point, the minute you can difuse the tension the less chance of a roadblock.

I grew up with the neighbours from hell, they objected to my Dad's workshop even though he made little noise, their retaliation was DIY on the party wall (terraced house) at 6am on Sunday morning and frequent over the fence battles. If we had friends round till late we were phoned by them to stop talking so loudly! (12" solid brick party wall, go figure!)

Edit: 'party' wall has nothing to do with parties, it's the dividing wall between ajoined properies in the UK.

 

I have the same problem, although not quite as bad yet.
I am situated in a cove, and my yard is pie-shaped.
My driveway is on one side of the house, which puts all my foundry activity pretty close to the side of my neighbor's house.
For my last pour, she came out of her house and said "What the heck is that noise?".

Luckily I repair all of her appliances (heating/HVAC, riding mower, etc) and anything else that breaks, and machine her custom metal parts for what breaks, so she is on my side.
One day she will move away though (she is not young), and the new neighbor will not be so tolerant, and I will have to do things differently.
One thing in my favor is that I am backed up to the expressway, and so the ambient db level is very high already.
But the furnace makes a deep rumbling/roar, and is distinct from the expressway sound.

So far the neighbors behind me have not complained (a row of condos), but they are down a hill from my house, and the sound may go up above them, or they may just ignore the sound.
The are quite a bit further away than my immediate neighbor.
I am right in the middle of the city, and while my neighborhood is not as dense as many, I still have a lot of neighbors near my house.

I had to put rubber pads under my air compressor to prevent hammering the concrete floor (this was recommended by the air compressor manufacturer), and this seems to work well, but the air compressor noise is mostly high frequency (2-stage unit).
I am going to try rubber pads under the three wheels of my furnace, to break the ground transmission of noise.

As someone above mentioned, I am going to move my leaf blower inside, and that will help with the high frequency whine, but it is the low frequency that is the main problem.

There was a jet engine test stand at the airport about 5 miles away (this has been a few years), and they figured out how to direct the engine noise up into the air.
One person told me that sometimes the noise would bounce and come down in unexpected places, but I have not been able to verify that.

In my case, a concrete block wall would make more sense, if it would indeed direct the noise straight up.
They have installed concrete walls along much of the expressway near my house, but not along the stretch near my house, and I have stood next to one of those walls (on the side away from the expressway), and they do a remarkable job of stopping the road noise.

They are working on bridge replacement today, and so the expressway next to my house is closed.
The difference in noise level is shocking when the expressway is closed.
If I did not make so much noise myself I would move to a more quite neighborhood.
I often do hot work indoors at night just to limit the noise (grinding, welding, hammering), and there is a risk of burning down the shop, but a greater risk if the Code people show up and start asking questions. Once the Code people show up, it is pretty much all downhill from there; they are not reasonable or logical people by any stretch.

I have also considered a swing-away elbow/pipe that would be cooled by a leaf blower, but what a complicated affair that would be.
The vertical stack is a pretty good solution if the ceramic blanket holds up over time.
3,000 F ceramic blanket is probably not cheap, but cheaper than a Code cease and desist notice I guess.

A few things I have noticed is that there seems to be less noise with less compressed air.
I am running 30 psi compressed air, and that is as low as I can run.
The leaf blower I use is a Toro variable speed, and it is quieter than many blowers, but still makes quite a bit of noise (I can put it inside the shed).

A slower combustion air flow seems to be quieter, but I have not used a db meter to verify that.

I will have to ponder my next move.

I have seen a few guys in England that use furnaces inside their shed/workshop (due to weather I guess), and the exhaust goes out the back side of the furnace, out a tuyere that is up high in the furnace.
The exhaust goes up a big iron stack outside the shed.
The problem would be with overheating the elbow coming off the furnace, but it could perhaps be air cooled to keep it in a reasonable temperature range.