I need to make a muffler for my furnace vent

We discussed similar issues in Alt.Energy.Homepower. One design that works, more or less universally, well, is to terminate the pipe into a pile of gravel. You may want to put a piece of plywood on the pile, to keep it from freezing into a solid mass in winter. For 3" outlet, you can also try a suitable muffler from a farm and fleet store, I believe that they do have 3" muffs and all kinds of adaptors.

Pipe it into a bucket as you did, but fill it with pea gravel and drill a few drain holes in the bottom so it can't fill up with rain water. That should difuse it pretty well.

Pete C.

Grant Erw>

For openers, QUIT USING THE WATER BUCKET before you CO poison yourself!! If the vent fan has to work very hard it will just back flow the furnace fumes!!!

A couple others mentioned using gravel. Around here it gets down to the

-20F to -30F, the water vapor in the exhaust is enough to form a pretty big mound of ice around the outlet. I can't imagine the gravel not getting clogged with ice in cold weather.

Most of the noise comes from the high velocity of the ejected gas. If you can double the size of the pipe you get 1/4 the velocity. Add > I have a boiler in my house which is vented to the side at ground level

I just discovered that even after emptying most of the water out of my bucket so that the fan could move the water and "blow bubbles", there was still too much back pressure and the boiler wasn't igniting. Once I took all of the water out of the bucket it ignites fine. So I need a real low back-pressure muffler. And I was wrong, the ducting is itself black 3" metal tubing, it's just the terminating 90 elbow that looks like plastic.

So I'm really stumped. I have to quiet this thing down and my best thinking has so far gotten me very little.

GWE

Ah, that makes sense. Run a short piece of 3" into a 3-6" conversion joint then into a 6" 90 going sideways .. I'll price that out tomorrow, thanks.

GWE

Do you have a decibel meter? They are about $50 a pop on ebay. That will help. If you live in Chicagoland, I can lend you mine.

So, you have a 3" diameter pipe coming out of your house. What kind of noise does it make? How far out does it stick?

If you build some sort of a wall in front of it, it may substantially reduce sound transmission towards the end opposite of your house wall.

I reduced noise from my Onan DJE generator, in the needed direction, by 10 decibel, doing a few simple things. I am optimistic for you, you have a much smaller noise source.

How hot is the exhaust gas?

Grant -

How about a muffler ? You might want to put a ICE detector and a heater to prevent ice buildup. These are very common on air conditioners (commercial) that run in high humidity areas.

Auto or such - even if it rusts out every 5 years, there is a replacement.

Mart> I have a boiler in my house which is vented to the side at ground level

Grant, Try venting it into a plywood box lined with 6" fiberglass insulation (no paper backing) and with several 1" holes drilled around the bottom for the air go get out, it should quiet it down pretty well. Ken.

Backpressure on an exhaust vent is *BAD* and can be dangerous. If you really need positive pressure venting you need a burner that is designed for it such as the Riello units. Even then you have to be extra careful that all the stack connections are actually sealled in that case.

The bucket of pea gravel should diffuse the flow and hence the sound with little increase in back pressure. I've heard of similar setups used on a larger scale for big diesel gensets.

Pete C.

Grant Erw>

On Sun, 20 Feb 2005 21:57:33 -0800, the inscrutable Grant Erwin spake:

Maybe the truck junkyards are open today. Look for a diesel muffler and stack which you could run vertically above the house. The vertical direction would take away a lot of the noise from her house and the muffler portion would further help, with little to no backpressure.

I think your concern is commendable, but if you restrict your flue you could end up killing yourself and your family.

Greetings Grant, If the gravel idea given doesn't work fpr you, how about a piece of 6 inch diameter duct rilled with fiberglass insulation. It seems like the back pressure would be low and if the outlet points down it wouldn't get rain in it. Eric

Think of how mufflers on trucks and cars are made. Run your 3 inch pipe into some six inch pipe, but modify the coupling so the pipe can extend into the six inch pipe. Now drill a bunch of holes in the three inch pipe so they will be inside the six inch pipe. Put a cap on the three inch pipe and drill a couple three holes in it too. Now make another three inch pipe that looks the same. The drilled section should be a couple of feet long or longer. Now your furnace exhaust will run out in the three inch pipe, out thru the holes into the six inch pipe Along the six inch pipe and then thru the holes into the other three inch pipe and out to the air. Some fiberglass insulation inside the six inch pipe would make it better especially between the ends of the three inch pipe.

Dan

Grant Erw>

Even easier is make a glasspack. 3" pipe drill as many small holes as you can(3/8" on a 1" grid). Put screen around that followed by loose fibre glass insulation, surround that with another pipe. your exhaust path is straight through the 3" pipe, the glass adds damping for the pressure pulses. Pat

You've already gotten fair warning about messing with the back pressure of your furnace--how about just putting up a sound barrier around, or in front of, the outlet? Either a 3-sided box of foamboard, 16" each side, and 8 ft high, against the house--or 32" per side, 4 ft high; or just a full sheet of foamboard, propped up horizontally in front of the vent.

Ken Grunke

I'm not convinced any of the holes are necessary. My built in vacuum cleaner came with twin mufflers, which are exceedingly affective. They're a square box, approximately 4" on a side, and perhaps 10" long. It's lined with foam rubber, maybe ½" thick. Vacuum discharges in one end, and the opposite end ducts to atmosphere. The amount of noise reduction is unbelievable. It could be that it's suppressing high frequencies instead of lower ones, so I'm not sure it would work as well for a burner. Perfectly simple, and little, or no back pressure. Fiber glass could be substituted for the foam rubber.

Harold

I was going to chase you up on a new thread, but here will do... You said in a different thread that you were using the built in vacuum cleaner for the shop while you build the house. and this caused me to go looking in Internet land. Do you get any problems with turnings or oil loaded material at all?

Regards Mark Rand RTFM

(snip)

I wonder if just redirecting the flow straight up for 10 feet or so with flue pipe wouldn't solve the problem. Boiler burner noise isn't like engine noise.

A vertical stack would not add any appreciable back pressure, and higher frequencies would propagate up rather than sideways.

You may have a situation where the distance between houses creates a reverberant space at some lower frequencies. That can amplify noise quite markedly. I once lived about half a mile from a stock car track. On Saturdays, if I stood between my building and the adjacent building the noise was very loud, though it was barely noticable in front or back yards.

If you have that situation, venting the exhaust up and above a reverberant space would greatly attenuate the acoustic coupling.

Aside form doing one really stupid thing, having two lines come together with a wye, pulling from opposite directions, no I have no trouble. Not that it can't happen. It's not real smart to vacuum stringy chips. Short ones work fine, even the heavy ones. If, by chance, you manage to get a blockage started and your lines are buried, as mine are, you may not be able to undo the problem once it gets sufficiently blocked off. You usually don't know that's happening until it's too late.

Our vacuum is killer big and tough--with two motors. It's rated for something like 18,000 ft of commercial building--so it picks up damned near anything that comes near. I spent an afternoon retrieving a short parallel it sucked up a couple years ago. I was lucky, I got it back and didn't lose the use of the system. Had I not, it would have plugged it off and rendered that section worthless.

Oil doesn't appear to be a problem, but I also don't vacuum up oil, only what may be on chips. Our vacuum does *not* have a filter bag, and that's by design. I've used enough shop vacs to know one with a filter is a mistake. Of course, if you must discharge in the shop, you may have no choice. Our system has a cyclonic separator that traps all but the finest of dust, which is discharged outside. So far, any time I've emptied the canister, it's very dusty inside, with no sign of any oil. I'm of the opinion they'll handle a tiny amount without any negative consequences.

If you're interested, ours is a Vacuflow 960. They may be the only vacuum on the market without a filter bag. Dunno. I chose it because of that feature, and I have no regrets. Top notch piece of equipment, although not cheap.

Harold