Noisy bolier flue

[ephraim_pule]

I have recently had an oil-fired CH boiler installed in an outhouse adjacent to the house. The flue runs vertically up and thru' the roof about 12 feet or so high. Unfortunately, it's very noisy. It seems that the boiler's fan and the burner resonate with the flue so that it amplifies the noise to the discomfort of our reasonable neighbours. It emits a loud low rumble which is a tad annoying first thing in the morning. It is an exceptionally quiet location so the noise can be heard a long way off.

I am wondering if wrapping the flue with a layer of something which won't catch fire such as loft insulation glass wool would help damp the sound. The flue gets quite hot so whatever I do has to use something that won't burn.

Has anyone had this problem? Solutions?

EP

[noelogara]

Is it a tin roof? Most of the noise is probably coming from the boiler. Try insulating the roof of the outhouse and the door which should be well sealed and of course any window. put a seal of heat sealant around the pipe where it exits the roof to stop those sound vibrations. If you had a few fire resistant sheets to put around the boiler itself that would help greatly.

[ephraim_pule]

The flue goes up thru' a corrogated roof but I'm pretty sure that it isn't the roof that's the problem. It really is the 200mm (or so) diameter flue that's resonating. So it needs damping,

EP

[Andy]

Your options are to:- (a) change the length of the flue to alter its resonant frequency (b) coat the flue in sound absorbent material to damp the vibrations (c) Add mass to the flue to change ( lower ) the resonant frequency away from whatever is exciting it and lower the amplitude. (d) Stiffen the flue to raise the resonant frequency above whatever is causing it.

I am assuming it is the flue vibrating, and that the noise is not caused by the flue acting like an organ pipe. If that were true, the solutions would be different.

If you lag the pipe, that will both increase its mass and ( if you use high-density rockwool perhaps ) damp it too. Changing the length of the flue would be difficult and no guarantee of success, and stiffening it might only be a little easier.

Andy.

[Andy]

PS:, A pipe 12 feet long could expect to have a fundamental resonant frequency of 45Hz, ( half a wave length ) so it might indeed be acting as an organ pipe. If all the sound is coming out of the open end, as opposed to being emitted by the pipe itself, I can only imaging that there is some sort of problem with the way the combustion chamber is interacting with the length of pipe - a tricky one to solve, I can only imagine lengthening the pipe or having some kind of muffler on the end working. If you have a few feet of similar pipe and can stick it on the top, if the noise changes pitch that'll be a clue.

Andy

[noelogara]

What length of flue is inside the shed and what is outside? Is there a lid at the top of the flue and is it securely bolted on?

[The Natural Philosopher]

You CAN get a double walled insulated flue..but it ain't cheap.

I am concerned that you don't seem to have an insulated flue already..how hot does it get?

I THINK rockwool will take a decent temperature. BUT its probably better to use MASS rather than padding to get the noise down.

Is there any chance you can box it in with brick or concrete?

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember ephraim [e-mail] saying something like:

Are you sure the burner isn't pulsating? A common type of fault is mis-matched flue and burner settings causing a pulsating flame and sounds like a low rumble. Betcha the installer didn't set it up right.

Similar thing can happen if the burner is running on diesel instead of kerosene - check the fuel.