This summer we replaced our boiler (hot water baseboard system) with a Buderus natural gas boiler. The unit heats very well, but we have recently noticed that in the afternoon, when the boiler is not (and has not) run for a while there is a single, very loud noise (shakes the house) and lasts for maybe a second.

I have read a lot of posts about such a noise when the unit comes on, or when it has just shut down, but this appears to be when the system is largely cooled (hours after it was last on). I had thought it may be air in the pipes (the system had been drained to evaluate the pressure gauge which is defective and will be replaced, and then the water was turned back on but the system was not purged). However, the plumber was out last week and purged the air and the noise is still there.

We had also suspected that it could be a backflow valve noise, as the fill valve also appears to be defective (we also get water leakage out of the pressure relief valve intermittently unless I shut off the water supply manual shut off), however, I left the water supply on today and the noise still occurred.

Any thoughts on what this could be? We just bought the house in the Spring, so it could have always had this noise, but I was not sure if anyone had heard of this.

 

This could be the piping cooling off and contracting...

Do you ever hear a similar noise, or series of smaller noises, as the system heats up?

By the way, you get the prize for being my 10,000 th customer! I rolled over the odometer and didn't even notice!

 

No that is what is odd. It is only something we hear during the day after the system has been off for a while. It may be that when it is heating up it is gradually moving against some part of the house, but when it cools it does it all at once creating the noise?
It is just odd in that it is pretty loud, and strong enough to feel through the floor. However, this could just be because when it happens we are downstairs (on the main floor) and the heating pipes run under that floor, in the walls of that room and above on the second floor.

 

That's pretty much exactly what happens... almost like pressure building on a fault line and it alla sudden let's go. Those expansion/contraction noises can be explosively loud! then the piping 'telegraphs' the noise and vibration through the whole structure.

Is much of the piping exposed in the basement? If so, examine any that you can see to try and determine if the installers shoved a pipe through a too small hole, or if they are strapped tightly to framing members, etc.

Finding these noises and correcting them can be extremely frustrating... you never know exactly when it's going to happen... so you sit and wait... as soon as you get up to get another beer... BANG! ... but you need to try and localize where it's coming from.

Loosen clamps, lubricate holes... one trick that has worked for me is to cut plastic 'shims' from milk bottles and slip them into the tight spots. Some folks have used a silicone spray lube with success, but BE CAREFUL! that stuff will STAIN flooring, furnishings, etc.

This is even harder than determining which smoke detector is 'chirping' !

 

Lukily the basement is open- so I will start there, but I havea feeling it is one of the pipes on the second floor which will make it much tougher of course! At the end of the day it is once a day, and we are not generally home so it is not that bad.

Thanks for your help!