Banging Pipes - Slightly Different

[alho]

My son occupies a rather large victorian dwelling.

The heating is open vented system with rads, heated by gas fired boiler. (which only does the heating) Hot water is supplied from a gas water heater, seems rather like a combi without the heating side, mains cold in one end hot out the other. This supplies about 4 taps and we think a shower. The mains cold feeds 3 wc, 5 taps and the shower. All pipework copper.and not really excessively long runs. Mains pressure is nothing spectacular. Every so often there is a quite loud bang from the pipework, sometimes 2 in succession, loud enough to be worrying. It still happens when the heating is off, so I think we can discount a problem there. He has tried to narrow it down as to when it happens, and the nearest he gets is that it always happens about 15 minutes after going to bed (no comments please!), then the only water that has recently been used is cold for teeth cleaning before retiring. (from a monobloc basin mixer, with a lift up handle) It does happen at other times, but during the day it is more difficult to pin down what taps have been used.

Any Ideas, especially as to why it happens about 15 mins after all taps turned off. Thanks Alec

[crb]

I had a similar problem years ago. I traced it to an intermittently- sticking float valve in the cold water tank that occasionally did not fully turn off the inlet flow completely.

CRB

[Cicero]

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It might be associated with the monobloc basin tap which effectively takes mains pressure water from two directions - one flow through the water heater (multipoint) and the other direct from the main.

If the basin risers have accessible isolating connectors you could experiment by turning off the hot flow at the isolating valve an hour or two before the last use of the basin. I've no idea if this will definitely pinpoint the source of the banging but it's a simple thing to try.

The same probably applies to the shower so you could try that also at a different time.

Cic.

[SS]

I have something similar, I get a loud clunk about 5 minutes after flushing the toilet, I only hear this when lying in bed as the cold water tank is up above in the loft . I have put this down to A a faulty valve thingy in the cistern or B a loose pipe somewhere, I have 2 other toilets but only happens with one of them. I gave up trying to trace it. It seems to occur after the cistern refills and switches off by way of the valve.

So maybe look in these areas, try and eliminate the loo as possible reason.

[Peter Parry]

1. Ghosts (common in Victorian houses with murky reputations).
2. Ball valve in cold water header tank in attic. The tank is refilling slowly and as it finally closes you can get water hammer as it oscillates between closed and open, this is heard as a series of loud bangs. Especially common in a hard water area. You can change the washer and clean it all up but it is often simpler and quicker just to change the whole valve and float assembly.

[alho]

Thanks for your thoughts gents

Will experiment with mmonobloc.

Not valve in CW tank as no CW tank.

Did myself think of ghosts or burglars one night, evwen got up and checked house inside and out. Noise is not like normal water hammern it is really loud and sounds dangerous.

Thanks anyway. Alec