We have a combi boiler, and recently when you run the shower after turning the hot tap off there is a large banging in the pipes. It only seems to happen with that specific tap (as far as I can tell).

I read online about water hammer, and I think that could be what's happening. I think I might need to turn of the water supply and drain the system, but I'm not sure if I need to turn the taps off/on in a certain order?

2 Answers 2


If you already have water hammer arrestors (you probably do, since this is the only tap that hammers), you'll need to turn off the water supply and drain the system to the lowest point in the system. You'll need to leave that drain point open for quite a while, depending upon the arrestors you have. When it's fully drained & the arrestors are all full of air again, just close the drain & turn on the supply again. No particular order to things.

Do expect quite a lot of sputtering at each tap the first time you turn them on.


Easy fix, if it is water hammer, is to install a water hammer stub -- just a capped pipe on a T downstream of your boiler. If that doesn't fix it no harm done.

You should also ensure that you have an expansion tank in the system. You'll need to check your boiler manual to determine if it has a backflow check valve, in which case the expansion tank also needs to be downstream.

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