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Central Heating Undue Noise From System 


Hi Jeff


I’ve just read your article on water hammer.


it was very interesting but didn’t quite relate to my situation.


I’ve got a conventional boiler with a feed and expansion tank in the loft.


I get no noise when the central heating is on on its own.


When the hot water is on on its own things are different. The whole process is noisier with sounds of water running around and some whining. After roughly twenty minutes the banging starts and sounds very loud and violent. After about 3 minutes this dies down and goes away, there are no real noises to speak of.


When the central heating and hot water are on together there is no undue noise.


I have recently had the ball cock replaced in the f&e tank but the same symptons remain.


There is a  heating valve in the airing cupboard which is fairly new and doesn’t appear to be making too much noise.


I would very much appreciate your comments on what may be going on and what I need to do to sort this out.


Many thanks


Kind regards


Hi Jeff.


It sound like you have an acute circulation problem in hot water mode.

When the heating is ion there is a lot more of the system to circulate in, when it's hot water only it's just the primarys and the circuit to the hot water. 

A circulation issue will lead to over heating a modern boiler will sense this on heat gain via thermal stats and produce a fault code etc, an older boiler wouldn't cut out so easily so you may be hearing the result of thermal movement, convection of the water may be causing the noise, an air lock is possible, or the pump may have failed causing circulation issues if the heating system is old enough it may be that the heating still manages to work in gravity mode, without the pump. 

Any ketteling, water heated above 100 degrees Celsius, to produce steam will make very loud banging noises the moment the water becomes steam you would hear a bang steam is 1,600 times bigger than water. Is the water next to the heat exchanger is not moving , no circulation, it could be ketteling and producing instants of steam. 


Hope that helps. I would advise you you call an experienced gas or heating engineer if you haven't already.






Hi Jeff,


Many thanks for the reply – it’s very kind of you to get back to me.


I’ve had the BG Engineer round and he replaced the pump and also the ballcock in the expansion tank.


Unfortunately the problem remains and he has concluded that there is dirt in the system and has advised that it needs to be power flushed.


Although I am happy to take his advice if this will solve the problem I am still a little confused. When the hot water is on on its own it runs fine for at least 20 minutes (the new pump is running smoothly at around 45W). To be fair 20 minutes heats the water to a usable level. So for 20 minutes the water has been circulating with no restriction in the pipes.


After this time the problem starts and the vibrations and drumming begin, when this happens the pump drops to around 5W (water not moving?) After a few minutes it recovers to 45W and things go quiet again. Sometimes this process repeats.


Given what you are saying about the hot water turning to steam could it be possible that there is a problem with the thermostat on the hot water cylinder and the water overheating? I have noticed the boiler clicking on/off/on quickly when it has been set to hot water only.


Anyway Jeff thanks again for your help and advice which I very much appreciate.


Best regards,




Hi Jeff.


I think you need to change your boiler stat/overheat stat. My theory after 20 minutes it reaches an adequate heat but the stat has failed so it continues to heat and the system overheats. Possible the over heat stat is working as the boiler cycles on-off-on-off, but the boiler stat has failed. Change them both way cheaper than a power flush, so if it doubt try the significantly cheaper option first.





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