Clanging heating- water hammer? HEEELP!!

15 Nov 2006
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United Kingdom
Hi all,,, got the above problem, 50% of the time the heating starts, pipes clang alarmingly. Its definitely water hammer, as I can replicate the sound with opening/closing taps violently. The system is cold when it happens, always. It makes no difference if the controller is set to 'heating and hot water' or just 'hot water'. All rads are well bled, no air has been present for a year or two. Some time I have to bleed the inlet hot pipe to the indirect cylinder, which is inconclusive as to whether it makes any difference as it won't always clang; my system is 20 yr old and has an expansion tank in the loft. No water ever comes out of the overflow from that tank. Sometimes not often-one rad will not get warm, no air present, and turning one or sometimes 2 others (random) off will start the non heating one up again, whereon the ones turned off can be put back on, and all work from then on. This need to turn off and back on does not hapen often. Otherwise, it all works OK, but the clanging!!I will add, that it was powerflushed about 5 years ago, crap readings afterwards were low, (mangnesite etc) it's not kettling as it runs quiet when working, it's just right on cold sart, for upto 5 seconds only, or less. Any ideas? MTIA!!
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i bet my xmas turkey it's not water hammer, sounds more like the boiler is firing before the pump kicks in

safe bet mind you i haven't got a xmas turkey
Hi CBF,, thats what I thought, but if I put the heating on, stood near the boiler, when its quiet, I can hear the boiler start up, also the sound of that is not that loud, so I can hear the pump start too at the same time. I thought underrun/overrun, but don't know where the stat is on the boiler; its a thorn appollo 40b. If it has one, I would replace it and see what happens. Though the pumps seems to stop in time with the boiler going off. Hmmmm.
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Just replaced overrun stat, made no difference- found pump does not overun! it goes off as soon as timer shuts heating off. found also one radiator is now stone cold, have bled the system all over many times and no air in it. the cold rad I have bled with 1- both inflow and balance valves shut, 2- alternate valves open on their own 3- both open together. water within is same colour as rest of system, and comes out with as much force as any working rad, i.e. healthy flow. so assume no blockage. just now, we need it on but it won't play. grrrrr only other change apart from stat is put boiler output from lo to hi. Anyone know what may be up with it? also, why the pump does not overrun? it looks like the old stat was ok then, but can't think why pump stops dead with boiler going off @ set time, instead of running for cool down period. Heeeeeellp!

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