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!!