My Chaffoteaux & Maury Britony combi has an intermittent problem. About a minute after turning hot water off, it starts to make a g-g-donk sound, about once a second for about 30 seconds. Next time I turn a hot tap on, it doesn't try to ignite, locks out (red warning LED lights) after 8 seconds, and has to be reset. It then starts normally. The selector switch is set to DHW only, i.e. central heating is turned off. I think the noise comes from the bottom right corner but can't be sure. Does anyone know what could be wrong?

Tony, August 2008

This answer is from the guy who put the question in the first place. My boiler is now working perfectly.

I found 2 problems. First I suspected there might be air in the system, so I replaced the air separator. The old one was well and truly gunged up, so it might not have been working properly, but it didn't fix the symptom.

Second I bought some descaler from my local boiler repair place and descaled the secondary heat exchanger. Loads of black stuff came out of it. Bingo - problem fixed!

Tony, February 2009