G'day people. I couldn't find a previous reference here, so I hope I'm not covering old ground again. If I am, I beg forgiveness and hope somebody can help. I have a 6-year old Potterton Kingfisher CF60 boiler that is overheating. It is in a simple system with gravity hot water and pumped heating. When programmed for hot water only - no heating - it occasionally (gradually becoming more frequent and now occurring about once or twice per week) boils so that water is forced out of the vent pipe into the header tank. If switched off completely and started again from cold (typically left until next day) it seems to work ok again - until the next time! I am unable to do the work myself, so a couple of weeks ago, I had a Certified chappie service the boiler and put some dispersant/detergent goop into the system to stir up the black sludge ready for draining and refilling. The overheating problem was in evidence before this, and it did not suddenly get worse after, so I don't think there is any connection between the events but thought I should mention it anyway.
After the usual eruption last night, SWMBO turned the boiler off, the way she does, by means of the control on the front of the unit. I switched it on again this morning, and it seems to be working ok again as expected. One thing I did notice, however, is immediately after the gas shut off (after a fairly short heating time, as the control is on a low setting) there was a clicking sound, like a relay operating, five or six times, as if it was trying to re-light, which of course it didn't do until some minutes later, after the heat exchanger had cooled down. This clicking only occurred for a couple of seconds right after the flames shut down, it wasn't continuous.
The User Guide, explains that the boiler is fitted with a "safety thermostat to protect against overheating of the water" which if it operates shuts the boiler off, requiring a 'Reset' button to be pressed to restart it. The red warning light has never illuminated, the boiler has never "locked out" like the Guide says it should and it can always be restarted without the button being pressed.
I found several very similar posts to this in other groups on the Interweb, but none that I could see explained the cause and and how to fix it. I am concerned that this problem seems fairly common - especially in boilers of a similar age - and I am worried that the 'overheat protect' system allows the water to boil. I called my CORGI (or whatever they are called this week) registered man a couple of days ago and he is coming tomorrow to flush out the system and look at the faulty boiler again. I would be most grateful if someone here could help me resolve this problem. I have, of course, told my CORGI man about it, but the boiler operated faultlessly - like they do - when he came before, and he was (probably quite correctly) reluctant to spend money on buying replacement parts until he is sure that they will effect a cure. I won't say cost is irrelevant, because obviously I don't particularly want to waste money, but I am anxious to get it fixed. If items 'A' or 'B' could equally well be the cause but we don't know which one actually is, I would rather replace 'em both than spend more time trying to find out.
Help! Please?
Mike. (Hatfield, Hertfordshire)