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)