fred wrote:

In article , Geoff Clare
writes
My boiler is working (it's heating the water that goes through it),
but something isn't right with it because quite often I can hear
the main gas valve turn off, and then on again about half a second
later, while the boiler is running.

It's due for a service, so hopefully the engineer will be able to
work out what's wrong when he comes, but I was hoping I might get an
idea of what to expect from the knowledgeable folks in this group.
All my attempts at Googling for info on the problem have only turned
up other kinds of problems, not mine.

By "quite often", I mean that it typically happens once or twice in
the time between the boiler starting up and shutting off. They are
usually isolated occurrences, but occasionally I've heard it happen
two or three times in a row with a few seconds in between.

The flue fan stays running, and I think the pilot light is staying
lit, as I don't hear the igniter spark, although I could be wrong if
the spark happens at the same time as the valve turning off (the
sound would be masked by the solenoid click). I tried watching the
pilot light through the inspection window for several minutes until
the boiler shut off, but as luck would have it the problem didn't
occur while I was watching.

The boiler is a Potterton of some sort, rebadged by British Gas as
a "Celsia F1", and is 15 years old. It had the main PCB replaced
about three years ago, but other than that has not had any faults.


That is not a boiler I am familiar with.

It looks very similar to these:

http://www.partsarena.co.uk/baxi/Sys...999/page23.gif

http://www.rwl-supplies.co.uk/img/boiler1.jpg

if that's any help.

Are you sure it is turning off as it could just be modulating down.

I can hear the characteristic soft "whump" of the gas igniting after
the valve opens again.

Some older boilers can modulate their output but only step between high
and low rather that the continuously variable modulation of modern
boilers. The noise you are hearing may just be the modulation solenoid
switching between high and low which would be normal if it had been
running for some time.

That sounds like normal operation for such boilers - which means
it would have been doing that all along, but it has only started
doing it in the last few months.

Even switching the burner off completely may just
be the boiler thermostat reaching temperature and shutting down the
flame.

Okay, but then it would be odd for it only to be off for half a
second.

What normally seems to happen when the thermostat reaches temperature
is both valves (main burner and pilot light) close and the fan stops,
so just the pump (which is located elsewhere) is left running. Then
when the circulating water has cooled enough, the boiler starts up
again (fan first, then pilot light, then main burner).

--
Geoff Clare