On 22 Jan 2007 08:51:03 -0800, "adder1969"
wrote:


T i m wrote:
Hi All,

Some good friends asked me if I knew any good heating engineers and
pre passing any names on thought I'd have a quick look at it myself (I
had helped them change the blower in it a few months ago).

It goes like this ..

Power on, fan spins (good blast of air outside) .. 30 sec later gas
control solenoid pulls in (presumably 'pilot stage?) and electronic
igniter tries to fire up the pilot. After a few tries (it's always
been like that apparently) the pilot lights but immediately (.75 sec)
goes out with a loud sort double clunk from the main gas valve and the
cycle repeats.

I've checked all the obvious (air pressure / flow? valve switching
over, wire all intact, pipe all in place etc) and am about to get /
fit a new HT lead as I thought I heard it arching out somewhere and
also though it was part of the flame failure detection cct (and
hopefully it's cheap) but I don't feel confident it is that?



Hmmm 3/4 of a second isn't enough for the "thermocouple" to react so
I'm not sure what to suggest really.

I don't think this design uses a 'thermocouple' as in what I'm used to
(a probe sticking into the pilot flame) but some for of 'sensing'
between the igniter lead and using a side effect of the pilot flame to
'sense' if it's lit or not.

On mine it was fairly easy to
check and/or hardwire all the sensor inputs to both give some
supervised heating and also to see where the problem was.

I had thought of doing similar but without any idea what goes on in
the control unit (and therefore the in / output logic) I didn't want
to make anything worse.

for me it
was the themrocouple whcih was actually a mercury vapour switch and
usually a second or two to react. I got mine from america new for £15
when they were otherwise £100 in the UK.

I changed a std thermocouple in my Mum's boiler a while ago and it was
under a tenner.

So, boiler on, control calls for heat, fan blows/sucks which I guess
then allows the pilot to open and the ignitor to spark, pilot lights
then something goes wrong.

Yup, spot on ;-)

So what's the next step in that process?

Well, I think what should have happened is after each spark is it
checks to see if there is a pilot flame and if there is it opens the
main gas valve to the burner (inverter flame detection). I believe it
does that using the same lead as it uses for ignition to ground?

Boiler temp input?

Currently calling for both heating and water and boiler stat is
working (tested).

High stat input? ..or do they come at the beginning?

Not sure, I would assume the high limit stat (thermocouple?) would
just cut the gas off somewhere (being a safety rather than std
'regulation' thing)?

Strangely though, we rang a relative of theirs who had just been
through similar, he had bought a new control unit and found it no
better. Called a heating engineer in who diagnosed a low gas pressure
setting (not sure what / where), suggesting that it had never been
installed properly in the first place but had been working fine for
the last 3 years?

All the best ..

T i m