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. 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. 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.

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. So what's the next step in that process?
Boiler temp input? High stat input? ..or do they come at the beginning?