Been having a few recent problems with the old boiler - no not 'er indoors!

My c.1990 Baxi Solo WM70/4PF has intermittently failed to fire in recent
weeks. I think I've now fixed it, but the whole topic is moderately
interesting - to me at any rate!

It was perfectly ok for 99% of the time but occasionally - usually when
it was hot from having run normally for a while, when it next needed to
fire (either on its own stat or the room stat) the pilot would light ok
but the main burner wouldn't - so it would sit there, making all the
right noises, with the fan running, but virtually no heat!

AIUI, the start-up sequence (when there's a call for heat and its
internal stat is not satisfied) is as follows:
1. The fan is switched on
2. The pressure switch detects pressure
3. The gas valve's pilot supply is turned on
4. The spark generator is turned on
5. The pilot flame lights
6. The flame ionisation detector detects that the pilot is alight
7. The spark generator is turned off
8. The gas valve's main burner supply is turned on

In my case, all of this appeared to be happening except for the very
last step. Looking through the inspection window, I could see that the
pilot was alight (albeit a bit weakly perhaps) and there was no sparking
- so the flame detector seemed happy that it was alight.

Duff gas valve, I hear you cry. But no - no voltage was being applied to
the main solenoid. And, if I temporarily bridged the pilot and main
terminals (don't try this at home, children!) the main burner would
fire, and away it would go - accompanied by some relay clicking on the
sequence board, which would stop after a few seconds - whereupon I could
remove the bridge and the boiler would continue to run ok - until the
next time.

The diagnostic chart in the installation manual suggested that the
sequencing board was duff, but I wasn't convinced because the problem
had first started when we'd had some very high gusty winds - and the
pilot flame hadn't looked quite right since then, suggesting that some
dislodged crud might be affecting it.

So, a couple of days ago, I decided to have a close look at everything
connected with the pilot flame. The pilot burner is very easy to remove
(a couple of screws on the front of the combustion chamber). I took it
all apart and cleaned it very thoroughly, including blowing through the
supply pipe and injector (small cup-shaped affair with two small holes
in the bottom) with an air line - and then put it all back together and
it now seems fine (say he, with all fingers and toes crossed!). The
pilot flame certainly looks a lot healthier.

This raises an interesting question in my mind. Any experts out there
(PING Geoff) care to comment? I had assumed that a single bit of logic
which detects the presence of a pilot flame fulfilled the dual functions
of turning off the spark generator *and* turning on the main burner. But
this appears not to be the case because - when the fault was present -
the sparking had stopped but the main burner hadn't been turned on.

So, are there two separate bits of logic? Or, maybe, two different
levels of DC current needed to (a) turn off the spark generator and (b)
turn on the main burner?
--
Cheers,
Roger
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