In an earlier contribution to this discussion,
T i m wrote:

Hi All,

I had another look at Mums 'old but going to br replaced soonish., the
pilot won't stay alight' Ascot / New World (Main?) gas boiler again
yesterday and think I found the upper limit cutout / inhibitor (?) had
triggered? I pressed what looked like a rest button in the middle of
it but as didn't have any real tools / meter with with me (only the
trusty Leatherman as always) I couldn't measure anything so shorted
the extra stat out (for testing) and tried to light the pilot.

Hold in the contril knob, hit the piezo, nice little pilot flame, hold
knob in 30 seconds, release, pilot stays alight (hah huh, so it was
overtemp cutout ..?).

Turn the gas knob round to 'On', pilot goes out?

Wait 3 mins, relight pilot, turn knob to 'On' pilot stays on this
time, turn mains power to boiler, main burner lights up but then main
pilot and burner goes out? Try this a few times with similar
(intermittent) results. So, it could just be that the thermocouple was
'marginal' and as they are 'cheap' ....

So, I removed the thermocouple (amazing what you can do with a
Leatherman PST) and this morning picked up a replacement. It's not the
'correct' part (long since obsolete) but sufficiently close where it
matters to 'do'.

I just did some comparisons between old and new probe using my DMM and
the gas stove and noted the following with either (probe, not DMM) in
the gas flame (simulating pilot).

New device .. open cct voltage 30mV, short cct current (DMM on 2mA)
250uA.

Old device .. 23mV and 200uA ?

So, from that very crude test the old thermocouple isn't 'dead' but
maybe down on performance (especially when not in a 'strong' flame ie
only a 'pilot' not a low gas ring) enough to make a difference?

I'll see what happens when I ahem, sorry, 'the Corgi bloke', put's
it back later... "You know anything about parachutes ...?" ;-)

All the best ..

T i m

If the pilot stays on after releasing the button, it sounds as if the
thermocouple and flame failure device are working ok. If the pilot goes out
when the main burner comes on, it suggests that it's being blown out by the
sudden rush of air. In this case, the pilot probably isn't quite powerful
enough - either because its height needs adjusting (is there a separate
little regulator for this?) or because there's a bit of crud partially
blocking its jet.
--
Cheers,
Set Square
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