On Tue, 06 Sep 2005 09:49:37 +0000, 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

An open circuit voltage of 25mV would indicate a working t/couple.
The gas valve solenoid+t/couple might typically total 1 Ohm thus giving
about 25mA current.

If there is some doubt about the t/couple and it gives 25mV when heated,
I'd reckon on it being OK.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
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