In article ,
T i m wrote:
[snip]
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' ....

Very similar symptoms to those we had with our factory
hot-air heater, trying to relight it after being off
all summer. Perched on the top of a 12ft ladder in
my case.

Press and hold button, light the pilot, wait, release
the button, looks ok. Turn on the main burner, looks
ok..... then the whole lot goes out. Repeat, and repeat,
and repeat, etc..... until it finally works.

After a few years of this I finally cottoned-on to what
was happening. The main feed pipe had been shut off at
the meter, for about 6 months, and air had crept into it.
The solution was to simply purge the pipe beforehand.

--
Tony Williams.