"Andrew Gabriel" wrote in message
...
In article ,
"Stoney" writes:
This my third go at getting somewhere with my ailing boiler.
The problem now appears to be that the thermocouple is failing at high
temperatures. So it will stay on all night but it will stop working
after
the boiler runs for 10 minuites or so.
Does anyone have any experience of this?
I have put in a new thermocouple but the same thing is happening.

Yes, I've seen the same problem with a gas fire which Eastern Gas fitted.
Problem was too large a pressure drop when the main jets were lit,
which resulted in pilot light shrinking. They had to come back and
take a feed from the gas main, rather than running it from an old
gas lamp pipe.

Have you checked the pilot flame's size/shape against the servicing
manual? It might be that it's misshapen due to partial blockage,
and the change in airflow when the main burner lights is taking it
away from the thermocouple. Another possibility is the thermocouple
isn't mounted at the right place/height. Another thought that occurs
to me is the flue's blocked, causing the flames to drift into the
wrong place due to lack of airflow (and consequent risk of CO release).

On my old gas boiler, apart from the occasional thermocouple (2 in 20
years), the problem has been minute particles of dirt in the gas supply
slowly blocking up the pilot light.

The symptoms (as suggested above) are a pilot light that is small, and does
not really cover the thermocouple well.

Stays in overnight but doesn't survive after the first long burn.

The cure for me is to remove the pilot jet, and clean it out with fuse wire.

Crude, but effective.

Once you have done it, you can easily see the difference in the flame.

You could even get a spare jet (cheap enough) and fit the new one then clean
the old one as a backup.

HTH
Dave R

P.S. Christian - do they do direct replacements i.e. ground floor boiler in
the centre of the house using chimney flue, or do all modern boilers vent
directly (or nearly directly) to the outside?
There is a vast difference between the strategy for direct replacement and
moving all the pipes to an outside wall.