"Jay" wrote in message
...
I have an oil boiler, HR Wallstar which stops when it is most needed.
Typically it will fire up in the morning and heat up the house etc. Then
with the Amber light still lit and demand from the thermostats it will
simply stop. The thermostat on the boiler is not tripping out and the
boiler stays off even when everything is well cold. The thermostat on the
boiler is up high.

this only happens when it is really cold, suggesting that may be the
boiler
is doing a bit of overtime and something is cutting out. The 'lock out'
relay is not operating so oil shortage is probably not the issue...?

When in this stopped state I power off the isolator and power it up again
it
will start the ignition cycle very briefly and then stop.

After an hour or two it comes back on and works OK for a while. Typically
on warmer days it doesn't seem to go wrong. The boiler service man can't
find a fault.

Any ideas??

Jay

Check the oil feed line - is much of it exposed outside? There *may* be a
little drop of water trapped in the line that will settle at the lowest
point, and if in an exposed part of the run may freeze on occasion causing
starvation.
Just a thought.

Dave