In message , The Natural Philosopher
writes
geoff wrote:

In message , Jay
writes

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??
Interesting post. I repaired an oil control module for someone a
couple of weeks ago and it's come back with the same problem.
The customer has tried putting it in the fridge and then installing
it, then heating the module and then installing it. It seems that it
won't work when very cold (which sort of defeats the object)
I'll be getting the freezer spray out tomorrow to investigate

I had a very old boiler that used to do this. Would get worse and
worse, maybe locking out as well.

It was always a carboned up nozzle and carbon on the pghotodetctor that
detected the flame.

Its probably worth servicing both items anyway, but if it persists, its
something in the control circuits as geoff says.


I have a theory ....

Maybe it's condensation.

I have liberally sprayed the pcbs with conformal coating and tomorrow
I'll attack it with freezer spray again.
--
geoff