Back boiler overheat stat

[Paulo1]

Hi

I have a Glow-worm back boiler which is giving me grief (I would have upgraded it by now but its a listed building and there's issues with venting through the external wall and I'm reluctant to go with an electric boiler instead (because of the consumption cost)). It's a sealed system.

The problem is that overheat stat keeps activating.

Sometimes it lasts a week without a problem and sometimes (like this morning) it lasts about 30 seconds before tripping.

The heating engineer has been out a number of times and, amongst other matters:
1. Changed the overheat stat
2. Changed the thermocouple
3. Changed the gas valve
4. Bled the radiators
5. Checked the pump, TRV, pressure vessel, pressure gauge, cylinder etc.

There is a small leak somewhere in the pipework (as the pressure gauge, which is set at over 1 bar, drops by apx 0.5bar every 24 hours); the plumber is coming to look at this although not sure if this will deal with the boiler problem.

Seems very odd that it's so intermittent (e.g. how can an overheat stat work fine for an entire day and then activate 30 seconds after firing on the next day).

Not sure if air in the system would cause the stat to activate (irrespective of the boiler temperature) although no sign of air in rads.

I'm at my wits end with this so any input would be greatly appreciated.

Thanks

Paul

 

[ianmcd]

Hi Paul when it trips, feel the pump to see if it is running, sound like it is intermittantly sticking

 

[Paulo1]

Thanks Ian.

I've just fired it up again.

The pump was vibrating and hot water was pumping through it (although the pump and pipes from it are very hot to touch).

It tripped again after about 5 minutes, this time after the boiler stat reached its temperature and was cooling down before re-firing.

ATB

Paul