Primary circuit water boiling over

[Brooksie]

One of my customers has a standard vented hot water and heating system. The boiler is oil fired, the pump is on the return to the bolier and it has a 3 way diverter valve with normal controller, room stat, cylinder stat etc.

When the hot water or the central heating or both are called for the bolier fires up and within about 10 mins 'boiling' water overflows from the vent pipe into the central heating feed and expansion tank with lots of 'pipe banging' from the airing cupboard. After about 10 more minutes the boliing water overflowing stops and then 20 mins later it can happen again. The central heating seems to work and there is hot water at the taps.

The system has recently had a power flush thinking it could be a blockage in the primary circuit but post the flush the symptoms are the same.

Can anyone help or suggest further test we should do?

Thanks
Martin

 

[unguided1]

have you checked that the pump is still working
Regards
Mike

 

[Puddle]

Well if heating and hot water both working ,would think boiler thermostate faulty causing overheating

 

[bobpape]

Have you checked the temperature of the primaries, I would suspect the boiler
thermostat is faulty.

 

[Bernie2]

Pump or overheat stat sounds like.

Check boiler flow and return temps against boiler stat setting perhaps?

Let's be honest a boiler should shut down quite a bit before the system temp reaches 100C of boiling?

So why isn't it shutting down?

Could be an airlock, I've known things like that happen on systems, but I assume all rads have been checked for air and circulation?

So if it was working okay and now its not and there is no sludge or air in the system and proper circulation is taking place, the cold feed from the expansion tank is clear, I suppose it then seem to point to something having gone wrong with a system component.

The most likely are the pump, diverter, thermostats or the boiler shut down mechanism not responding to the thermostat.