Boiler overheating then extinguishing

[meander]

Apollo 65/80B wall mounted gas boiler very gets hot indeed and goes out. This happens on high or low settings and the pilot light then has to be re-lit to start the boiler up again. All thermostats have been replaced. Could this be a gas valve problem ?

 

[DP]

Unlikely (unless someone has been tinkering with it). Is the pump running?

 

[COG]

IF its getting to hot then its got poor circulation ie slow pump or blocked heat ecchangers, the boiler has told you its too hot by over heating why did you assume the stats were ALL faulty ?

 

[meander]

Unlikely (unless someone has been tinkering with it). Is the pump running?

Pump is certainly working OK. All radiators get very hot and pump still runs when boiler has gone out.

 

[meander]

IF its getting to hot then its got poor circulation ie slow pump or blocked heat ecchangers, the boiler has told you its too hot by over heating why did you assume the stats were ALL faulty ?

Boiler was very noisy so I flushed it out, removed a radiator that was not getting hot and flushed out lots of murky sludge fom it. Boiler now runs sweetly with very little noise but the overheating and extinguishing persists. The themostats were all replaced as a matter of general maintenance because it is not too costly by DIY. I am now wondering if I may have a faulty overheat thermostat.

 

[CH4]

... and pump still runs when boiler has gone out.

The boiler has a pump overrun thermostat, which keeps the pump running until the boiler has cooled down.

As COG suggested, a low water flow rate through the boiler is the likely cause of the problem. The pump could be running slow because of a build-up of sludge on its rotor or its bearings may be worn out. Also, the heat exchanger could be constricted by heavy scaling or baked-on sludge.

 

[meander]

... and pump still runs when boiler has gone out.

The boiler has a pump overrun thermostat, which keeps the pump running until the boiler has cooled down.

As COG suggested, a low water flow rate through the boiler is the likely cause of the problem. The pump could be running slow because of a build-up of sludge on its rotor or its bearings may be worn out. Also, the heat exchanger could be constricted by heavy scaling or baked-on sludge.

Many thanks for your help. Still having peculiar problems. The pump must be working OK because all radiators get too hot. The pump shaft is free (removed vent plug and checked), and it responds to each of the three speed settings. If the thermostat is turned down when radiators are far too hot the boiler goes out leaving the pilot light on but the pump NOT RUNNING. Then, after a few seconds, the pilot light goes out (so the overheat thermostat must be doing its job). Any ideas would be appreciated.