Pottertom Prima 80F Pump Overrun Issue?

[vincentbrown]

I am looking for some advice on an issue I have with pump overrun please. I am not a plumber, but I am an engineer, and like to troubleshoot things myself before calling in the experts.

My Boiler (Potterton 80F) is about 20 years old but still in good working condition. The only issue with it is that about every 4 to 5 years it needs a new PCB board.

My issue now is that it has cut out on the overheat stat a few times over the last couple of weeks so I am trying to establish what might be causing this.

It is a Y plan system but with an added complication of an underfloor heating circuit with its own pump etc which is linked into the central heating circuit, so the wiring is fairly complicated.

What I have established so far is the following. and I am not sure if this is as it should be or not...

• Heating / Hot Water / Underfloor Heating all operate correctly and independently as they should.
• When a call for heat is applied to the boiler, the boiler fires and the pump runs. When the boiler thermostat operates, the flame stops and the pump continues to run - there is obviously a call for heat on the boiler still at this time via the switched live.
• However, as soon as the call for heat is removed from the boiler switched live, the pump stops immediately. I am guessing that this is causing a build up of heat in the boiler causing the overheat stat to operate occasionally.

My question is, should the pump continue to run on the overrun stat once the call for heat has been satisfied by the thermostat turning off? If so (and I am guessing that it should) then I am I right in assuming that the fault must lie within the boiler itself rather than the wiring elsewhere? The boiler does have a permanent live to enable this and the only external control that the boiler has is the switched live. Or, is it possible that there is a wiring issue somewhere and I have just never noticed that the pump stops as soon as the switched live is off.

Thanks!

 

[scoobydo123]

Do what I done. Install a timed relay on the pump. Never had a overheat trip since.

 

[Agile]

Your boiler has an internal stat to provide pump over run.

This may have failed or possibly the pump is not correctly wired back to the boiler pump output connections.

Tony