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See post 16.

I arrived at that conclusion, with hindsight, and having subsequently got hold of the the functional diagram. I didn't base my decision to replace the component based on that but rather the advice received here and elsewhere. Also, I don't think I said I performed the fix myself. The diagram (I...

With the benefit of hindsight and having got hold of the boiler installer manual, the overrun thermostat is unique in that it seems to be the only component that works independently of the pcb. The pcb seems to be the most prominent, therefore probably the most sought after, part for this boiler...

I'm curious, what happens in these overrun thermostats to result in poor contact to switched/permanent live? Is this metal fatigue in a bimetallic strip or something else?

I managed to find that part new. Fitted today and now all working properly. Many thanks for the help.

Just seen the pump lose power and a variation on the issue I have not witnessed until now: No demand for CH or HW. Boiler off. Pump off. Demand for HW arrives. Zone valve opens, pump comes on, boiler fires. Pump ran for about 10 seconds then went off. The temporary light also went out. Boiler...

I have wired up a small bulb to PL in the airing cupboard junction box. Next time the pump doesn't run, I plan to get there quick and see whether the bulb is lit.

This boiler has had one control board replaced to my knowledge. Probably within the last 5 years. I wished I'd asked to keep it and got it repaired, as a spare! But I agree, this issue does not appear to be a control pcb problem.

Thanks Ian. I just managed to find the Potterton installer's manual {original paper copy from when the boiler was fitted). According to the fault finding chart, the pump overrun not operating -> replace overrun thermostat. And since I also have the intermittent failure to power pump (still to...

I'll update this once I find out the problem. I do have homecare so will probably get an engineer out. But intermittent problem means sods law and it will operate fine when they come. Something that occurred to me whilst looking at this. With HW demand, the return near the boiler is about 10C...

Thanks Ian. Next step I guess is to check for power at the pump during one of these events.

Thanks CBW. I guess I need to monitor for power arriving at the pump during an event it fails to start up. Is an intermittent fault with that make of pump likely to manifest itself as a totally blank pump display? It don't even flash/blink. Normally this grundfos Alpha goes through quite a...

I have an old Potterton Prima 50F in an S Plan system and an intermittent issue. Occassionally, becoming more frequent, the following happens: When there is demand for CH or HW, the relevant zone valves open and the boiler fires. However the pump (a Grundfos Alpha 2 25-60 130) does not power...