Lazy pump overun PrimaF causing overheat stat trip

30 Nov 2008
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United Kingdom
My potterton is lazy on the pump overrun. All radiators get hot so no suspected blockage. I can stop the trip by turning the boiler Stat down to around 2.5 but obviously this does not generate enough heat during this cold spell. I can set the boiler to MAX and whilst heating is calling for heat no trips occur.

I get nuisance trips both during the night and day if boiler stat is above 2.5. Overrun does work albeit the main burner switches off followed by in around 5 seconds the overrun stat kicking in. I suspect that sometimes the overheat stat has tripped prior to the overrun stat kicking in. Not sure if this can be caused by a wiring issue, lazy stat or perhaps a sludged H/E?

I have attached a breakdown of how the terminal block is wired, I have a drayton mid postion valve and Landis and Gyr RWB200. VAL=mid postion valve, Prog1= cable 1 to programmer, Prog2 = cable 2 to programmer (live)

Any help very much appreciated.

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I think that the overun on your boiler is operated by a thermostat on the flow pipe in the combustion chamber.

Its possible at this age the stat is not operating very well and would overcome the problem if it were replaced.

However, if I am right then this would need to be done by a registered engineer as its inside the combustion chamber.

A better solution for someone with electronics knowledge is to power the pump through a delay off relay to give it a timed 2-3 minutes of pump over run.

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Fitted my delayed off relay yesterday and have completed a full 24hrs without overheat trip. System can now run on MAX without tripping out when all call for heat is satisfied. Cost was £35 which is not too expensive and actually works better than the original design.

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