Potterton Statesman Flowsure+ - Overheating

[Datcher]

Recently my Potterton statesman Flowsure+ started to trip out on the limit thermostat - generally during the night without the CH calling for heat - and therefore required manual reset in the morning.
Couldn't fault the system as never actually saw a overheat on the temp gauge and I would have liked more time for fault finding but the 'tribes ' were turning up for xmas dinner! Due to the time of year and possible unavailability of parts over this period I have replaced both the Limit Stat and the Boiler Stat.
Things ok for 2 days then same problem. This time witnessed overheat. Reset Limit Stat - fine. Then next day no CH - system calling for heat with boiler temp of approx 65 degrees. Overheat lamp not illuminated. Pump (CP53) and all pipes close to boiler were hot - but obviously no flow. Carried out pump bleed and rad bleed -system ok. Noticed arcing noise coming from pump control box, stripped it down and found poor contact between selector switch rotary and stationary contacts. One cause for open circuit giving no flow.
I have since had the same problem a number of times but have found the pump switch to be made( Speed III) and the only way to get the pump running is to perform a manual start ie by unscrewing the nut at the end of the motor shaft.

1) I know the selector switch issue is a problem - can I buy just the control housing?
2) The system is fitted with a pump overrun stat and from the wiring diagram the pump should be running all the time - I am going to wire the pump from a 3-pin plug to eliminate the stat. Good or bad idea? Could it be a cause?
3) Could there be an airlock stopping pump? If so how do I get rid?
4) Am I barking up the wrong tree?

Thankyou for your assistance.

 

[Strachan]

From your description, there`s power to the pump when called for. how old is it? It may be old and tired enough not to be able to overcome the initial inertia from a standing start. This would cause a lockout for overheat.

 

[Datcher]

Thanks for your reply.

The boiler was installed in 2002 and I believe it's the original pump - Circulating Pumps CP 53.

 

[oilman]

It might be the diverter valve actuator, I'll find a link here it is.

 

[Datcher]

Thanks Oilman.

Have you any ideas for testing the pump and the diverter valve actuator to determine which is at fault, or is a matter of fault finding by component substitution?

 

[Datcher]

Further investigation -
had the fault condition today (no CH, no Overheat indication, DHW ok.) Removed the diverter valve actuator - valve spindle and actuator drive in corresponding position - at the time DHW. Operated room stat and observed actuator drive rotation - this took 10 secs to begin to move and then 20-25 secs to complete the 90 degree turn. Operated stat again - rapid movement back to DHW position - is this movement via spring tension?

Through all this I left the diverter valve spindle in the DHW position and this eventually gave an overheat indicated - as you rightly predicted Oilman!

Is the actuator on it's way out - indicated by slow rotation or is this speed ok? Would water pressure acting on the diverter valve be enough to stop actuator rotation?

Thanks

 

[oilman]

Does the valve spindle move easily?

 

[Datcher]

I can easily move it with my fingers.

Ordered new CP53 pump - this was cheaper than the valve actuator and with the speed switch problems I was having, would like to rule it out.