Potterton Prima pump overrun question

30 Nov 2008
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United Kingdom
Can anyone confirm if the pump overrun should not allow the pump to stop when all heating has been satisfied and then start about 2 seconds later? I understand that the pump overrun stat is active if the temp in the boiler is above 80 degrees. My boiler is shutting down then around 2 seconds later the pump overrun stat kicks in but occasionally it is not "quick enough" before the overheat stat kicks in. Or is the symptoms pointing to another issue elsewhere (scaled/sludged H/E)?

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Had the boiler running on Max and it worked ok for a couple of days then tripped. I can reproduce a trip condition by letting the CH get up to temp and then turning the roomstat down just before the water temp stat kicks in. The boiler switches off (as does the pump) followed by a small gurgle in the HE then the pump starts up again but the trip cuts in before the excess heat can be dawn away. In general circulation seems good (all rads fine, boiler reaches temp without cycling) does this point towards a sludged HE or scaled HE or something else? I would imagine if I could get a pump overrun stat set to a lower temp then I would be fine (although this is making the issue). I can get it to run trip free by running the boiler at around 3 on the thermostat but in the winter this setting is not really high enough I find.

because all the 'stats (sensor parts) are inside the sealed case you will need a RGI.
Is the pump wired directly into the boiler pump connection ?
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Will check wiring but from memory yes direct to wiring block within the boiler but will double check. Twgas in your professional opinion do you feel it is a lazy sensor or a problem of a localised heat spot sludge in the bottom of the HE that once the pump switches off causes a surge of heat that is closer to the overheat sensor thus not letting the pump overrun catch it in time? It will run for a couple of days fine but all depends when the roomstat or cylinder stat is satisfied with respect to what temp the boiler is at. If mid cycle then all is fine as the temp in the boiler is nowhere near the overheat trip, but if it is near to the end of the on cycle the pump will go off and then the trip can happen.

Flow temp (at top of boiler) when measured with an infrared heatgun which is not that accurate on a shiny surface is 67c (cant find my contact one at the moment). PS have run the unit on pump setting 1 and it still did not play up unless the room stat or cylinder stat was satisfied whilst temp was reasonably high (ie boiler about to cycle off due to water temp stat reaching high), assume this shows circulation in general appears to be good maybe not that great in the HE. Was thinking of reverse flush of the HE see if I can budge what ever is causing the hot spot or do you think I am barking up the wrong tree.

I think your barking up the wrong tree with the heat exchanger, the o/heat stat changes over from the switched live to the permanent live about 60c, so there shouldent be a lag when the burners shut down, I would look at how the permanent live is connected, it sounds like its being switched when the switched live goes off (if that makes sense).
Will check through the wiring and post up how it is wired tomorrow. Just heard the roomstat click off and went upstairs to check if it had tripped (about 30 seconds had passed) and I then heard the pump start up (overrun stat for about 10 seconds)but it had tripped boiler. Below is the spec of the overrun stat.

Pump Over-Run Thermostat

Will keep the pump running after the boiler has shut down, as long as the water temperature within the boiler is above 80°C.

The thermostat is preset and no adjustment is possible.

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