Slightly different CH problem with Puma 80e

[Shivering Code Monkey]

I have a roughly eight year old Puma 80e. HW is working but CH is intermittant. When heating is turned on in the morning boiler just does not fire up even though it is completely cold. Sometimes after running HW the CH starts working normally (i.e. pump running and flame on and off with a cycle time of a couple of minutes). The pump definitely spins when HW is running and for some time after. When CH is switched on but does not start pump is not running and has no power. CH temperature sensor has resistance of 7.25 k Ohms when fairly cold going down to below 1 k Ohm when hot.

Any thoughts?

 

[baxpoti]

Dripping taps?

 

[Shivering Code Monkey]

No there are definitely no dripping hot taps -- only three to check here.
Even if there were, I would expect to get cold boiler to fire up once and then fail to get heat flowing out to the rads.

 

[Shivering Code Monkey]

Just in case anyone is still wondering or might find it useful, I can tell you how it has ended. Eventually I tried bridging across the HW flow switch to see if that would get heat out to the rads which it did for me that completely ruled out diverter valve etc. I spent time trying to work out what could be wrong with the circuitry. Eventually I took the modulator PCB out and found a burnt dry joint on one of the connector pins. This was the link from the common pole of the CH Mode Switch to the wiring panel. Since it is my own boiler I could just decide to link the switch directly to the siring panel and eliminate two unnecessary contacts and two doubtful solder joints. Now it is working.