Potterton Kingfisher MF Purge/Ignition Problem

[AJed]

I have a recently serviced Potterton Kingfisher MF CF80. the problem is the boiler sometimes does not ignite. The conditions are: room thermostat calling for heat, pump running, boiler begins to start up , fan runs, pilot light lit then it just stays there on the purge cycle with the fan running continuously but no main ignition. This can go on for hours and then it will eventually decide to fire up. Curiously this seems to only happen during the daytime (Yes the timer is on!) morning and evening startups seem to proceed normally.
Does anyone have a suggestion for the cause of this? Could it be related to burner pressure settings following the service?

AJed.

 

[weareleeds]

After press sw made & pilot lit does sparking persist if so suspect blocked pilot inj or burner, damaged electrode or lead or pcb not sensing lit pilot

 

[AJed]

Thanks for the reply, No more sparking occurs it just sits there with the pilot lit and the fan running for upto 2 hours before it lights the main burner apparently of its own accord.

AJed

 

[Agile]

faulty PCB or sticking/faulty gas valve?

Have you measured for volts on the gas valve solenoid?

Tony

 

[AJed]

Not yet, next time it occurs I'll put my DVM across it. Thanks.

AJed

 

[AJed]

Yep only 6V dc at the main gas valve, 200v at pilot. Could this be the flame sensor on the pilot faulty?

 

[AJed]

Ah-Ha tapping the pilot assembly gently produces 200v at the main gas valve and ignition. Guilt proven. Thankyou everyone.

AJed

 

[croydoncorgi]

If you 'tap' the pilot assy itself and the thing takes off, the problem is probably poor earth connections on the boiler (you need a good return from the burner area to the PCB for flame sensing to work properly. The actual sensing is done by the PCB, not at the pilot assy as such).

 

[AJed]

Many Thanks, I'll look into it.

Ajed