Wickes 90 (Halstead Finest) CH problem

[past3bar]

Boiler operates correctly in DHW mode.
In CH mode it starts ignition sequence ,fires ,runs for approx 30 seconds then gas valve shuts down pump overrun continues then boiler goes into anti cycle mode for 3 mins. It will fire normally in dhw and provide endless hot water or start again if isolated from the mains.
Boiler does not go to lockout.(Lockout light doesn't illuminate)
Burner on light seems to operate normally.
Overheat stat does not need re-setting.
I have replaced CH thermistor.
I have changed the pcb link to isolate the clock from the equation. There are no other external controls.
I have cleaned the venturis.Sensing pipes seem ok
APS works fine in dhw mode.
Halstead technical man no ideas, his "senior" engineer phoned me back & said try new APS but already spent hours on this boiler and reluctant to buy parts without being confident of fault.
Am I missing something simple?

 

[Gasguru]

Does the flow pipe under the boiler get hot?....are TRV pins jammed down?

Shut off the gas and run the boiler in DHW mode to cool down the plate heat exchanger (reset it several time if necessary).

Now run the heating....does the plate get hot?

 

[past3bar]

Thanks i'll try those tips tommorrow

 

[AntoniaB]

Hi past3bar

I have EXACTLY to same problem !

Any help would be appreciated.

Kind regards

Antonia