Pottertom Puma 80 PP modified overrun

[Noisypuma]

With Hot Water and Central Heating OFF, the boiler fires up as soon as the switch is moved from Standby to On. So it burns for 30 seconds, the water overheats, and the safety cutout shuts it all down. Is it the pcb or a sensor failure ?

 

[corgiman]

when you say stand by to on

are you turning the heating on or running a tap

 

[Gasguru]

Stuck water flowswitch?...pop off the front panel and see if the red lights on in the midlles of the circuit board.

 

[Noisypuma]

"Standby to On" is the top rocker switch, and this fires up the boiler, even with the HW turned off and CH switch set to off.
The red LED on the pcb is on

 

[ChrisR]

The red LED on the pcb is on

Stuck flow switch it is then. Plenty here if you search..