Isolate Mains Power to appliance
as per TOP's P4

Remove the PCB cover
Locate jumper/jumper position, next to connector M17 on the main PCB

Observation:

Is there a jumper in location JP9 on the main PCB?
If preheat is enabled boiler will fire to maintain heat exchanger temperature. Firing temperature range 40-55 degrees C (approximately).
Note:
Hot water preheat - enable add jumper to JP9, remove jumper to disable.