Set Appliance State:

Mains Power Off
Mains Reset Switch OFF
Ensure the boiler is cool so that there is demand for preheat
Set DHW temperature selector to ECO
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Mains reset switch On.
Allow pump to run.
After approximately 10 - 20 seconds does the combustion fan start to run?