Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Check for mains voltage across the brown and the blue wires at the fan.