Set Appliance State:

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

Measurement:

Check for mains voltage across the live (brown wire) and the neutral (blue wire) connections at the fan.