Set Appliance State:

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

Measurement:

Selector Switch to DHW position.
Measure voltage across the live (brown wire) and the neutral (blue wire) connections at the fan.