Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Mains Isolator Switch (ON/OFF Switch) in ON position
Selector Switch in DHW Summer position
Hot Water Temperature Control set to Maximum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across fan connections White/Red wire and Blue wire at fan.