Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Rotary Switch to DHW position
DHW Temperature Control To Maximum
Hold in Boiler Case Switch
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure approximately 90V AC across Red (L) and Black (N) wires of (top) Fan Connector.