Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Ensure external timer and room stat are calling (if fitted)
User display control set to HW and CH
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure Mains Voltage between the black (switched live from controls) and the neutral at the incoming mains terminal strip.
Expect Mains voltage.