Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
CH temperature control to max
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across terminal marked 4 and the terminal marked N on the controls installer interface.
Expect mains voltage.
If no external controls are fitted then a link should be present between terminals marked 3 and 4 on the installer interface.