Set Appliance State:

Mains Power Off
Ensure all external controls are calling for heat
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage at terminal 9, marked L (brown lead) and terminal 10, marked N (blue lead) at the terminal block.