Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Selector switch to DHW and CH position
CH temperature selector to maximum
DHW Preheat selector to OFF position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage across connection 3 (brown lead) at connector A3 on the PCB and earth.