Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Locate connector M4 on the PCB and measure the voltage between pins A and D of the plug.