Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
Selector switch to HW position
CH temperature selector to Min
DHW temperature selector to Max
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure voltage across connection 7 (red wire) on connector X3 on the main PCB and a suitable neutral.
Expected mains voltage.
Note:
Up to 190V AC may be measured with no demand.
Voltage measured must be full mains voltage.