Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Operating Switch in Heating & Water position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Is there Mains voltage across Connector X2 connection 4 (L) and Terminal block X4 connection 2 (N Blue)?