Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Boiler On/Off Switch On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage at connector CN1 bottom connection (brown wire) on the control PCB and a suitable neutral.