Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
DHW Demand
Turn on boiler and allow fan to run
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Test for voltage between the red wire on connector X15 on the PCB and neutral (blue)
There should be 15vdc.