Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
DHW Demand
Gain access to the X20 connector on the right hand side of the PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

When the fan is running measure the DC voltage
between pin 12 on X20 to a suitable earth point.
Note:
Colour of wires may change but test points remain the same.