Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Access the X20 connector on the right hand side of the PCB
Disconnect the connector to the fan and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset appliance as necessary.
Measure the voltage between the wire 4th down from the top
on the RHS of the connector (may be grey or yellow
depending on age of harness) to a suitable earth point.
Approximately 20 seconds after the appliance is switched on the voltage should change from 0V DC to approx. 22V DC.