Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Voltage Set meter to measure AC Volts
Gain access to PCB (use Insulation Mat as required)
ON/OFF switch in ON position
CH Demand OFF
DHW Demand OFF
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between the red and black wires at the transformer.
Expected voltage approximately 19 volts AC.
Note:
This is transformer output.