Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
Ensure there is no demand for either CH or DHW
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Check voltage across the lower (smaller gap) pair of pins on the transformer.
Expect approximately 30v AC.