Set Appliance State:

Set Multimeter to DC Volts Set meter to measure DC Volts
Isolate Mains Power to appliance as per TOPs P4
DHW temperature selector to 0 position
CH temperature selector to 0 position
Lower control box cover
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

All heating controls calling.
Measure voltage across the CH 24 volt control terminal and the main neutral on the PCB.
Note:
The output of the 24 volt connector is the lower of terminals on the connector.