Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Mode Selector Switch to C/H position (after turning to Lockout/Reset position)
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure approximately 23V DC across connection 3 (White/Black +ve) on the Terminal Block and connector M4 connection 17 (White/Brown -ve).