Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
Turn Hot Water Tap On
Selector switch in OFF position
CH temperature selector to MIN
DHW temp selector to MAX
CH temperature selector to MIN
DHW temp selector to MAX
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
With hot water tap on, turn the selector switch to the water heating position.
Allow the burner to ignite and, after approximately 3 to 4 seconds, measure the voltage across the connections of the modulating solenoid.
Expected voltage is approximately 21 Volts DC.
Allow the burner to ignite and, after approximately 3 to 4 seconds, measure the voltage across the connections of the modulating solenoid.
Expected voltage is approximately 21 Volts DC.
Isolate as per TOPs P4
and refit as necessary