Set Appliance State:
Set Multimeter to AC Volts 
Isolate Mains Power to appliance as per TOPs P4
Remove control box cover
Selector switch to heating and hot water position
CH temperature selector to MAX
DHW temperature selector to MIN
Remove control box cover
Selector switch to heating and hot water position
CH temperature selector to MAX
DHW temperature selector to MIN
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure voltage across the connection marked C, on the two connection connector (this is adjacent to the external control terminal strip), and the neutral connection on the main terminal block.
Isolate as per TOPs P4
and refit as necessary