Set Appliance State:
Set Multimeter to AC Volts 
CH Demand
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:
Ensure clock is in timed mode and in an on period.
Measure voltage across terminal 4, on external control terminal block marked room thermostat, and the neutral connection on the main terminal block marked N, on the PCB.
Measure voltage across terminal 4, on external control terminal block marked room thermostat, and the neutral connection on the main terminal block marked N, on the PCB.
Isolate as per TOPs P4
and refit as necessary