Set Appliance State:
Set Multimeter to DC Volts 
Isolate Mains Power to appliance as per TOPs P4
CH/HW switch in HW position
Ensure that the boiler is cold and ready for pre-heat
CH/HW switch in HW position
Ensure that the boiler is cold and ready for pre-heat
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Test for voltage above 20 volts DC across one of the terminals (grey) wires at the DHW overheat thermostat, and the Neutral (blue) on the mains supply connector block.
Repeat this test on the other terminal (grey) wire at the DHW overheat thermostat.
The DHW overheat thermostat is the lower of the two thermostats situated below the secondary heat exchanger.
Repeat this test on the other terminal (grey) wire at the DHW overheat thermostat.
The DHW overheat thermostat is the lower of the two thermostats situated below the secondary heat exchanger.
Isolate as per TOPs P4
and refit as necessary