Set Appliance State:
Set Multimeter to AC Voltage 
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Gain access to boiler terminal block
Gain access to boiler terminal block
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Measure the Voltage across the room thermostat terminal marked ON (connection 6) and the neutral marked N (Blue wire, connection 1) on the main connector block.
Note:
If a Room Thermostat/External Controls are not fitted there will be a Link Wire connected to the connection marked ON and the connection marked HW.
Note:
If a Room Thermostat/External Controls are not fitted there will be a Link Wire connected to the connection marked ON and the connection marked HW.
Isolate as per TOPs P4
and refit as necessary