Set Appliance State:
Mains Power Off
Set Multimeter to DC Volts 
CH Demand
Gas On; Pilot On
Boiler On/Off Switch ON
CH Switch ON
DHW Switch in INSTANT position
CH Temperature Control in Maximum position
Boiler On/Off Switch ON
CH Switch ON
DHW Switch in INSTANT position
CH Temperature Control in Maximum position
| WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure |
Measurement:
Allow the burner to crosslight from the pilot.
When boiler has reached a reasonable operating temperature turn down the CH temperature control thermostat while measuring the voltage across the brown wire and the black wire on the gas valve solenoid.
Does the voltage reduce as the boiler temperature rises and CH temperature control is turned down?
Note:
It may be necessary to reduce the heating load by turning off some radiators to reduce time taken for this test.
When boiler has reached a reasonable operating temperature turn down the CH temperature control thermostat while measuring the voltage across the brown wire and the black wire on the gas valve solenoid.
Does the voltage reduce as the boiler temperature rises and CH temperature control is turned down?
Note:
It may be necessary to reduce the heating load by turning off some radiators to reduce time taken for this test.
Isolate as per TOPs P4
and refit as necessary