Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to DC Volts 
CH Demand
Gain access to PCB (use Insulation Mat as required)
Ensure all external controls are calling
Ensure all external controls are calling
| WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure |
Measurement:
Measure the DC voltage between the right hand terminal of the 24v external controls connection and the earth point on the boiler chassis.
Expect 24 Vdc.
Expect 24 Vdc.
Isolate as per TOPs P4
and refit as necessary