Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to the flow and return pipes of
the primary heat exchanger (as indicated by the red arrows)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to run for central heating.
Measure the temperature differential across the flow and return pipes.
Expect a temperature differential of approximately 20°C or less.