Set Appliance State:

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

Observation:

Allow the boiler to run for central heating and the system to start to heat up.
Measure the temperature differential.
Expect a differential of approximately 20°C or less.
Is the temperature differential as expected?
Note.
Initial temperature differential may be greater than 20°C when system is heated from cold.