Set Appliance State:

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

Observation:

Allow boiler to run for at least 2 minutes while monitoring the differential between the flow and return pipes.
Differential should be approximately 15°C or less.