Set Appliance State:

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

Observation:

Allow ignition sequence to commence, burner to ignite and run for at least 2 minutes.
Measure the differential between the flow and return pipes.
It should be approximately 20°C or below.