Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach temperature clamps to the flow and return pipes as indicated by the red arrows
Appliance DHW thermostat set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow boiler to ignite and run for at least 2 minutes.
Measure the difference in temperature differential.
Expect a differential of approximately 20°C or less.
Is the temperature differential as expected?