Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature probes to flow & return
pipes as indicated by the red arrows
Ensure a good connection is made, it may be easier to use the probe style temperature sensor on the return pipe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Cancel purge cycle & if required, reset boiler.
Allow burner to ignite and CH to operate and reach a reasonable
operating temperature, approximately 55°C.
Measure the temperature differential across the primary heat exchanger.
Expect a differential of approximately 22°C or less