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
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Reset the boiler / cancel pump purge as necessary.
Allow burner to ignite and CH to operate.
Measure the temperature differential across the primary heat exchanger.
Expect a differential of approximately 25°C or less