Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
CH temperature control to MAX
Attach temperature clamps to flow and return pipes of the primary heat exchanger as illustrated
Note: Depending on access and the type of temperature clamps you have you may need to use a temperature probe for the pipe leading to the pump assembly.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow burner to ignite and flow temperature to reach a reasonable
operating temperature, approximately 60°C.
Measure the temperature differential across the primary heat exchanger.
Expect a differential of approximately 25°C or lower.