Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to the flow and return pipes of
the primary heat exchanger (as shown in the photo)
Note.
Ensure a good connection is made with the flow & return pipes
or misdiagnosis will occur
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to run for central heating and the system to heat up.
Measure the temperature differential across the flow and return pipes.
Expect a temperature differential of approximately 25°C or less.
Note.
1. Ensure all radiators are open and system is balanced correctly
to provide correct test results.
2. Initial temperature differential may be greater than 25°C when system is heated from cold.