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 indicated by the red arrows)
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

Measurement:

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 22°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 22°C when system is heated from cold.