Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Preheat OFF
Attach temp probes to the main Flow and Return pipes to the primary Heat Exchanger
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 fire for Hot Water and once it has reached
a reasonable operating temperature.
Measure the temperature of the main flow and return pipes to the primary
heat exchanger and ascertain temperature difference between them.
Expect a temperature differential of approx 24°C or less.