Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Selector switch set to CH/DHW
CH temperature control set to MAX
Preheat switched OFF
Attach temperature clamps to external flow pipe and internal return pipe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow appliance to run for CH demand.
Does the return pipe show a higher temperature than the flow pipe?