Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature clamps to the flow pipe and return pipe of the primary heat exchanger ensuring a good contact
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to fire and run for CH.
During CH operation measure the temperature of the
CH flow pipe and primary return pipe.
Does the primary return pipe temperature inside the boiler
reach a higher temperature than the flow pipe?