Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach temperature probes to the flow and return pipes near to the boiler
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Set boiler target temperature to maximum (82°C)
Allow fan to run and boiler to fire and run for at least 2 minutes.
Monitor the temperature of the flow and return pipes.
Is the temperature differential 20°C or less?
Note:
Differential may be higher on initial start-up.