Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Preheat OFF
Attach temp probes to the Flow and Return pipes as close to the boiler as possible
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow the boiler to fire for Heating and once it has reached a reasonable operating temperature turn off demand.
Using temp probes and Performance Tester (FGA) measure the temperature
of the flow and return pipes as close to the boiler as possible and
ascertain temperature difference between flow and return.
Expect a differential of approx 20°C.