Set Appliance State:

Mains Power Off
CH Demand
Boiler stat set to MID range
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run and boiler to fire and allow system to reach a reasonable operating temperature.
Turn off demand.
Using FGA temp clamp measure the temperature of the flow pipe close to the boiler observe flow temp falling as system extracts heat.