Set Appliance State:

Mains Power Off
DHW Demand
Boiler stat 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 telegan temp clamp measure the temperature of the flow pipe close to the boiler observe flow temp falling as system extracts heat.