Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Insert FGA temperature probe into flue sampling point
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the appliance to run in DHW mode.
Measure the temperature of the flue gasses while the appliance operates.
Expect a temperature of approximately 80°C or less.
Is the flue gas temperature as expected?