Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach FGA temperature clamps to the primary heat exchangers flow and return pipes as indicated by the red arrows
DHW flow rate of 12 litres/minute
DHW temperature selector to maximum
Attach a U gauge to the burner pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow DHW to run for a couple of minutes.
Expect a temperature of approximately 20°C or less?
Also monitor the burner pressure, it should remain steady
and not modulate down prematurely before the hot water temperature is reached.
Is the differential as expected?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.