Set Appliance State:

Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Purge Switch in NORMAL position
HW Switch in ON position
CH Switch in OFF position
HW Potentiometer set to maximum
Attach a U-gauge to the gas valve outlet pressure test point
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the Fan to run and the Pilot to ignite.
Does the Burner crosslight from the Pilot and operate with a gas outlet pressure of approximately 16 mb?
Note:
When U-gauge is removed pressure test point should be tightened & checked for gas soundness.