Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Attach U-gauge to gas valve outlet pressure test point
Selector switch to DHW position
CH temperature selector to Min
DHW temperature selector to Max
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run and ignition sequence to commence.
Does the burner ignite and go to full pressure of approximately 13.6 mb?
Note:
When U-gauge is removed pressure test point should be tightened
and checked for gas soundness.