Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Heating temperature selector to OFF
Hot water temperature selector to maximum
Programmer or selector switch set to hot water only
Attach U-gauge to burner pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run and burner to ignite.
Observe U-gauge, is the burner pressure approximately 14.5mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.