Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Attach U-gauge to gas valve inlet pressure test point
Selector switch to DHW position
CH temperature selector to minimum
DHW temperature selector to maximum
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to start and ignition sequence to run.
Observe U-gauge. Does the inlet pressure remain constant at approximately 20 mb?
Note: When U-gauge is removed pressure test point should be tightened and checked for gas soundness.