Set Appliance State:

Gas Off
User Control to Off Position
Isolate Mains Power to appliance as per TOPs P4
Attach U-gauge to gas valve inlet pressure test point
Ensure DHW is calling for heat
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

User control switch to ON position.
Allow fan to start and ignition sequence to run.
Observe U-gauge, does the inlet pressure remain
constant at a minimum of 18mb?
Note:
When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.