Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach pressure gauge to the inlet pressure test point
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow fan to run and ignition sequence to commence and burner to ignite
Does the pressure gauge show a constant inlet pressure of at least 17mb throughout the operation?
Note:
1. When "U" gauge is removed the test point should be tightened and checked for gas tightness.
2. Where the manufacturer states an inlet pressure that would not normally be acceptable please refer to TOP's for further guidance.