Set Appliance State:

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

Measurement:

Monitor the inlet pressure on the U-gauge.
Does the inlet pressure remain satisfactory while the appliance operates?
Notes:
1. When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.
2. Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.