Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach U-gauge to outlet pressure test point on gas valve
CH temperature selector set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler ON/OFF switch ON.
Allow boiler to ignite and ramp up to its set working pressure and reach a reasonable operating temperature.
Slowly turn down the CH temperature selector to minimum demand.
Does the burner gas pressure reduce as the boiler temp meets demand?
Notes:
1. It may be necessary to reduce heating load by turning off some radiators to reduce time taken for this test.
2. Modulation will not take place if system is satisfied at minimum gas rate.
3. When pressure gauge is removed the pressure test point should be tightened and checked for gas tightness.