Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach a U-gauge to the gas valve outlet pressure test point
Selector switch to DHW and CH position
CH temperature selector to maximum
DHW temperature selector to minimum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow burner to ignite and ramp up to full pressure and heat system to approximately 60 degrees C.
Slowly turn the CH temperature selector to minimum position.
Does the gas pressure decrease as the burner modulates?
Note: When U-gauge is removed pressure Test Point should be tightened and checked for gas soundness.