Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Attach pressure gauge to gas valve outlet pressure test point
Selector switch to DHW and CH position
CH temperature selector to maximum
DHW Preheat selector to OFF position
Gas On
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°C.
Slowly turn the CH temperature selector to minimum position.
Does the gas pressure decrease as the burner modulates?
Note:
When pressure gauge is removed pressure test point should be tightened and checked for gas soundness.