Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Set flow rate at tap to approximately 5 l/min
Slacken the upper test point screw of the Gas Valve and attach a U-gauge
Selector switch to OFF position
CH temperature selector to minimum
DHW temperature selector to maximum
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Selector switch to Hot Water position
Allow burner to ignite the Hot Water to warm.
Slowly turn the Hot Water temperature control towards Min position.
Does the gas pressure decrease as the control is turned?
Note: When U-gauge is removed the test point should be tightened and checked for gas soundness.