I have a Potterton Profile 100e
When there's a call for heat the fan runs and a pressure switch checks that it's running OK and the control board then opens the gas valve for the pilot flame and makes sparks to light it and when the control board sees that the pilot flame is good and hot the main gas valve is opened.
When there's no call for heat both valves are closed - pilot and main burner both go out.
Peccavi, March 2010
