This is to William.
For your information, a Stack Switch, which by the way was used for many years before they began using electronics, IS a Primary Control. A Primary Control is nothing more than the device that is activated by the thermostat,and which controls the burner. For a gas furnace, the Primary Control is the gas control valve. Oh, and by the way, not only did I work on those units for nearly 15 years, I also taught all aspects of heating for many years as well. In the future, before you attempt to act like you know what you are talking about, I suggest you do some research first. If you are a heating mechanic, then I suggest you go back to heating 101 as you did not learn much the first time!!