This is either a millvolt system or Intermittent pilot ignition (IPI). With a mv system, the pilot flame hits a thermopile, which generates ~500-700 millivolts dc, which powers the main operator in the valve. The wal switch is simply in series with the TH and TH/TP terminals on the valve.

On an IPI, the wall switch closes the contacts to the switching circuit in the module, which takes power either from a transformer or a battery pack. For instance, Hearth & Home Technologies uses a 3 volt dc supply for the controls. This allows you to run it off 2 D cell batteries if need be. In an IPI, the flame conducts a tiny microampere signal to the module and rectifies it into dc, which the module sense as presence of a flame thus allowing gas to flow. Should anything distrupt the electrical connection (yes, flames can conduct electricity), the signal is lost and the unit either shuts down or goes into a re-light mode. Even though the unit requires 110vac, the wallswitch is low voltage. Also, the IPI should be unswitched, which makes for trouble when the AHJ calls for an emergency disconnect switch.

HTH,
Bob

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