The Fireplace is ran by a low voltage module. The switch on the wall just open and closed a circuit between two contacts on the module. That circuit is normally a millivolt circuit.
-The G2 smart switch has 120 vac in to power it.
-The switch on the wall is then connected to the G2 external switch terminals. The external switch terminals are not controlled by the G2, it controls the G2. This means that you have remote control via the app to control the G2 as well as an external switch. The two together act just like a three way switch.
- The G2 output terminals have 120 vac on them as well, therefore they can NOT be connected directly to the fireplace module.
This is where the dpdt relay comes in to play. It is powered by the 120 vac from the G2. It is a dry contact relay (no physical contact between the two sides). So, 120 vac on one side, and the low voltage module on the other.
Now you can safely power the low voltage fireplace from the ST app, from the wall switch, and from Alexa.
You do not have to connect the external switch, but I wanted a physical switch to kill the fireplace is I had two.