Doorbell Mystery Solved

I'm so ashamed.

I solved the mystery of the amazing thermostat-controlled doorbell. It's mundane and not nearly as exciting as it was when it was still mysterious.

If you recall, we had an old thermostat and an old doorbell mounted a few feet apart on the same wall. A pair of twisted wires ran directly from one to the other, along the wooden molding, held down with insulated staples. No other wires in sight. And the doorbell worked. Huh? How's that supposed to happen?

I finally pulled both units completely off the wall, peeling up the wires as I went. Turns out the wires from the thermostat ran along the wall about half the distance and then disappeared through a tiny hole in the molding directly under one of the staples. And the doorbell wires came up through the same hole. The crossover was totally invisible under a tiny little staple. Impressive.

The doorbell still works (and the thermostat doesn't). But at least now I know why.