Trying to get an Ecobee thermostat to work in my old house where I only have two wires going to my thermostat. Ecobee needs a common, and the remedy they suggest is a Fast-Stat Commonmaker, which has two parts - one that lives up near the thermostat and one that lives down in the furnace cabinet. Here's the before and after they describe:
What do these black boxes actually do? How does this work?
