This post covers how I upgraded our home thermostat from a battery-powered two-wire setup to an Ecobee3 Lite supporting both heating and fan-only modes. I wanted the fan-only mode to circulate air in our two-level condo where hot days often result in a hot and stale 2nd floor and a frigid 1st floor.

Note that I’m neither an electrician nor an HVAC pro and it’s very possible that what I did is a very bad idea. But it worked for me, so I thought I’d share.

Our house only has two wires running from the thermostat mount in the condo to the furnace in the garage, just enough to complete an electrical on/off circuit used to tell the furnace whether to heat or not. This is the dreaded “no C-wire” situation with no way to power a smart thermostat and no way for the thermostat to tell the furnace to just run the air circulation fan. Our furnace is relatively modern and has more wire terminals, but running additional wires from the condo down to the garage was not really an option.

To overcome this I bought two items:

• A 24V transformer that’s plugged into an outlet near the thermostat mount inside the condo. This powers the Ecobee
• A Fast-Stat Model 1000. This gizmo consists of sender (inside) and receiver (furnace) components. It works by multiplexing additional control signals (for fan-only, in my case) over the single installed wire-pair. Higher-model-number Fast-Stats can provide more virtualized wires, but I just needed one

The first step was to install the Fast-Stat. It comes with easy-to-follow instructions and wiring it into our furnace’s clearly labeled bread-board-like circuit board was relatively simple.

With the Fast-Stat installed I could run both heating and fan-only modes using the old dumb thermostat, validating that it’s working correctly.

Next, I mounted the new Ecobee and wired it up with the wires from the Fast-Stat and from the 24V transformer. The first time I did this, I got it wrong. I wired the transformer wires to C (“Common”) and R(c) (for “Red-Cooling”, I believe) and put the black wire in R(h) (for “Red-Heating”). I guess I thought that the Ecobee wanted it that way because it’s going to be running the heating system (hence R(h)) and the transformer instructions said to connect to C and R(c) wires.

With that ready the Ecobee turned on fine and all the wires showed up in the Ecobee configuration interface. Heating even worked! I couldn’t make the Ecobee run the fan-only mode, however, and at this point I actually gave up on fan-only for a couple of months, happy that I could at least control heating using the fancy new smart thermostat.

This weekend I had a chance to fiddle with the thermostat some more, and managed to get everything working. First I tried just connecting the G (“Green”) terminal (which runs just the fan) to R(h) with a piece of wire and the fan duly started whooshing air around. This was not surprising since the old dumb thermostat could to that too, but at least it showed that the wiring and connections on the Ecobee mount were OK.

Then I tried simply reversing the inputs to the R(c) and R(h) terminals so the transformer wire went to R(h) and the furnace control wire to R(c). In that configuration the Ecobee wasn’t getting any power and wouldn’t turn on. The breakthrough was to simply jam both the transformer and the furnace control wire into the R(c) terminal of the Ecobee mount. Re-reading the Ecobee instructions that makes some sense because the Ecobee wants to always use the R(c) terminal for systems with only one R-wire.

In spite of much googling I never found complete instructions for combining a 24V transformer and a Fast-Stat to make an Ecobee work for both heating and fan-only with a two-wire system. I hope this post helps others with the same setup.