My existing thermostat has three wires. G, W, R. Can I install a ecobee thermostat with this configuration? I have a gas furnace with heating only.
Okay, if there are only three wires at the thermostat location, that means you have two options to wire up the device.
Option 1: Use the G wire as the Common Wire. Remove the wire that is currently running into the G terminal on your old thermostat and terminate it into the C terminal on the Ecobee3. Take the other two wires, R and W, and place them into the RC terminal and W1 terminal on the Ecobee3. There is a control board located inside the furnace, usually behind the bottom panel.
Remove the panel and take the G wire from the thermostat bundle off of the G terminal and run it into the C terminal on the control board. Once you are done, close up the panel and go back to the thermostat location. Place the thermostat on the wall mount and the device should power up.
Important! If your furnace requires fan(G) signal for a heat cycle then this option will not work.
During 1st Run Configuration make sure to select RC only, not RC/RH.
Your ecobee will detect Rc, W1, and C. With this configuration, the ecobee will automatically assume you have a boiler installation and a different heating algorithm will be used. To avoid this, during the wire detection process, select "NO". Select "Configure" and manually tap on the G terminal to allow the ecobee to think you have a G wire. Doing so will ensue the "Furnace" heating type is available.
Option 2: Run a new 4 or 5 wire bundle from thermostat location to control board inside the furnace.