If you have a heat-only system, you may notice that your thermostat has three wires—R, G, and W. In this scenario, the Power Extender Kit (PEK) is not compatible. There are a couple of options available to create a C wire and make the ecobee compatible.
1) Run an additional wire to use as a C from the furnace control board to the thermostat.
2) Re-purpose your G wire as your C wire, connecting it to the C terminal at both your furnace control board and your ecobee.
Original furnace wiring After re-purposing G as C
With option two, once your ecobee powers up, you will be asked to configure your equipment. Make sure to select "RC only" and not "RC and RH" during this configuration process.
Your ecobee will detect RC, W1, and C. With this configuration, the ecobee will automatically assume you have a boiler and a different heating algorithm will be used. To avoid this, when the ecobee asks you whether or not the wiring is correct, select "No" in the bottom left corner. Next, select "Modify in the bottom right corner--this will allow you to manually select terminals. Tap the G terminal to make sure the G is selected so that the ecobee will recognize your equipment as a furnace.
You will need to be sure to set fan control in heat mode to "HVAC/furnace" so that the furnace can engage the fan on a call for heat.
To adjust this setting, go to MAIN MENU > SETTINGS > INSTALLATION SETTINGS > EQUIPMENT > FURNACE > FAN CONTROL IN HEAT MODE
If you require further assistance with your installation, please reach out to our Support team. To ensure prompt service, please have wiring pictures at the thermostat and furnace control board handy.