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 three options available to create a C wire and make the ecobee compatible.
Option A: Use an additional wire as the C
If you have an extra, unused wire behind your thermostat, you can use it as the C by connecting to the furnace control board. If you don't have an available wire, you might be able to run a new one from the thermostat to the furnace (this may require the assistance of a technician).
Option B: Install the Fast-Stat Common Maker
In case you cannot run a new wire, but want to retain independent fan control, you can install a third-party device called Common Maker, made by Fast-Stat (more details here: https://www.fast-stat.com/).
Diagram taken from Fast-Stat's official manual
Option C: Re-purpose your G wire as your C wire
With option two, you will move the G wire to C on both the ecobee and equipment sides (see diagram above). The only drawback is that you will no longer have independent fan control (meaning that the blower fan won't be able to run without heat).
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.
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.