Diagnosis
The failure of the fan to trigger during a heating cycle on the Ecobee Smart Thermostat Lite typically stems from a mismatch between the thermostat's software configuration and the physical control logic of your HVAC system. In most modern forced-air systems, the furnace control board manages the blower fan based on heat exchanger temperature. However, if the Ecobee is configured for "Thermostat Control," it expects to send a 24V signal via the G-wire to trigger the fan. If this signal is missing or the setting is incorrect, the fan remains idle until you manually override it to "On." This issue is common during new installations where the "Fan Control" setting defaults to the wrong mode. You can confirm this by switching the fan to "On" manually; if the blower starts immediately, your G-wire is physically intact and the issue is purely a logic setting within the Ecobee software. If the fan does not run even when set to "On," you are dealing with a physical wiring break or a blown fuse on the furnace control board.
How to Fix It
To resolve the automatic fan trigger, you must align the Ecobee's equipment settings with your furnace's hardware capabilities. Follow these exact steps:
- Remove the thermostat from the wall plate and verify that a wire is firmly seated in the G terminal. Ensure there are no stray copper strands touching adjacent terminals.
- Snap the thermostat back onto the wall and wake the screen.
- Tap the Main Menu (three horizontal lines) in the bottom left corner.
- Navigate to Settings and then select Installation Settings.
- Select Equipment.
- Locate the Fan Control in Heat Mode option. You will see two choices: "Furnace" and "Thermostat."
- If your furnace is a modern forced-air unit, select Furnace. This tells the Ecobee to let the furnace control board decide when to start the fan based on the heat exchanger's temperature.
- If you have an older system or a heat pump where the thermostat must trigger the blower, select Thermostat.
- Exit the menu and run a "Test" by raising the temperature setpoint 5 degrees above the current room temperature. Wait 3-5 minutes; if set to "Furnace," there is often a delay before the blower kicks in to prevent blowing cold air.
If That Didn't Work
- Check for a missing C-wire: If you are using a Power Extender Kit (PEK) or have no C-wire, the G-wire may be sharing a circuit that isn't providing enough voltage to trigger the relay. Verify the C-wire is connected at both the thermostat and the furnace control board.
- Inspect the Furnace Control Board Fuse: Open your furnace panel and look for a small automotive-style fuse (usually 3A or 5A) on the control board. If this fuse is blown, the 24V signal from the Ecobee's G-terminal will never reach the blower motor.
- Verify the G-wire Continuity: Use a multimeter to test for continuity between the G-terminal at the thermostat and the G-terminal at the furnace. A break in the wire inside the wall will prevent the fan from running automatically regardless of software settings.
- Perform a Factory Reset: In rare cases, a corrupted configuration profile prevents equipment changes from saving. Go to Main Menu > Settings > Reset and perform a full reset, then re-run the equipment detection process.
Heads Up
- Avoid "Fan On" Constant Use: Running the fan in "On" mode 24/7 will significantly increase your electricity bill and wear out the blower motor bearings prematurely.
- The "Cold Blast" Phenomenon: If you set Fan Control to "Thermostat," you may experience a blast of cold air at the start of a cycle because the fan starts before the heat exchanger has warmed up.
- Voltage Warning: Always shut off the breaker to your HVAC system before removing the thermostat or opening the furnace panel to avoid shorting the 24V transformer.
- Filter Maintenance: If the fan runs but the heat doesn't reach the rooms, check your air filter. A clogged filter can cause the heat exchanger to overheat, triggering a "high limit switch" that shuts down the furnace for safety.