ecobee premium says it's heating but no heat is coming out, the furnace isn't firing up

Diagnosis

The Ecobee Smart Thermostat Premium is functioning as a switch; when it displays "Heating," it is closing the circuit between the R (power) and W (heat) wires. If the screen indicates heating but the furnace burners aren't igniting and the blower isn't engaging, the failure exists in the signal transmission or the furnace's internal safety sequence. This is typically caused by a "broken chain" where the 24V signal is leaving the thermostat but failing to trigger the furnace control board, or the furnace is receiving the signal but is locked out due to a safety fault. To confirm this, check the furnace control board's LED diagnostic light. If the light is flashing a specific code (e.g., three rapid blinks), the furnace has detected a problem (like a blocked flue or failed flame sensor) and is ignoring the Ecobee's request for heat to prevent a dangerous situation. If the LED indicates "Normal Operation" but the burners are off, the issue is likely a loose wire at the W terminal or a failed transformer.

How to Fix It

Follow these steps to isolate the break in the signal chain and restore heat:

1. Perform a Hardware Test: On the Ecobee screen, navigate to Main Menu > Settings > Installation Settings > Test Equipment. Select "Heat." You should hear a faint mechanical click from the wall plate. If the screen says "Heating" but nothing happens at the furnace, proceed to the wiring check.
2. Inspect the Thermostat Wiring: Pull the Ecobee Premium off its wall plate. Ensure the wire inserted into the W or W1 terminal is stripped to exactly 1/4 inch and is firmly clamped. Tug gently on the wire to ensure it hasn't slipped out of the push-terminal.
3. Verify the Furnace Door Switch: Ensure the furnace blower door is completely closed. Most furnaces have a safety interlock switch (a small plunger button) that cuts all power to the control board if the panel is loose. Even a 1/4 inch gap can prevent the furnace from firing.
4. Bypass the Thermostat: To determine if the Ecobee is the problem, turn off the power to the furnace. Remove the thermostat wires from the wall plate. Use a small jumper wire (or a paperclip) to momentarily connect the R wire to the W wire. Turn the power back on. If the furnace fires up immediately, the Ecobee is faulty or configured incorrectly. If it still doesn't fire, the problem is inside the furnace.
5. Check the Air Filter: A severely clogged filter can cause the furnace to overheat during the ignition sequence, triggering a "Limit Switch" trip that kills the heat call. Replace the filter and reset the furnace power by flipping the breaker off and on.

If That Didn't Work

• Check the Flame Sensor: If the furnace starts for 5-10 seconds and then shuts off, the flame sensor is likely carbon-fouled. Use a piece of fine sandpaper or a Scotch-Brite pad to gently clean the small metal rod located in the path of the burner flame, then reinstall.
• Inspect the C-Wire/Power Extender Kit (PEK): If you are using the PEK because you lack a C-wire, check the wiring inside the furnace cabinet. A loose connection on the PEK's "W" output will prevent the heat signal from reaching the furnace board despite the Ecobee showing "Heating."
• Test the Capacitor: If you hear the furnace trying to start but the blower never kicks in, the blower motor capacitor may have failed. This prevents the system from clearing the "heat-up" phase, causing the control board to shut down the burners for safety.
• Verify the Fuse: Locate the low-voltage fuse (usually a small 3A or 5A automotive-style fuse) on the furnace control board. If this fuse is blown, the Ecobee will not have the power necessary to trigger the relay, and the system will remain dead.

Heads Up

• Never jump the R and W wires while the power is on without ensuring your jumper wire is properly insulated; a short to the chassis can fry the furnace control board.
• Avoid using "thick" 12-gauge wire for thermostat connections; the Ecobee Premium terminals are designed for 18-gauge or 22-gauge thermostat wire. Over-stuffing the terminal can cause intermittent connectivity.
• If you have a multi-stage furnace, ensure you haven't accidentally wired the heat to W2 while the Ecobee is configured for Stage 1 (W1) heating.
• Regularly clean your flame sensor every autumn to prevent the "false start" issue where the furnace ignites but shuts off immediately.