ecobee keeps saying 'heating' but it's blowing cold air, the furnace isn't igniting.

Diagnosis

When your ecobee SmartThermostat Enhanced displays "Heating," it means the thermostat has successfully closed the circuit between the Rc (power) and W (heat) terminals. The thermostat is doing its job by sending a 24V signal to the furnace control board. The fact that the blower is running but the air is cold indicates that the "Inducer Motor" is likely spinning, but the "Ignition Sequence" is failing. This typically happens because the furnace's safety loop is broken. If the pressure switch doesn't detect a proper draft or the flame sensor is coated in carbon (soot), the control board will kill the gas valve to prevent a dangerous buildup of unburned gas in your home. You can confirm this by listening to the furnace: if you hear a humming sound (the inducer) followed by a clicking sound (the igniter/gas valve) but no roar of flame, the issue is internal to the HVAC unit, not the ecobee software or wiring. If the blower starts immediately without any attempt to ignite, your furnace may be in a "lockout" mode due to repeated failed ignition attempts.

How to Fix It

To resolve this, you must move from the thermostat to the furnace cabinet to identify the specific hardware failure.

1. Shut off the power to the furnace using the dedicated breaker or the toggle switch located on the side of the furnace cabinet.
2. Remove the upper blower compartment door. Note that most modern furnaces have a "door switch" (a small plunger) that must be pressed in for the unit to power on; if the door is off, you may need to manually hold this switch in to see the diagnostic lights.
3. Locate the LED diagnostic light on the control board (usually a red or green LED). Count the number of flashes in a sequence. For example, a steady blink of 3 times followed by a pause often indicates a pressure switch failure, while 4 blinks often point to an igniter or flame sensor issue.
4. If the light indicates a flame sensor issue, locate the flame sensor—a thin metal rod protruding into the heat exchanger. Use a piece of fine-grit sandpaper (400-600 grit) or a clean Scotch-Brite pad to gently scrub the oxidation off the metal rod. Do not use a heavy abrasive, as you can scratch the protective coating.
5. If the light indicates a pressure switch error, inspect the black rubber hose running from the inducer motor to the switch. Ensure it is not cracked, pinched, or clogged with debris.
6. Restore power and set the ecobee to "Heat" with a setpoint 5 degrees above room temperature to test the ignition sequence.

If That Didn't Work

• Check the Gas Valve/Supply: Verify that the gas shut-off valve leading into the furnace is fully open. If you have a gas meter, check if the meter is spinning. A failed gas valve solenoid will prevent the furnace from igniting even if the igniter is glowing white-hot.
• Inspect the Hot Surface Igniter: Visually inspect the igniter element. If you see a visible crack or a "burnt out" dark spot on the element, the igniter is dead and must be replaced. This is a common wear-and-tear part that typically fails every 5-10 years.
• Test the C-Wire/Power Stability: While unlikely if the screen is on, use a multimeter to verify 24VAC between the Rc and C terminals at the ecobee backplate. If the voltage is dipping below 20V, the furnace control board may not trigger the relay consistently.
• Clear the Lockout: Some furnaces enter a "hard lockout" after three failed starts. Turn the power off at the breaker for exactly 60 seconds, then flip it back on to reset the control board's internal memory.

Heads Up

• Safety Warning: Never bypass the flame sensor or pressure switch to "force" the furnace to ignite; this disables critical safety features and can lead to a gas explosion.
• Air Filter Maintenance: A clogged air filter can cause the heat exchanger to overheat, triggering the "High Limit Switch" which shuts down the burners while keeping the blower running to cool the system. Change your filter every 30-90 days.
• Condensate Line: If you have a high-efficiency (condensing) furnace, check the PVC condensate drain line. If the trap is clogged with slime, water will back up and trigger the pressure switch, preventing ignition.
• Voltage Check: Always use a digital multimeter when testing wires. Never touch the Rc wire to the C wire, as this will blow the 3-amp or 5-amp fuse on the furnace control board instantly.