just installed an ecobee, it won't turn on at all. did i wire it wrong?

Diagnosis

The ecobee SmartThermostat Premium is a high-draw device because of its integrated screen and sensors; unlike old mercury thermostats, it requires a constant 24VAC power supply to operate. If the unit won't power on, the root cause is almost always a "broken" power circuit. This happens most frequently when the system lacks a Common (C) wire to complete the electrical loop from the furnace or air handler to the thermostat. Without the C-wire, the device cannot pull the current necessary to boot the processor and illuminate the display.

Another common failure point is a blown 3-amp or 5-amp fuse on the HVAC control board. This often occurs during installation if the wires touch while the breaker is on, causing a momentary short. To confirm the diagnosis, check the HVAC control board inside your furnace for a blown fuse (the glass tube will be blackened or the filament broken). If the fuse is intact but the screen is black, the issue is likely a missing C-wire connection or a faulty Power Extender Kit (PEK) installation.

How to Fix It

Follow these precise steps to establish a stable power connection and boot the device:

1. Shut off the power to your HVAC system at the main circuit breaker. Verify power is off by attempting to turn on your heat or AC; if it doesn't engage, it is safe to proceed.
2. Remove the ecobee unit from the backplate by pulling it straight toward you.
3. Inspect the wiring. If you have a blue or black wire that was tucked into the wall, this is typically your C-wire. Insert it into the RC terminal if it's a power wire, but specifically, ensure the Common wire is seated firmly in the C terminal.
4. If you do not have a C-wire, install the included Power Extender Kit (PEK). At the furnace control board, locate the R, G, Y, and W terminals. Disconnect these wires and run them through the PEK adapter.
5. Connect the PEK's labeled wires (R, G, Y, W, and C) back into the corresponding terminals on the furnace board. Ensure the PEK's "C" wire is connected to the furnace's C-terminal.
6. At the thermostat backplate, move the G-wire to the PEK terminal and ensure the R-wire is in the RC terminal.
7. Snap the ecobee unit back onto the backplate and restore power at the breaker.

If That Didn't Work

• Check the HVAC Fuse: Open the blower compartment of your furnace. Locate the low-voltage fuse (usually a purple 3A or orange 5A automotive-style fuse). If the filament is broken, replace it with an identical amp rating. A blown fuse will prevent the ecobee from receiving any voltage, regardless of wiring.
• Test with a Multimeter: Set your multimeter to 24VAC. Touch one probe to the R wire and the other to the C wire. If you don't see a reading between 24V and 28V, the issue is with the transformer or a break in the wiring inside your walls, not the thermostat itself.
• Verify Transformer Capacity: If you have an older system with a very small transformer (under 40VA), the SmartThermostat Premium may trigger a power trip. You may need to install a 24V 40VA transformer to provide sufficient amperage for the device.
• Inspect the Backplate Pins: Ensure the gold pins on the back of the ecobee unit are not bent or pushed in. If a pin is misaligned, it won't make contact with the backplate terminals, leaving the device powerless.

Heads Up

• Avoid "Power Stealing": Never attempt to use a "power stealing" workaround or a third-party adapter not approved by ecobee; this can cause "short-cycling" where your furnace turns on and off rapidly, potentially damaging the inducer motor.
• Wire Stripping: Ensure only about 1/4 inch of copper is exposed. If too much bare wire is showing, it can arc or short against adjacent terminals, blowing the furnace fuse.
• Firm Seating: When inserting wires into the push-terminals, give them a gentle tug to ensure they are locked in. A loose wire in the C-terminal is the #1 cause of intermittent power failures.
• Voltage Warning: Never connect a 120V or 240V high-voltage line to an ecobee. If you see thick black wires with wire nuts, you have a line-voltage system and this thermostat will be instantly destroyed if powered on.