How to troubleshoot your ecobee thermostat installation

With the holiday season in full swing, I'm certain many of our readers will be receiving some awesome tech gifts from loved ones. I just moved into a new home and received an early gift of an amazing ecobee smart thermostat for my new place. Although in my last home the installation was as easy as swapping some wires from the old system to the new ecobee, my new home wasn't as kind to us. But with a bout of googling and posting to HVAC support forums, we persevered and are enjoying the features of a connected thermostat!

Here's how you can set up your ecobee thermostat if installation isn't exactly going smoothly.

How to install your ecobee thermostat

We've briefly detailed how to go about the installation before. If everything goes as expected, here are the steps in short form:

  1. Power off the breakers for the HVAC system (heating, ventilation, and air conditioning).
  2. Remove the cover of the old thermostat.

  1. Prior to unscrewing anything, label all of the wires with the provided stickers. Each wire should have an associated connector that is marked by a letter depending on what kind of additional services you have, like heating, A/C, humidifiers, etc.).
  2. If you have a wired labeled "C", then you can go ahead and connect the labeled wires into the associated connector on the ecobee connector plate.

  1. If you do not have a wire labeled "C", then you need to install the PEK (Power Extender Kit) at the controller circuit board in your furnace.
  2. Once all is connected, you can plug in your thermostat and go on with the app installation detailed in our guide.

Running into snags

Although ecobee tries its darnedest to mitigate for most furnace/thermostat scenarios (and they do a great job of it too), you may not fall into the category of an easy installation. I fell into this unfortunate category in two ways. First, I had no wire labeled "C".

The "C" wire is a powered wire that supplies 24VAC to your thermostat. Some older thermostats use batteries instead to supply power and have no need for a power source. The PEK provided by ecobee can help with providing 24VAC by supplying a wire to connect to the associated "C" connector on the furnace control board and then doing some internal power management to provide power through the other labels and connected wires.

My next obvious step was to install the ecobee-provided PEK. So down to the furnace room I went, opened the side furnace hatch to get access to the internals and was greeted with this nightmare:

Secondly, not only did I not have a wire labeled "C", but I also did not have an associated furnace control board with any labeled wires. Yikes!

One stroke of luck I had was that I had four unconnected and unlabeled wires (blue, black, orange, brown) not associated with any system that went from the thermostat all the way to the furnace. Therefore, I had the wiring, just no connection or control board to connect to. But fear not; here are some steps we took to get things working.


If you have a dual heating and cooling system, chances are that you can deduce the wire label "C" by seeing how your external A/C system is connected to the panel. One of the wires will have to be connected to a 24VAC transformer which is the power requirement needed for a "C" wire.

  1. Turn the furnace breakers off.
  2. Trace the wiring from the thermostat to the furnace. Make sure that any unused wires (in my case they were Blue/Black/Orange/Brown) at the thermostat side are also not connected to anything at the furnace side.
  3. Trace the 2-conductor wiring from your OUTSIDE AC unit to the furnace. One of the conductors will be connected to the YELLOW conductor from the thermostat (most likely using a marrete). The other conductor will be connected to C side of the 24VAC transformer. My 24VAC transformer was in the very back behind all of the wired mess. But once I pushed the wires aside (DOUBLE CHECKING THE BREAKERS WERE OFF!) I could visibly see a rectangular transformer with it's 24VAC specifications listed upon it.
  4. Pick any one of the unused conductors (the blue conductor its what I picked) from the thermostat wiring, and connect it along with the other conductor from the outside AC unit to the C side of the 24VAC transformer (they should all be in the same marrete).
  5. Make certain there are no exposed wires and turn the breakers back on.
  6. Test the connection with a multimeter at the thermostat end of the wiring by seeing if you detect 24VAC between the wire you just connected at the furnace (in my case blue) and the RH red wire.
  7. Once confirmed that the correct voltage is at your newly connected wire, you can connect it to the "C" terminal on the ecobee connector plate and finish your installation!

One last thing

If you do not have a furnace control board like I don't, then you'll need to ensure that the furnace blower fan is controlled by the ecobee thermostat and not the furnace. This can be done on the ecobee thermostat itself under preferences.

Final thoughts

Do you have any "smart" installs-gone-wild stories you'd like to share? Let us know how you fixed them in the comments!

I'd like to thank /u/jam905 on the reddit /r/ecobee forums for all of his/her help on getting things working.

Anthony Casella