Looking to install Ecobee thermostat. Only 4 wires coming from unit. Ecoboo has power extender kit but there's no control board on the HVAC unit, so I can't locate a c wire. Any help would be appreciated. Pics of wires aren't great, they're all in the same spot.

One group of wires goes to the fan. Other group goes into smaller main box, took the top off and it's just more relays.

enter image description here

Top right wires from thermostat, go down through connector on right. Red turns into blue and goes top left.

Wires to fan.


  • First thing to do is identify what those four wires connected to in your old thermostat. Make sure they are what you think they are. Aug 1, 2019 at 18:25
  • Can you post a photo of where the thermostat wires are connected to the air handler at? Aug 1, 2019 at 23:02
  • add some insulation to that black wire in the foreground
    – Jasen
    Aug 2, 2019 at 6:05
  • Red wire turns into blue and goes into transformer (top left second pic, Green goes to fan relay(bottom left second pic), blue and yellow run down below and go into heat pump (background of third pic, i can take the top off but there isn't room to see inside)?
    – Joe Unser
    Aug 4, 2019 at 5:41

5 Answers 5


After looking at the schematic for the unit and your picture of the wires with the transformer, I can help identify what "would" have been a c terminal if it was brought to a connection screw. In the middle of the schematic, the wire on the secondary side that is "common" to the relays is referred to as the c wire. Now in your picture of the transformer is writing on the transformer which identify the secondary side wires.(primary is the high voltage, secondary is the low voltage) Of those 2, the blue wire connects to the red wire of the schematic (not the c wire), don't touch. The yellow wire connects to the white wire from the schematic ( this is the c wire). Obviously before you start , be sure to turn off the power to the unit!

And be sure to set the correct system settings in the thermostat. Thermostat electrically behaves differently set for a heat pump vs fan coil vs furnace. Good luck!

  • If c-wire is just common ground to keep continues power from red to power the unit, couldn't I just ground straight the the chassis Like posted below? Or could I get different volts?
    – Joe Unser
    Aug 4, 2019 at 6:32
  • Otherwise, the yellow wire coming out of the transformer will be my ground, my c-wire. If it's connected to something else do I just splice into it? To test that it's right I can use a volt meter and connect to the red(at thermostat) and yellow wire from transformer to confirm 24vac?
    – Joe Unser
    Aug 4, 2019 at 7:11
  • C wire is not ground. The furnace controls are an ungrounded system. The transformer reads as white wire is primary neutral, red is primary hot for 208v , orange is primary for 240v. Secondary side is 24v and is a blue wire and yellow with white stripe wire. The yellow with white stripe wire from the transformer connects to 2 white wires( connect there!) That is the c wire connection!
    – user68386
    Aug 12, 2019 at 23:25

The diagram shows the C wire connected to ground, there's a group of 6 quick-connect "spade" terminals welded to the chassis to the right of the second photo Connect to that for the C wire.

The green wire is probably the safety ground so don't mess with that, if you must splice splice into a wire do the white one. but I'd use a connector on one the empty lugs.


I hate to post just a link to detailed instructions, so I'm quoting small parts of the page I found for you. Basically, "follow the instructions that came with your ecobee" . There are a lot of clear photos at that page.

Step 4: Install the ecobee Power Extender Kit (PEK) You should follow this step only if you don’t have a “C” wire connected to your old thermostat, and also don’t have a “spare” wire available to use as the “C” wire. The most common example of this would be if you only had R, G, Y, and W wires at your thermostat.

This step also requires you to crack open your HVAC system and connect something to its main control board. Relax, it sounds way scarier than it actually is. As long as you’ve killed the power to your HVAC, you’ll be fine. However, if the thought of that still freaks you out, contact ecobee’s support crew, or hire a professional to install your ecobee3 for you. The PEK section in the installation manual that came with your ecobee3 also does a great job of explaining how to do this step.

In cases where you only have four wires available, the PEK (which is included with your ecobee3) will create a “virtual” C wire to help reliably power your ecobee3 thermostat.

Those five white wires in the above photo (labelled R, C, G, W, Y) connect to your furnace’s control board, and the four wires coming from your thermostat location connect to the other end of the PEK.

Focus on locating the four wires that are currently running to your thermostat location. They’ll almost certainly be attached to the R, Y, G, and W terminals on the control board. They’ll eventually group together and head off toward your thermostat location.

Loosen the terminals for those four thermostat wires, then disconnect the wires. If there are other wires attached to the same terminals on the control board, leave those other wires connected. You should only remove the four wires that run to your thermostat location.

Next, connect the five white wires (with the colored labels) from the PEK to your HVAC’s control board terminal that matches each wire’s label (R, W, Y, G, C). Again, if there are other components already connected to those terminals, make sure they also stay connected, so that whatever they’re controlling will continue to work. Be careful when tightening down the terminals. You want them snug enough to hold the wires in place (gently tug on the wires to make sure they won’t come loose easily), but don’t over-tighten them and break something.

With one side of the PEK wired to your control board, connect the four wires coming from your thermostat location to the appropriate terminals in the PEK’s cover

And so on.

  • I can't really get into the second part of the furnace. I top the top off but can't see inside without removing the condenser. I think the water lines need to be disconnected from the pump also. Doesn't seem like it's worth it.
    – Joe Unser
    Aug 1, 2019 at 19:22


blue in top left

is the hot side of the 24v transformer. Reading the label, it looks like yellow is the common side. Follow the yellow wire coming out of the transformer on the top left and see if there's somewhere you can jump into it (terminal, wire nut, etc.) As always, make sure you read 24vac across R and what you think is common.

  • Can you explain how to read the transformer? White - Red - Orange ?
    – Joe Unser
    Aug 4, 2019 at 7:16
  • That's the primary side. I believe it means you would use white and red if you were transforming 208v and white and orange if you're transforming 240v. White is the common of the primary side, but you need 24v common.
    – Matt
    Aug 4, 2019 at 14:48

I see this question is 3 years old but it definitely has me cracking up. Those Spade connections that are welded to the unit you were talking about, are all common if you need a spot to hook common to or to get common power from Just connected to that. With these systems that have only four wires just make sure that you get the pek device when you order your thermostat. With that device you will plug in your four wires that you have and it turns it into five wires. You wire the four wires to where they were originally and now you wire that fifth one to Common.

  • What makes you think those wires are welded to the frame? They look like they've been unplugged and are just sitting there/resting against the metal.
    – FreeMan
    Jan 25, 2023 at 15:26

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