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I want to connect an Ecobee to my house, under the constraints imposed at the thermostat (pictured below); you will immediately note that there is no yellow/Y wire (because there is no air-conditioning system here), and that there is no blue/C (common) wire either. That leaves:

  • red; connected to Rh and Rc (power; the two are bridged)
  • white; connected to W (heat/furnace), and
  • black; connected to G (fan)

The instructions on the Ecobee website show how to use the provided PEK (Power Extender Kit) to do a 4-to-5; i.e., it assumes you have a Y wire, which I do not have.

My question, as you can already guess, is how I can get this system up and running without running a C all the way from the furnace to the thermostat; it's an old house and the path seems ugly. I already checked to see if by chance a C was lurking behind the wall, wrapped around the existing wires—alas, no.

Next idea then might be to provide 24V AC locally (not from the furnace, but from the nearest AC power near the thermostat. To do that, maybe I could do something like this...

At the thermostat:

  • 24V/AC ⇨ Rc, C
  • Red ⇨ Rh
  • White ⇨ W
  • Green ⇨ G
  • let the Y port to remain unfulilled

At the furnace:

  • Follow the standard 4-to-5 documented approach from Ecobee here

One issue is that I've still not really address the missing yellow wire. But at this point (i.e., with the proposed setup above), do I need to?

Note that a goal here was to not repurpose the G wire as the C wire and lose independent fan control.

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Will this work, or will some very bad things happen—either immediately, or over time?

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If you have only three wires for a heat-only system, you have three options that don't involve running new wires. These are all described in the ecobee documentation.

  1. Use your wires for R, W, and C. You will no longer have independent fan control. This is a supported solution. If you do not need independent fan control, this solution makes no other compromises.

  2. Install using a Common Maker. This removes the need for a separate C wire and will allow you to retain independent fan control.

  3. Install a local 24V AC power source near the thermostat.

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  • I don't have enough rep to upvote, but thank you. The link for 3, which seems like the simplest approach, is broken. But knowing what to search for, I found the page here: support.ecobee.com/s/articles/3-wire-heat-only-thermostat-R-G-W Commented Jan 11 at 2:03
  • Sacrifice G is the easiest, #1
    – Mazura
    Commented Jan 11 at 2:58
  • @NimaTalebi you're allowed to accept the answer. Also I'm not sure what link is broken, I only linked to the main answer and the link still works for me. It looks like the same address you wrote as well.
    – KMJ
    Commented Jan 11 at 3:38
  • @Mazura the question specifically said the goal was not to lose fan
    – KMJ
    Commented Jan 11 at 3:40
  • The broken link I'm referring to was from a page this, stating If your system has three wires (R, W, G) go to My thermostat has three wires. Am I compatible with ecobee? ⇦ that is the broken link. Commented Jan 11 at 18:09

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