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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?

Nima Talebi
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1 Answers1

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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.

KMJ
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