I'm trying to replace existing Honeywell controller for my furnace with Ecobee 3. The Honeywell controller does not use a C wire, however there are a black wire and blue wire in the wall behind the controller. From furnace board it seems to me the black wire is C wire so I use it in Ecobee wiring. Unfortunately Ecobee starts with wiring error message.

Attached are wiring on Ecobee, furnace board and furnace diagram. I also noticed there is "Fast-Stat Common Maker Thermostat Wire Extender" installed in the furnace.


From what I can see, the brown wire connects thermostat and furnace. It is split into white and red, and then fed into Fast-Stat 5000 transformer.

Furnace wiring ↑ Furnace wiring

Furnace wiring ↑ Furnace wiring with Fast-Stat 5000

Brown wire ↑ Brown wire that goes to the thermostat

Furnace diagram ↑ Furnace diagram

Ecobee wiring ↑ Ecobee wiring

Honeywell wiring (previous working thermostat) ↑ Honeywell wiring (previous working thermostat)

FAST-STAT 5000 Diagram ↑ FAST-STAT 5000 Diagram

Fast-Stat 5000 sender behind the wall ↑ Fast-Stat 5000 sender behind the wall

  • 2
    First you should shorten the amount of bare wire going in to each terminal - easy to create a short like that.
    – Solar Mike
    May 14, 2021 at 7:01
  • Can you get us a close-up of where the wires from the thermostat cable connect at the furnace, please? May 14, 2021 at 11:41
  • Why do you think there is not a common? It is listed on the schematic? How far is the furnace from the thermostat? The wire extender also needs a common. Watch out playing with brown and blue if not part of the thermostat cable they just might be line voltage. I agree with solar mike trim those wires stat shorts usually are not a big deal but if your supply and common get shorted it may cook the transformer.
    – Ed Beal
    May 14, 2021 at 14:04
  • @ThreePhaseEel Sure, just updated. It is a two-wire brown wire that is fed into the transformer.
    – zlx
    May 15, 2021 at 4:41
  • What is the picture of wires coming out of the wall into a backplate, posted after the picture of the Ecobee? What is that backplate and what are those wires? In the picture of the Ecobee's backplate, is the Faststat receiver in the wall? You need to fish it out to make sure you know what you're dealing with. The furnace end appears to wired correctly.
    – jay613
    May 15, 2021 at 20:03

1 Answer 1


The Fast Stat device consists of two boxes, one located at your furnace and the other located at your thermostat location. They connect to each other using your existing two-wire thermostat cable, and they provide 6-wire connections between furnace and stat. You need to read the Fast-Stat documentation and make sure that the furnace and Ecobee are each connected to the Fast-Stat in a way that meets the needs of all three devices ... i.e. the furnace, the Fast-Stat and the Ecobee.

None of your photos shows the thermostat half of the Fast-Stat. It might be buried inside the wall. You need to find it, and you need to figure out which color wires are being used with it to correspond to the Ecobee's needs.

They have a web site with instruction manuals available for download.

If you have specific questions about how to apply the Fast-Stat installation instructions to your particular furnace, you can follow up here. But I think it's probably already installed correctly ... you just have to find the other end and map out the colors.

  • That's a great point. I do notice there seems to be another converter inside the wall behind the thermostat, I cannot fetch it though. I found the FAST-STAT wiring instructions on website and included in original post, however it seems the wiring on thermostat side is expected, with black wire as C wire.
    – zlx
    May 14, 2021 at 18:35
  • @zlx -- can you please post a close-up of where the thermostat cable connects to the wiring inside the furnace? (You may have to move the black power cord aside to get a good shot) May 15, 2021 at 1:23

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