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.
Update:
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 with Fast-Stat 5000
↑ Brown wire that goes to the thermostat
↑ Furnace diagram
↑ Ecobee wiring
↑ Honeywell wiring (previous working thermostat)
↑ FAST-STAT 5000 Diagram
↑ Fast-Stat 5000 sender behind the wall
Best Answer
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.