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I've read How can I add a "C" wire to my thermostat? and checked out the Venstar Add-A-Wire Accessory. I've watched Honeywell's YouTube video on "DIY Wi-Fi Thermostat Substitute G-Wire for C-Wire," but I don't have a terminal block to move the G-Wire (Fan on) as described in the video. This is where I'm stuck.

What options (if any) do I have to re-wire this to work with a wireless thermostat that requires a C-Wire?

Old thermostat: Honeywell RTH2310

New thermostat: Honeywell RTH6580WF

Wiring diagram w/old thermostat

Furnace

Junction

Old Thermostat

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  • Did you try calling the 800 number in the instructions? When I installed my wi-fi thermostat I called them and they were very helpful.
    – OrganicLawnDIY
    Dec 23, 2013 at 0:25
  • Didn't check if they're open on weekends (that's the only time I have to work on this). I have some hesitation trying to explain my entire setup over the phone. I feel like I would get more effective help by showing pictures of the setup.
    – Kermit
    Dec 23, 2013 at 0:28
  • What? You work in a coal mine or something all week with no phone access? They're not going to walk you through it over the phone. You tell them your system, the wiring on your old thermostat and they tell you how to hook up the new one. You write it down and do it on the weekend. But if you feel you'll get better advice from random people on the internet after not providing enough info, an incomplete wiring diagram and pictures of a mess of wires I guess that's an option. One day I'm going to invent a way to smack people in the back of the head Gibbs style online. :)
    – OrganicLawnDIY
    Dec 23, 2013 at 0:49
  • I might as well work in a coal mine. I would love to update the wiring diagram and add more info if you could elaborate on what's missing.
    – Kermit
    Dec 23, 2013 at 1:35
  • What type of heating system do you have? What type of cooling system do you have? Are they 2 separate systems or one? Your diagram implies separate but your thermostat wiring implies a single transformer system. Your wiring diagram shows 4 wires connected at only one end. It shows a green wire connected to Rc on the thermostat but there's no wire connected to the Rc terminal in the photo of the thermostat wiring block. Just the jumper wire from Rc to Rh which you may not need for your system. The video you linked to is for single heat systems. I hope you didn't try to follow it.
    – OrganicLawnDIY
    Dec 23, 2013 at 1:50

1 Answer 1

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The C-wire substitution requires you to use the G-wire. When using the G-wire as a common you have to change it to C at the thermostat, and then to a common in the bottom panel of the unit. In your situation, your specific unit uses an older style control board made by White Rogers. This early revision of the board did not have a terminal bus on the board the thermostat, air conditioning, and fan speed 24v connections. Wires were used instead if a terminal bus, and only has 4 connections (your W, G, R, Y wires). To have your new thermostat work, you will need to remove the thermostat wire at the unit. Then you must either ground it to an exterior panel of the unit, or using spade, attach it to one of the open neutral buses on the control board. After doing this and adding the G-wire on C at the thermostat, it will work fine.

Now that the thermostat is working, you have lost the ability to control the fan. A little trick if you want to retain a way to control the fan without having to run a new 5-wire is to get a piece of 18/2 wire and run it into the control panel of the unit. Attach one wire to R and one wire to G. Attach a switch box to the side of the unit and run the wire into it. Connect the 2 wire to any light switch and mount it into the switch box. Throw your switch cover plate over it, and you have yourself a home made, manual fan switch that you can activate by going to the unit.

For Reference:

  • W - Heating
  • R - Continuous 24v Power
  • G - Fan
  • Y - Cooling

  • C - Common

    R is used to energize, C is common (For Optional Accessories like A/C, Etc..)

  • Closing R and W will initialize the heating cycle
  • Closing R and G will initialize the blower to run on low speed
  • Closing R and Y will initialize the cooling cycle and the blower on high speed
  • Closing R between Y on certain units will only energize the A/C relay and not activate the blower. These units require the thermostat to energize the Y and G terminals together to initialize the cooling cycle and the blower on high speed. Make sure to check that the Air Conditioning is working properly, and that the blower is running on high when the Thermostat is calling for cooling.
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  • Very nice idea to just use the 'fan-only' wire as common, since it's practical to live without fan-only (whereas clearly it is unthinkable to live without wifi connectivity). One note though: on my furnace, the fan-only (G) connection actually runs the fan at high speed (verified by measuring the current draw using an ammeter). Particularly interesting is when heat is called for while fan-only is on, the furnace slows down the fan and then fires up the burner.
    – gregmac
    Apr 14, 2014 at 15:05
  • Sorry for the late reply. Your air handler is an older model. A lot of new furnaces just require the thermostat to close the terminals between Y and R. One possibility is to jump the g and r terminal at the furnace, or wire it with a switch in between. Your fan will continue to run constantly, but it will not use a ridiculous amount of power due to not having to stop or start, and you will get constant air circulation throughout the home. This helps to keep all the rooms within a close average temperature.
    – Mnc123
    Aug 2, 2014 at 6:00

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