I'm hoping to get some help with my thermostat problem. We bought our home brand new and it came with a very basic thermostat (non programmable and runs on batteries). I want to upgrade it to a wifi programmable thermostat so I bought a honeywell one. Our home has a honeywell ventilation system that pulls fresh air from the outside and circulates it through the home. enter image description hereenter image description hereenter image description hereenter image description hereAs I read the instructions to use the g wire for the c wire it seemed easy enough. I am handy enough that I finished my own basement electrical and all do I figured it can't be too hard. So I pulled the old thermostat off after labeling the wires. Put in the new wifi one and used the g wire for the c. Went to my furnace and I have wires running all over. enter image description hereenter image description hereInto the furnace from upstairs is just a 4 wireenter image description here, I have no extras. All wires are connected to the furnace board except the g wire is connected to a wire that runs up to the honeywell ventilation system and then another set of wires returns back to the furnace board. I figured out which of the return wires is the g wire and attempted to move that to the c wire in the furnace board then out a jumper in between the y and g on the same board. At first I thought this made it work because I was able to setup the wifi thermostat because it was receiving power. I setup the wifi and set the temp and the ac kicked on so I figured I was good to go. About 10 minutes later though the power to the thermostat was gone and I blew the 3amp fuse in the box. I tested it again and sure enough, after 10 minutes the new fuse blew. can anyone help me understand what is going wrong and what I need to do to fix it so I can use my new thermostat? I did call the person that installed my system and he didn't seem too thrilled about answering my questions.


2 Answers 2


I think you're out of luck, since the G wire passes through the ventilation control system. You have to leave the G wire as a G wire, or remove/bypass the ventilation system. The easiest solution, would be to run a new 5 wire cable. Or don't use a fancy thermostat.

If you follow the G wire through the system, you can get a better picture of what's going on.

G wire through the system

When the thermostat energizes the G terminal, the Gt terminal on the ventilation control receives the signal. The ventilation control does whatever it does, and eventually energizes the Gf terminal. When the Gf terminal is energized on the ventilation control, the furnace receives the signal and starts the blower.

From what I can tell based on the images, this is what the whole system looks like.

Whole System

On a side note. The installer that left that rats nest of wiring in the furnace, should be ashamed of himself.

  • That's what I was afraid of. Thank you for the response and help. I hadn't really worked with wiring before but I'm glad to know that it really is a mess of wires because that's what it looked like to me.
    – user21975
    Jun 12, 2014 at 3:48

enter image description here

The picture below you would pull the green wire off of Green at both the stat and furnace and move it to C/Common, then you jumper Yellow to Green at the furnace.

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