trying to install RTH9580. My thermostat has only 4 wires so I am trying to substitute the G wire instead of C wire. On the furnace I put a jumper between Y and G wires and moved G wire to C wire location. See pictures? I still get no power. Any ideas on the wiring on the furnace side?

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  • The pictures appear to be the same. – Tester101 Feb 10 '16 at 17:56
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    What's the model number of the furnace? – Tester101 Feb 10 '16 at 17:58
  • Again you just said what I did above Tester 101, can't you find an original answer of your own?? The proper answer is to run a five wrie thermostat cable instead of trying to bastardise your system. No it isn't easy but after instally three to four central air conditioning systems per day and doing it the proper way, you don't have to worry about too much current draw from adding what could be a heavy condenser contactor draw. If you are going to give others information at least have the decency to give the right stuff. It's up to them after if they want to screw up the warranty on an expensiv – Richard Feb 14 '16 at 16:07
  • @Richard Please learn to use comments. – Tester101 Feb 16 '16 at 19:27

Check your voltage from R to C terminals on the board to make sure you have 24 volts there. If you don't mind the feel of air moving, you would be better off just putting a short jumper from R to G. This will energize the fan relay to keep it running continuously and the best for you for constant air filtration and if equiped with a power humidifier will maintain the house humidity properly. Then using the G wire as proposed should work. Before you did this, did the fan start when flipping the fan swich on the thermostat to on for continuous fan operation? If you never tried it, it might be a good idea to change things back and do so to eliminate the chance that there's a break in the green wire. Or if your new thermostat is in place, leave the power off change things back on the Furnace and disconnect the red for power and the green and just hold them together with the power back on for a few seconds at the thermostat to see if the fan starts, again to verify no breaks in the green wire.

  • If you do this, the fan will always run, unless you turn the furnace off at the breaker/serviceman switch. – Tester101 Feb 12 '16 at 16:09

The question is missing the second picture showing the modified wiring. But giving the information provided it sounds like you made the correct modifications to allow the common wire to supply constant 24 volts to the wifi thermostat, bypassing the ability to operate the fan-only "green wire".


  • Check the transformer is supplying power and is not bad.
  • Check ohms of any fuses.
  • Check wires are tight and not touching other wires.
  • Check ohms through TSTAT wires as sometimes the tiny wires can break and a entire new TSTAT wire is required.

I have a furnace that is at least 3 decades old. It did not have a C wire. I picked up a 24 volt transformer and used that. SOrry I don't have the specifics but there was some info on the web about it. Just be sure everything is in code.

THe transformer provided the required power for my Honeywell wifi theromostat. It's been working well for over a year.

  • I suspect I know what you did, but this answer is lacking details others may need. How would someone hookup this transformer? Do they install it at the furnace or directly wire it at the thermostat? If at the thermostat, how do you handle the furnace wiring on one transformer and the thermostat trying to close the circuits with another transformer? – BMitch Apr 15 '16 at 12:01

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