How can I modify a 4 wire thermostat to a new thermostat requiring c wire?

Question

I attempted to replace my thermostat with a fancy new Trane z-wave enabled model. To my dismay my home was only wired with a 4 wire setup with no 5 hidden in the bundle.

I followed Insx on the web and moved the g (fan wire) on the furnace to the c terminal. Then I put a jumper wire to connect the g and y terminal at the furnace board.

I wired the thermostat as shown in the Trane manual while assuming the new C wire in its appropriate position and leaving the fan terminal empty. The Trane unit is internally jumpered between Rc and Rh.


The thermostat powers up fine but as soon as I go to do a heating call, or a cooling call it blows the 3A fuse on the furnace board. I also tried wiring the R wire to Rh on the thermostat but the same result. I have a York TG9S series gas furnace and no other hvac system (no a/c). A Pro1 T725 was used as the previously thermostat even though it's really for a heat pump. I've included pictures of before and after wiring as well as furnace schematics.

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Answer 1

You moved the Green to C on one end but the picture with the transformer in the top left shows Green still on G there it should be on C.

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.

Answer 2

Testor 101 answered, since there is no cooling unit use the cooling wire as C and leave the Y terminal on the thermostat empty.

Answer 3

Your pictures have no 24 volt wires going to an AC unit, you show 4 wires at the stat and the 4 wires at the air handler with the G jumped to Y and the Green wires used for Common that looks ok, but nothing from the air handler to any AC unit outdoors as in Yellow and Common. You have a direct short in heat or cool modes yet for cool mode nothing is wired up to an AC unit outside, unlesss there is a splice you are not showing us. Remove the 24 volt wires from the air handler and jump red to green. Does it work or blow a fuse? Jump red to Yellow, if the fuse blows you have a direct short in the board or the 24 vot wiring for the heat mode, since you say either modes blow the sue yet no AC units 24 volt control wiring is wired up I suspect a bad board. bypass the thermostat and jump red to Greenfor fan, the red to yellow for cool then red to white for heat. If it runs fine your stats bad, if it wont run and blows a fuse the stats not the culprit or its wiring, by jumping red to white at the air handler you remove the thermostat from the equaton. See the Yellow and Blue 24 volt wires going outside to the AC unit? You do not show these anywhere.

Answer 4

If you don't have cooling, why are you using the Y terminal? If you have only heat, you can use Red for R, Green for G White for W, and Blue for C`. Also, you'll want to check the rating on the transformer (in volt-amperes) in the furnace and the thermostat (in milliamperes).