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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.

Trane thermostat Trane thermostat wiring York Furnace C wire substitute wiring Pro1 Original thermostat wiring York Furnace original wiring Furnace schematic York info

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Thanks Testor101, I don't have a lot of experience with HVAC so I just wired it as suggested on the web. I subbed the Y wire directly to C, and now everything is working fine w/o blowing any fuses. Thanks for your help!!! –  user19152 Jan 11 '14 at 21:52

3 Answers 3

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

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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.enter image description here

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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 forC`. Also, you'll want to check the rating on the transformer (in volt-amperes) in the furnace and the thermostat (in milliamperes).

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