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.
Click for full size image