Furnace

t-stat wired wrong

It sounds like the thermostat was installed incorrectly..... There is an industry standard color coding of wires, but unfortunatly this is not always followed. The format is Rh and Rc are a red wire (these terminals may be jumped together if you have gas heat), which is 24v hot, W is a white wire, which is 24v common, Y is a yellow wire, which is power to the outdoor A/C unit, and G is green, which turns on the high speed fan. I must assume that your old thermostat would turn the fan on, since you mentioned it above. Your best bet is to check at the furnace to see which wires go to where
Please Note that if you have oil heat the jumper between Rh and Rc should be removed. Rh is power for the heating circuit (usually red), and is connected to the oil burner primary control. Rc is power for the cooling circuit (usually blue) and is connected to the fan relay. I hope this helps