I've replaced my very old (mercury) Honeywell thermostat with a new one (round, Model: YCT87K1011), but I can't get it to control the temperature. If I connect the two wires one way, the furnace comes on but won't shut off. If I reverse the wires, the furnace doesn't come on at all. I've actually tried two of these replacement thermostats with the same results. Any suggestions? John

Hey John, man that ole T87 has been around for a hundred years and I have installed many of them, but I cant remember the letters on the wiring. Seems to me that it is R-W-Y, any way you will use the R and W. I have a t87 at home still in the box and I could wait till I got home to help you but Im sure its the R and W terminal. Dont forget to set the heat anticipator. Set it the same as the current value of your gas or oil valve.