Hi, What is happening is the control wiring from this thermostat is touching on the heat side from somewhere in the wall back to the furnace, so it calls for heat. It gets its power from the transformer on the furnace. Try disconnecting it at the furnace and it will shut down. If this is heat only, should have only 2 conductors and they would have to be touching somewhere. Look carefully at the wiring behind the stat and around the furnace for it may have melted together or bare spots where it is touching. This is the only thing that would make it come on with out the stat. Please let me know what you find. You can also ohm the red and white out to see if there is continuity, and if so, they are touching and you will either need to find it or run a new set of stat wires to solve this. Get back to me. Good Luck,
Shastalaker7
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