I have a Coleman D615MH furnace. It will not kick off. I have to turn both breakers off. I just replaced the thermostat, thinking that was it. I turned the breakers on & it was still running. I turned the thermostat off & it continued to run. So I have turned the breakers off again. Any ideals on what it is. The furnace works, so when it reaches the temp I want I just kick the breakers off. When it gets chilly I turn the breakers back on. Any help would be appreciated..
Sounds like a stuck relay or a bad control board at the furnace end; or a short in your thermostat wiring. You could try disconnecting the thermostat wiring at the furnace end. If it still runs with the thermostat disconnected entirely, you're looking at a stuck relay or bad control board.
If it doesn't run with the thermostat disconnected, check for shorts in the thermostat wiring between the furnace and the thermostat. You could do this with a multimeter. If you don't have a multimeter, try removing the thermostat from the wall and connecting the thermostat to the furnace with short length of wire. If the system behaves properly with the short length of wire, then you've isolated the problem to the thermostat wiring between the furnace and the thermostat installation.
If your system is that old, you probably only have two wires hooked up to the thermostat. Pull the thermostat from the wall and if it shuts off, then the problem is wiring from the thermostat, or the thermostat itself. A newer furnace with all new wiring is safer and tons more efficient that old pig you have in basement. It's cheaper now to replace then later.