The household thermostat have relays (which is why you hear the click) when the heat comes on and off. Now I haven't looked in depth into it but most furnaces that I have seen have a transformer on them that step the voltage down which will run to the thermostat and back to the furnace. Although I believe the relay is powered by the batteries of the thermostat the relay will not handle the current for heating in most cases. You might get away with a small heat pad but it wouldn't be worth the time or risk in most cases. Then you are still stuck with the temp sensor inside the case so you would have to desolder it and wire it so it was in a appopriate spot to measure temp. Also household thermostats have a degree or better swing so you'd probably be better off just buying a inexpensive zoomed 500r. If you want better regulation you eventually have to move up to a proportional thermostat.

Dion Brewington
Owner, Spyder Robotics