I have a forced air HVAC unit that gets heat from the hot water tank. There is no limit switch on the fan -- it comes on with the heat. Last week the thermostat would not shut off the heat whether I reduced the temperature setting or switched it off. When I switched it to cool, both the heat and AC ran. I pulled the thermostat off the wall and the heat stopped. Replaced the thermostat and it worked fine for about 5 days. Now the heat is running nonstop again. The only way to shut it off is at the breaker. Before I call Honeywell to blame the new thermostat, I'm thinking there must be something in the wiring or HVAC unit.

Check the voltage to your thermostat. It may be too high and burning the thermostst up. It should state the voltage on the transformer. Never seen it before but...