It would be pretty rare for a 9V battery to last for 15 years of continuous operation. In my thermostat, which has features similar to yours, the battery is just for backup in case the power goes out, and the thermostat normally runs off the 24V power from the furnace circuit (it works fine if you remove the battery). It&#x27;s possible that your thermostat works the same way and your battery has been dead for many years.