What Is a Thermostat?

A thermostat reads the room temperature and closes a circuit when it falls below the setpoint (heat) or rises above it (cool), which starts the furnace or air conditioner. Most also have a fan switch with two settings. AUTO runs the blower only while the system is heating or cooling, and ON runs it constantly, which is why a thermostat left on ON makes a furnace seem to blow cold air between heating cycles.

Basic models run on two AA batteries, and a blank or dim display almost always means they are dead. Smart thermostats need a common wire (the C-wire) for steady power, and without one they can reboot or lose settings in cold weather. If the thermostat shows a call for heat and the furnace does not respond, the fault is downstream. A thermostat that reads the wrong temperature, because it sits in direct sun or over a heat vent, makes the system short-cycle.