The thermostat is the command center for an HVAC system. The thermostat is what will switch the equipment on and off. Most people are familiar with the thermostat on the wall for their heating and cooling, but they’re in almost everything from water heaters to freezers and kitchen equipment.
Modern thermostats sense temperature using a thermistor. The thermistors resistance value changes depending on the surrounding temperature. A small microcontroller converts the measured resistance to temperature and decides based on desired settings if heating or cooling should be switched on.