A 120V line voltage thermostat is a device used to regulate the temperature of a space or area. It is designed to be connected to a 120V power source, such as a wall outlet, and can be used to control heating or cooling systems. The thermostat works by measuring the temperature of the space and sending a signal to the heating or cooling system to adjust the temperature accordingly. The thermostat can be set to a specific temperature, or it can be programmed to follow a specific temperature schedule. It is important to note that the thermostat must be properly installed and maintained in order to ensure it is working correctly. If the thermostat is not installed or maintained properly, it can cause damage to the heating or cooling system, or even cause a fire.