Line thermostats turn on and off electric baseboard heaters. Unlike most home installation thermostats that control turning the furnace or air conditioner on and off via low voltage, these actually either control a higher voltage, 120 or 240 volts. You see, the low voltage circuits pull in a relay that completes the furnace circuit, whereas this thermostat breaks and makes the circuit that directly feeds the furnace. The thermostat breaks the “line” side of the circuit to disconnect it and connects the circuit to turn the heater back on.
On 120-volt circuit feeds, the power is fed in on one side of the line thermostat and out on the other. It basically is a single-pole switch that interrupts the circuit feeding the thermostat. The twist is that the contacts are pulled in when the thermostat calls for heat, but the switching is done internally in line thermostat, making it an all-in-one device.
A 240-volt line thermostat acts pretty much in the same way, only the device makes and breaks 240-volt connections feeding the baseboard heater. This requires two incoming feeds and two outgoing feeds. This device would be similar to a single-throw, double-pole switch. That just means you flip one switch to control two poles, or circuits, in order to control a circuit.
Baseboard thermostats make adding heating controls to a baseboard heater easy for installation. These thermostats can either be mounted on the baseboard heater itself or in a box mounted around switch height in your home. Depending on the installation and voltage your baseboard heater runs on, this is the simplest heating installation you’ll probably tackle as a do-it-yourselfer.