When it comes the programmable thermostats there are two major types: Line voltage and low voltage programmable thermostats, it’s very important you know which type you need before you purchase a new thermostat.
A line voltage system uses direct current instead of only 24 volts, which is also known as a low voltage line. The direct current will be either 120 or 240 volt electrical line most commonly used for electric heaters or a direct-wired electric furnace.
Are you still confused? No problem, let me try to explain a little more: In the simplest of terms, a line voltage thermostat is connected to a two or three Romax wire that will be either 14, 12, 10 or 8 gauge in thickness. And low voltage thermostat will use 24v bell wire is much smaller is size so you will be able to tell very quickly if you are taking an old thermostat off.
To go into more detail if we start from your “fuse box or “circuit breaker panel” the electrical wire from the panel to your heater will be 120 or 240 volts and the wire is measured in gauge thickness starting at 14 gauge to 8 gauge and once you electric wire gets to the heater you will need another wire to connect to your thermostat. A line voltage thermostat will use the same type of wire from the heater to the thermostat a low voltage thermostat will go through a transformer and a relay which knocks down the power from 120 of 240 volts down to 24v. A 24v wire is what is called “ bell wire” it’s very thin and you will be able to tell the difference between 120 and 240 volt wire and once the wire goes from the heater to the thermostat.