We use these Thermocubes in a few places on the farm.  They are great because they have no electronics and no way to make mistakes! They have been working for 3 winters so far.

They come in different temperature ranges. This one turns on at 2C and turns off at 7C. We have a heater plugged into it.  That way when the temperature drops below 2C, power flows through the Thermocube and turns on the heater.

If you need to prevent an area from going below freezing, they work quite well!