Southern_Man is correct, insulate the building. I'm also in TN (Cumberland Plateau) and my well house is 8x8. I've got R13 in the walls and R19 in the ceiling. I run one flood light during the winter and it keeps everything above 35 during the winter. I've got a freeze alarm that will text msg my cell phone but that has only happened once when it never got over 35deg for about 8 days straight (rare in mid TN).