I, too, never put heat in the coop. But last year, I read a book by a British chicken keeper and they stated that they don't take the dirty litter out of the coop but just keep adding clean on top of it so the chickens have a near clean floor in which to walk. In doing so, the litter and poop on the floor begin to compost which generates a small amount of heat which can add just enough heat for the roosting chickens, as heat rises.
I tried this last winter and I think it worked out pretty well. I use straw on the floor and in the early spring when I cleaned out the house, the litter was dry- no mold, no moisture and it did not stink as bad as I thought it would- behold the wonders of composting. But you do have to be careful not to let the litter get too deep, if I recall, the highest point it should reach is seven inches and then be mucked out. If there is moisture present on your floor, don't do it, you can get mold and that is bad for the chickens.
For this winter, I have built a "poop tray" under their roost, last year it was deposited on the floor so this worked really well but now with the tray, I will clean it out regularly and not to do this for a source of heat, but I will not supplement with a lamp either. They will just have to be chickens and use the coat nature gave them.