Do not heat the coop. Chickens do really well in the cold. Just make sure it's well ventilated with NO drafts. You would want to make sure you have something to keep the water from freezing though. I have their water on a warmer outside in the run and it keeps it from freezing. You shouldn't store water inside the coop either since that's a source of moisture and will cause frostbite to the chickens while they're roosting in the winter.