Unless you are dealing with a particular breed of chicken (like a Serama, say) that isn't cold hardy, most chickens can handle the cold far better than they can handle the heat. It's all that down insulation that nature puts on them!
Again, speaking generally, most adult chickens don't need heat at all provided that they have a shelter that keeps them from getting wet, protects them from drafts, and is well ventilated so the air inside doesn't become humid (that's how they get frostbite on their combs).
On the other hand, people in warm climates frequently have to do lots of things to ensure their chickens can survive hot summers.