I really get a little aggrivated by postings that downgrade the use of heat sources. "Chickens don't need heat" is a real negative approach even though it holds true to decades of chickens surviving without it. Finding the right comfort zone for your chickens without the numerous winter maladies that affect them is the key factor to solving anyones worry of their chickens being warm enough and protected from the weather. "Sure", there are good hardy breeds that do very well in cold climates such as Alaska that have short summers and cold dark winter months and my hats off to those people and nay sayers. A lot can be learned by their methods and others as to how to raise chickens without the use of artificial heat souces.
The reality is that in todays age and era, many useful products have been made available to provide an alternative if we choose to. As many have mentioned, too much heat may not be healthy and could do more harm than good! No argument there, thats a fact! Once again I will use the term "Comfort zone. There are tons of debatable issues that all play a part in keeping your chickens in the "Comfort zone" during the winter weather that lies ahead of you. Your coop construction, insulation, air ventilation, and the list goes on and on are all the key elements to aide you in providing sufficient living quarters without using artificial heat. But then again, you may find it very useful to add some heat if you failed or lacked doing all that is necessary to live without it!
Whtever you opt to do, keep it safe and choose wisely!