My backyard I don't add heat to any of my coops unless there is chicks under 4 months old in them. It has been down in the single digits here before and I have never lost a bird to the cold. Now all that being said in different parts of the country where it gets say down to the 20s and stays there for ever then I would turn hear on around 28 or so and only then over the water to keep it from freezing. Now below zero is still a different story and I would try to keep the coop just above the freezing mark.