Heat in the summer is going to be a bigger problem with BO's, than cold. They are good cold weather birds.
The real problem with heat, is that since childhood, we have been taught to "close the@#$% door, are we heating the whole outside?"
So when people add heat, a lot of the time they close up the coop tight to keep the heat in is what they are thinking. But that is not what happens. What they keep in is the moisture. The poop stays thawed, the birds are in the coop longer because of shorter days, and they breath out a lot of moisture. All of that gets trapped in the coop.
The ammonia fumes are hard on birds, damp chickens are cold chickens.
Instead of thinking warm, think DRY!!!! Dry and out of the wind, and BO's will come through negative 30 degrees without a problem. Instead of adding heat, add wind breaks in your run. Add a large tote on its side, facing the sun, the front facing away from the prevailing wind. You don't have to wind proof the whole run, but giving them options to get out of wind on their choice is a good idea.
Chickens need more fresh air and sunshine. Give them choices, and follow their lead, they know more about being a chicken than we do.
Mrs K