I'm not going to try to talk you out of heating since it seems you have your mind made up, but I will tell you what I do and why. I do not heat, and I live where it does get down into the -20's F during the winter. We just had such temps last week, and when I went out to do chores, the chickens were all scratching and pecking around their straw like it was any other day. I don't heat because my chickens have their own down coats that they acquire starting in the fall when the temperatures start dropping. They wear these down coats 24/7 no matter what it's like inside or outside their coops. I have heated water dishes in the coops for them so their water is always thawed. (That might be a thought for you since you apparently have electricity to your coop).
Before putting heat out there, try this - put on ALL of your winter gear. Coat, hat, snowpants, boots, gloves and scarf. Go outside for an hour or so. Nice and comfy, right? Now come in but don't take off any of your winter gear. Leave it on for at least two hours. Still comfy? Or are you hot? Think about your chickens. They started growing their down coats this fall and are acclimated to the weather by now. They can't get out of their winter gear, and by the looks of the size of your coop, it doesn't look like they'd have much chance to get away from the heat you are putting in there, either. By not adding heat, you are helping keep it dry in there because the moisture in the poop freezes, so it's not in the air like it would be if it's thawed. I had a lot more frostbite problems when I heated my coops than I have since I stopped doing that. Pouring boiling water into their water dishes may be causing extra humidity, which would also make them cold.
May I ask why you are convinced that your chickens are cold? Are they shivering? All fluffed up? What are the signs? Do you have frost on the walls of your coop?
One other thing that is very much on my mind right now is: what happens when they get dependent on that heat, and you lose electricity? There are 19,000 people in South Dakota (I live 20 miles from the MN/SD border) who are without electricity right now, and it doesn't look like that's going to change soon. We had an ice storm on Christmas day, with very strong winds that knocked down thousands of power poles. Once your chickens get dependent on that heat, how would they manage for a week without it?
Ultimately, of course, it's up to you. Just remember that chickens are not like people and are more comfortable with similar temps inside and out, rather than going from outside where it's cool to inside where they can't take off their coats or get away from that heat .
