20 F is balmy to chickens. They'll happily peck about outside right down to 0 F. By heating your coop you are acclimating the birds to a warmer temp and they wont go outside. A sheltered area outside of the coop is all they need, toss hay down on the ice covered ground. My area is a lean too roof and two sides covered with plastic so that's three covered sides and a roof. They love it. Sure there are those days of -15 and lower they are reluctant to leave the coop. It's ok, there are not that many of those days in the grand scheme of things. Those are the mornings where it's so quite your foot falls are echoing in your ears. No animal is out and about, they are all puffed up waiting it out. Normal and only a few days of the year.
Well, it just makes me feel better about it, plus keeps the water in the coop from freezing.
They usually do prefer outside during the day when it's above 0, but I like for them to have the option to go inside to warm up if they want to.