Mine don't have heat. Last night was -18 F. They started laying wayyy late, at 31 weeks, day before Thanksgiving I got my first egg. We are up to 3-4 a day now and the size is gradually getting bigger. I found that when I give them some kind of protein, they lay more the next day...they are on a layer pellet, and won't go outside any more, so I also give them some scratch, in the afternoon, do give them something to do and get the bedding turned.
I thought they were going to wait for longer days also, but the wait was worth it, albeit a little worrying.
I have 15, 14 pullets and one cockerel. They were sold as hatchery BR's. Funny looking for BR's but very cold hardy!!!
ETA I also give them warm water, through out the day and on really cold night, some pellets, made into a warm mash in the am. They seem to like that and are more settled.