We had a few solid weeks where the temps didn't get above 10 degrees during the day, and went down to -5 at night. I checked for eggs in the morning, after I got home from work, and when I locked up for the night. I didn't have any eggs freeze this winter, and I do not heat my coop.
A few things help here. I have an open-front coop that is 16 ft deep. inside of my coop is white, and lots if sunshine during the day. This allows the coop to heat up when the sun shines. (The oak trees shade it in summer).
I don't know how many chickens you have, but, if you put your dummy eggs in just a couple of nests, the girls will line up to lay in those nest boxes. They are copy-cats and will see multiple eggs as an enticing place to lay, for some reason. Constant warm bodies on them keeps the eggs from freezing, but it'll be cold enough that they won't start to incubate. I put 3 golf balls in 3 nest boxes. If you're bored and get cabin fever, you can watch the egg piles move daily by moving the dummy eggs.
The water will freeze, too. I got a heated pet water bowl from
petsmart, and it works like a charm.
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