What if you put a cap on the water jugs? The plastic gallon jugs that some milk comes in? This:
If you put a cap on the water, it would give off heat but not steam!
This might be a silly idea, but if you live where it doesn't get to cold all the time, it might work at night. If you filled a waterer in the coop with extremely hot water and maybe even put gallon jugs filled with boiling water around the coop, would this produce enough heat (just for the...