Hmmm waterbed heater....It has been awesome. I have it plugged into a Thermocube (on at 35, off at 45) and the chickens love it. When I stick my hand into the coop when I let them out in the mornings it is nicely warm but not hot in there. I plan to only use it when it is going to be below freezing. I don’t want them to get frostbite on the their combs.
The waterer didn’t freeze at 10° so it is a success too.