That open air design is something I never would have thought of. Basically, with that they hardly need a run/yard. I believe you that temps in the teens and single-digits might be okay, but here we can see -20F, down to -35F. When it gets that cold, my bird are hermetically sealed (sic) in their insulated coop. I don't usually have to worry about frozen waterers unless the temps stay in the teens for days on end (quite common), and I just use rubber pans & change the water daily. I've tried several DIY heater ideas, but none can combat really cold temps. I even tried wrapping a hanging waterer with heat tape. The water in the can didn't freeze, but the water in the pan sure did!
they were used in those temps ; pg 4 Post 33 is a link to the book
I'm probably being over protective.