Use a toilet bowl filler/float. I've used them before for horse troughs.
My uncle had a dairy farm in Vermont, he used something similar. The winters up there, a few miles from the Canadian border, are way colder and stay that way most of the winter.
Mirafounts for sure!
Try these
Bar Bar A Home Page
They don't use electric, work off pump. Pressure fed.
...... might not be double waterer.
I would like an automatice horse "washer" - I have a grey - who thinks he should be a bay or a chestnut!!! I cant keep him clean for nothing!!!
My h ubby calls him a junk yard horse!
Jose say he water horse real good long time, long time. Sleep in stall until social security comes in.