Good trick for defrosting a frost free hydrant (or automatic waterers for that matter) is to use a hair dryer. They move a ton of air, don't generally melt plastic, and most people have one hiding away somewhere. For hydrants, I put a plastic barrel with ends cut out over the hydrant, point a hair dryer down into it, and cover with a blanket (just not the intake on the backside of the hair dryer obviously!). I do the same on the auto waterers, just pop the top near the valve and float and let the hair dryer do the work.
So far, we have had zero freeze ups, and it was -25 degrees here again this morning. The first weekend in October every year, we put new heat tapes on all hydrants, and all our auto waterers get new heat tape, new reservoir heaters and a new light bulb, and get back filled with gravel about 1/3 of the way up the outside of the unit (for most auto waterers, wind leaking in under the shroud is the main reason for any failures).