If a properly installed Frost free hydrant is freezing up it's leaking or the surrounding ground is saturated preventing it from draining.
Exactly. Once saturated, the leaking water goes up the standpipe and freezes. If your line is frozen (not the hydrant), then yes the line is too shallow.
Once they freeze- what a pain!!
The problem is you can't tell if there's a slow (or fast) drip at the buried valve (plunger/stopper) that is letting water go by and saturating the ground.
Adjust the stopper too tight and it deforms/rips the stopper. Too loose and it leaks by.
I have a Campbell hydrant that wears out the rubber stopper every 6 months. And the b*stards won't just sell the rubber stopper that wears out, they make you buy the whole kit.
I wish I knew of easier way to thaw them once frozen.
A couple years ago when it froze the line, it busted the elbow under the valve and I had to dig it up. At this point I put in A LOT of crush stone around the weep / drain hole.
I also put in a pressure gauge DOWNSTREAM of the shutoff valve at the source (house basement). The idea being that you can test if the frost free hydrant is leaking by seeing if the pressure goes down over time after turning off the source valve.
But seeing how it's my wife & kid who use the hydrant, and pay no attention to how the handle "feels" and won't performs any weekly pressure tests (or understand them), and I DON"T TRUST THE HYDRANT, we just have a 2 step policy of opening the source valve in the house when the barn hydrant is going to be used and turning both valves off when done.
The other things we did was:
1) Wrap the exposed hydrant pipe in heat tape and insulation and plug in on really cold days (<10F). Yes, the metal hydrant riser pipe is an excellent conductor for the cold to follow. IMHO, they should design part of this riser pipe with a PVC section to prevent this. .
2) Avoid removing snow cover on the area the water line runs under. When my line froze it was where it runs 12' under the lean-to on its way into the barn that gets no snow cover. Since then I've covered the ground over the pipe in the lean-to with old carpets and hay.