I had never though about this until I read the "How do I Unthaw yard
hydrant underground?" thread.
In my shop I have a standard yard hydrant that feeds the sink, stool,
etc. The hydrant is only about 6" from the wall so the underground
portion is very close to the footing. I keep the temperature in the shop
at 46 degrees F unless I am out there (which is rare in the winter) and
then I turn it up to about 68. In short, the hydrant is normally in an
area that is 46 degrees F and it is always turned on so there is water
in the standpipe at all times. Also, it might go at least a week without
any flow through it (stool not flushed, sink not on, etc.) so it does
not freeze because water is moving through it.
It is hard to believe that the simple fact that it is in a building with
limited heat would keep the pipe underground warm enough to keep it from
freezing but it has not froze in the six years since I build the shop.
Oh yeah, I live in SE Iowa so it gets down to zero sometimes (like two