- they aren't buried deep enough
- they have insufficient drainage underground.
Just above that plunger is a small drain hole in the pipe. The same plunger that opens and closes the valve, simultaneously opens or closes the drain. When the water is running the drain is blocked and when the hydrant is shut off, that drain hole opens. Any water in the pipe drains out of that hole via gravity.
A properly working hydrant should never have water in the pipe except when actually in use. I often see people wrapping insulation around their hydrants. While insulating the pipe may seem like the thing to do, it is generally a waste of time because the pipe should not have water in it.
If you do have water remaining in the pipe after the hydrant is shut off, you have a problem. If your hydrant is working correctly and you still have trouble with it freezing, the problem is in the ground, not the pipe. Placing a couple of hay bales around its base to keep the frost out of the ground is much more effective.
When installing a hydrant, it is crucial to protect the drain hole. You don't want it getting clogged with dirt and if you have a lot of ground water, like I do, you want to give the water a place to drain to as well.
You will need a backhoe or excavator to do this job unless you live in an area where there are no rocks and you can manage digging a deep hole by hand. My property is NOT one of those places. This little tractor backhoe was almost too small to deal with the rocks we have here.
A few of the things you will need are a hydrant, a plastic bucket with the bottom cut out, some clean, crushed stone, a bit of landscape fabric and whatever fittings are required for your size water line, which is typically 3/4 inch. Border Collie supervision is optional.
The bottom of the hydrant needs to be below the frost line. Plan for the absolute worst winter you can imagine and then go 6 inches deeper. Do not let a contractor tell you that 3 feet is deep enough because he "never digs any deeper than that and besides, it never gets that cold around here anymore".
Measure the hole carefully to make sure that it really is deep enough.
Once you go as deep as you can (in this case, we hit bedrock at about 40 inches. I would have liked this to be even deeper, but that would probably have required blasting. It is 10-12 inches deeper than it was), put the bottom of the hydrant into the bucket before attaching the water pipe to it. Once all the fittings are tight, make sure to turn the water on and test everything at this stage. You sure don't want to find a leak after you've refilled the hole!
If you look really closely at the little brass nipple on the bottom of the pipe (at the white arrow), you can see water coming out. That little brass bump sticking out is the drain hole that needs to be protected.
If you look close again, you can also see a bit of water in the bottom of this hole. That is ground water seeping in. It is what happens to every hole I dig up here. It is why this job HAD to be done when it is dry and it is why I may still have problems with my hydrant even after all of this. When it is wet, this hole would fill with water before you could finish digging it.
To help with drainage and to protect that pipe drain, fill the bucket and the area around it, with clean, crushed stone.
In order to help keep silt from working its way down into the stone, cover it with the landscape fabric.
This next bit is optional and may be overkill, especially of you have gone deep enough and have good drainage. Since neither is as good as I would like here and I had a piece of this blue-board foam insulation left over from another job, I put it over top of the fabric to help insulate the area and keep the frost out. This may be a bit of paranoia, but I've found that, when it comes water lines, a little paranoia is not a bad thing.
Once you are sure everything works and is in place, back-fill the hole and tramp it down well.
To finish things off, it is good to securely anchor the above ground portion of pipe to a solid object. There is a lot of leverage in a 7 foot pipe and even a small wiggle at the top can translate into damage down below. If your hydrant isn't near a wall or strong fence, install a sturdy post alongside it when you bury it so that it will have solid support.
If you've done everything right, your hydrant should give you a lifetime of reliable water flow no matter the temperature.