You asked me why your outside valve to the garden, froze and burst during the winter. You said the valve was freeze-proof, and shouldn’t have frozen.
I said “Not quite, it’s complicated”. You wanted to know more - - so here’s the explanation.
Terminology: -- A valve that exits a house, and is meant to attach to a garden hose, is called a sillcock (also spelled sill cock).
-- An anti-siphon mechanism prevents water outside the house (for instance if the other end of the hose is immersed in a pond or swimming pool), from accidentally being sucked backward into the house's plumbing. This can happen when there is a dramatic change in water pressure, such as when someone flushes a toilet. You do not want pond water mixing with your drinking water (also spelled antisiphon, anti-syphon, and antisyphon).
-- If the sillcock is manufactured to resist freezing in the winter, it is sold as frost resistant or frost proof (also spelled frost-proof, and also frost free) . Note that they only resist freezing -- they are not freeze proof.
The vast majority of sillcocks do have an anti-siphon mechanism built in. However, the vast majority of sillcocks are not frost proof.
How to tell the differences? An anti-siphon mechanism looks like a round cap, attached above and just behind, the handle that turns the water on and off outside the house. If that is missing, it is not anti-siphon.
To identify a frost proof sillcock, you must get inside the house and look at the plumbing pipe leading to the sillcock. A frost proof sillcock has a significantly larger diameter than usual plumbing. If you see a much smaller pipe coming from inside the house, and just before it exits the house it joins to a much bigger diameter pipe, then it is frost proof. If the pipe is the exact same diameter all the way to the outside wall, it is not frost proof.
There are two parts to the design of a frost proof sillcock, that make it able to survive freezing weather without bursting.
First, the entire unit is very long, typically 8" to 12". Although the handle that operates the sillcock is outside the house, the valve itself is all the way at the other end -- deep inside the house. This means the heat from inside the house should be warm enough, the valve itself will never freeze.
Second, the sillcock is usually fitted with an anti-siphon mechanism. If you shut off the valve, and remove the hose, all the water in the sillcock will drain out. That means the entire sillcock will be empty. There will be no water inside that can freeze.
Good in theory, so why are they not freeze proof? 1) The house must be heated. If the valve is in an unheated part of the house such as a crawlspace, or the building is entirely unheated, the valve will freeze. 2) The sillcock has been insulated. If you put insulation around any part of the sillcock, you will block the heat of the house away from the sillcock. The cold weather outside will chill the pipe and eventually freeze it. 3) The sillcock was installed wrong. It must be tilted slightly, so that water will drain out when the hose is removed. If the water doesn't drain out, it will eventually freeze. [This is what happened in your case.] 4) The hose was never removed. Before freezing weather begins, you must remove the hose from the sillcock, and let the water drain out. If the hose remains, water also remains inside the sillcock, and it will freeze.
If you recognize any of the above problems, or if your sillcock is not frost proof, you must take further action to prevent freezing. All pipes that exit a house, must have a separate valve that can shut off the water. Before freezing weather begins, check along the length of the pipe and find the shut-off valve. Close it off. Go outside, open the sillcock, and let the water drain out, all the way back to the shut-off valve. [Sometimes, the shut-off valve will have a small thumbscrew built into it to help drain the water. After you close off the valve, loosen the thumbscrew until air enters the valve and releases the water to drain.] When there is no water in the pipe, it will not freeze.
You may wonder, how will you know when freezing weather will begin, before it begins. Remove your hoses after the season's first frost. That's when the leaves drop, and pretty much everything in the garden dies. You can wait that long. Just not much longer.
