We have an outdoor antisiphon spigot because of the freezing here in Colorado. I am sure it is leaking inside the wall but here’s the thing … only when it is turned on. If the spigot is closed no leaks.
Can I unscrew it or otherwise replace the whole thing without getting into the wall just by unscrewing it and screwing a new one on?
Is it possible that only the inner stem is broken and I can replace it?
Am I screwed and need to call a plumber because it is definitely a pro job?
I think you’re gonna have to get in the wall. My experience with leaky spigots is it’s always worse than you think. IANA plumber and maybe you should call one.
No to 1 and 2. What has happened is that the water did not drain from the tube properly and the outer shell has cracked. Only when the valve is opened does it fill with water and leak. You need a whole new valve. This happens if you leave a hose on the faucet and it freezes or if it is installed without the tapered washer that makes it point downhill. The faucet extends into the heated space. If it is the type with a flared connection, easy peasy. Otherwise it needs to be unsoldered and a new one soldered in.
First, do you have a basement? The pipe should come through the base of the wall and you can probably see it from the inside if your basement isn’t finished. I presume the anti-siphon is built into the valve so you will probably need to replace the whole valve but you need to find out if it is soldered, threaded, pex tubing. or other plastic tube. If you have old galvanized pipe, make sure you have some time and money to do the job as the pipe can be so rotted that it crumbles instead of unscrewing.
The backflow preventer has set screws that are broken off after installation to make it difficult to remove. So you will not be able to remove it using a pipe wrench.
You will have to gently cut it out using a dremel. I have done it a few times and it is very easy.
Here is a video to show you how to do it : How to Remove and Replace a Stuck Vacuum Breaker [Backflow Preventer Fix] / Siphon Spigot leak Lowes - YouTube
If there are no leaks when the spigot is turned off, then there are no leaks in the pipe to worry about.
Don’t think that little doo hickey is the OPs issue. Rather it sounds like a faucet that reaches through the wall and has the shutoff inside the house. Water is supposed to drain out without freezing.
Yes to #1, in fact that would be the preferred way. You need to shut off the water to the line. Unscrew the old one so you can measure the length. Buy one the same size and screw it in with some Teflon tape.
The problems to look for would be that it isn’t a threaded spigot (most are) and/or there was some tension or twisting in the water line so when you remove the old one it shifts and makes it hard to find the threads 10 inches deep in the wall.
any outdoor spigot should be backed up by a shut off valve.
I have had barrel cracks a couple times and was able to solder the crack.
By the way, the outside spigot assembly, spigot and pipe, is called a sillcock.
Look at the second picture here: anti-freeze sillcock - Google Shopping
See how far back under the house it would go? That’s so the pipe supplying it doesn’t freeze. The valve (washer and seat) is located just in front of where the thing connects to the water supply. Yours may not be that long, but I bet it’s busted between the valve and spigot. Water is supposed to drain out of the part between the washer seat and the spigot, but sometimes people forget and leave hoses attached and that can cause them to break.