I recently moved into an in-law unit of a large old house. In the kitchen, there are two pipes coming out of the wall (one protruding out a little more than the other). I had assumed they were gas lines to connect my stove. But since the stove only connects to one pipe, I'm now wondering if they're actually water lines. Can anyone explain how I can tell the difference? On the other side of the wall is a large closet with the air conditioning system, water heater, and what I think is the gas shutoff valve.
You should be able to tell by the color, and connections used.
"Black pipe" is commonly used for natural gas, and is dark grey/black.
All connections will be threaded.
Water lines come in a variety of materials, some of the most common are.
This will be um... copper, in color.
While there are other ways to join copper pipes, the most common is solder.
Galvanized pipes will be a light grey.
And use threaded connections similar to black pipe.
You might also find various types of plastic-ish water pipes. These can come in a variety of colors. Red, blue, white, black, etc.
These may use compression type connections, crimp connections, or various other type or connectors.
Still can't decide?
Try using your other senses.
Touch the pipe.
If it's warm to the touch, it's likely hot water (may not be warm unless hot water was recently used). Hold the pipe and have a helper turn a nearby tap on/off quickly, to see if you can feel vibrations. Try running the water for a bit, then touch the pipe to see if the temperature changes. If the pipe gets colder/hotter, it's likely a water line.
Listen to the pipe.
Put your ear to the pipe, and again have a helper turn on a nearby tap. If you hear the water clearly in the pipe, you found yourself a water line. (make sure the pipe is not in contact with any other pipes when you do this, since sound could be transferred to the other pipe). This is not the most accurate method, but it can sometimes work.
Taste the pipe.
This won't help you at all. I just pictured people licking pipes in my head, and thought it was funny. You could tell your helper to do it, and then laugh at them when they do it. But it's not going to help you figure out what the pipe is.
What do the pipes connect to inside that closet? Trace from where the pipes penetrate the wall back. Do they connect to the water heater or other water pipes?
Compare the pipes coming out of your water heater (at the top, where the water side is) to the pipes coming out of the wall. If they match, thats a good sign its a water pipe.
A gas line will usually have a shutoff valve at the end of it. It's usually made of cast iron, steel, flexible steel, or copper.
Here are some pictures of typical stove pipe configurations.
If it just a metal pipe sticking out of the wall, ask the owner of the house what it is or have a plumber come out and look at it. Don't assume anything.