A stop-and-waste valve is a shutoff valve with a small built-in drain port. Closing it stops the water and lets you drain the pipe past the valve. That prevents a trapped pipe from freezing and bursting. It is common on outdoor faucet and sprinkler lines in cold weather.
A stop-and-waste valve is a shutoff valve with a built-in drain. It does two jobs in one body. It stops water flow like any shutoff, and it drains the pipe past the valve when you close it. That second job is what makes it useful in cold weather.
How the waste port works
The key part is a small drain hole in the valve body. One valve maker calls this hole "the key to the Stop and Waste Valve's functionality." When the valve is open, that hole stays shut and water flows through as normal. When you close the valve, you can open the drain hole. Water left in the downstream pipe runs out, and air moves in to take its place.
Draining the pipe matters because water expands when it freezes. An empty pipe has nothing to freeze and burst. The valve comes in two common styles. One is a globe-type body with a drain hole in the body. The other is a ball-type with a drain hole in the ball.
Where it is used
Stop-and-waste valves protect lines that face the weather. Common spots are outdoor hose bibbs and sprinkler systems. In Phoenix the risk is a sudden winter cold snap, not months of deep freeze. A hard overnight freeze can still split an outdoor line or an irrigation pipe that holds standing water. A stop-and-waste valve lets you shut the line and drain it before that cold night.
In summer you leave the valve open so water flows. In winter you close it and open the drain. For related help, see how to fix drip irrigation and why an outdoor faucet or hose bib leaks. Compare it with a plain ball valve or gate valve, which shut water off but cannot drain the line.