A stop-and-waste valve is a key irrigation fitting that helps keeps sprinkler lines from freezing in the winter. Many people have an automatic stop-and-waste valve serving as the main shut-off valve for their sprinkler system. The stop-and-waste valve is turned on and off with a meter key. When it is in the off position, it automatically drains any water in the line. When the water is shut off at the end of the season, all water in the lines drains away, thereby eliminating the possibility of lines bursting from freezing water.
A stop-and-waste valve has a lifespan and it will eventually fail. Unfortunately, if the water in the irrigation line does not drain away, you may not find out until the pipes are frozen. Water remaining in the lines when the water is turned off can mean the auto-drain in the stop-and-waste valve is not working properly or is clogged. Another reason to replace a stop-and-waste valve is when it leaks.
Stop-and-waste valves don’t always work. If your region is prone to heavy freezes, consider using a compressor (or hiring a pro) to blow out irrigation lines and winterize the system in case the stop-and-waste valve fails.
Source: The Spruce