Most sprinkler systems are not buried deep enough to protect them from freezing. Since water expands as it freezes, draining the system before cold weather strikes ensures that the pipes won't burst. Draining a little water may not be enough to prevent pipe rupture. Flush all the water from the system to avoid damaging pipes and fixtures. The method you use for flushing the system depends on they type of drain the installers included when they built your system.
Type of System
Installers add either manual or automatic drain valves when they install a sprinkler system. Check the owner's manual that came with your sprinkler installation, or give the installers a call to discover which drain system they included. If you no longer have the manual, or do not know who installed the system, turn off the water supply to the sprinklers. Locate the drain valves at the lowest points of each watering zone. If water is draining out, you have an automatic system.
Automatic valves trigger when water pressure inside the pipe falls below a set amount, typically 10 pounds per square inch. The valves open and water drains. Locate and check each valve to make certain they are all operating correctly. A drain cap installed between the main valve to the sprinkler system and the water backflow device allows drainage of water left in this portion of the system. Also, lift each sprinkler head by hand just to make sure any small amount of water left inside them escapes.
Manual valves are also located at the lowest points of each watering zone, where waters tends to collect. When you close the water supply, these valves don't open in reaction to a drop in air pressure. Instead, you must open all the valves by hand, allowing water to drain from the system. Your system may also have drain valves upstream of the zone valves. These let you drain water from the main irrigation line. Lift each sprinkler head manually as well, so that any residual water left in them drains away. After flushing the system, manually close all the open valves.
Professionals and some homeowners use compressed air to blow water out of a sprinkler system. This is the most thorough method of removing every drop of water from inside the pipes. However, the method requires the installation of specific valves, and it has another drawback. Should air pressure rise beyond 40 psi, it could damage the pipes and sprinkler heads. Compressed air for water removal is best left for professional yard sprinkler installers.