Sprinkler pipes and heads, like the components of any water system, are vulnerable to freezing temperatures. When residual water turns to ice in the pipes, it expands and blocks them, creating excessive pressure between the blocked section and the shutoff valve. If the pipe bursts, it usually does so in that section, not the one in which the ice is located. You can prevent bursting pipes by draining them for the winter. The procedure is easiest if you have a drain valve installed in an appropriate location on the mainline. If not, drain the system with compressed air.
Look for a drain valve. If there is one, it will be located at the lowest point of the sprinkler mainline. It may be a ball valve, a compression valve or even a plug or cap screwed to a fitting.
Turn off the main water supply and open the valve controlling flow to one of the zones in the system. You may be able to work the valve manually, or you may have to open it electronically through the controller.
Open the drain valve slowly -- the water may be under pressure. Use a wrench to unscrew a cap or plug. Let it drain into the ground, or if it's in the house, into a bucket.
Close the drain valve, and then close the zone valve and open the next one and repeat the draining procedure. Drain the water from all the zones in the same way.
Blowing with Compressed Air
Turn off the main shut off valve and open one of the zone valves, either manually or electronically from the control panel.
Attach a compressor hose to a quick-connect air fitting attached to the sprinkler mainline. If your system doesn't have such a fitting, you may need to install one. Cut into the mainline near the shut-off valve with a hacksaw and glue in a tee with plastic pipe cement. Glue a threaded female adapter to the perpendicular port and screw the quick-connect fitting to the adapter.
Turn on the compressor and let the pressure build until all the sprinklers in the zone pop up. Do not exceed 50 psi, or you could damage the pipes. Leave the zone pressurized for two minutes, and then turn off the compressor. Wait for the air to purge from the zone, close the zone valve, open another one and repeat the procedure.
Blow each zone at least twice. The pipes are purged of water when only a fine spray emerges from each sprinkler head when they are pressurized.
Leave one zone open while you shut down and unhook the compressor. Close the valve when the compressor is disconnected. If the system has an electronic control, turn it to "Off."