How to Winterize a Sprinkler System Before a Freeze

The lawn is still green, so the sprinklers feel like next month's problem. Water left in a buried line does not drain itself, and the piece that splits first is usually the backflow preventer standing above ground against the house, where a replacement runs into the hundreds before anyone touches the lawn. NOAA puts the first fall freeze across much of the country inside the next three weeks, so this is the window.

Look up the normal first freeze date for your area

NOAA's National Centers for Environmental Information counts a freeze as surface air temperature at or below 32F over a wide area for a meaningful stretch. Its 1991 to 2020 climate normals put the first fall freeze in the Midwest and Northeast between mid-September and early October, in the Northwest and Southwest anywhere from the start of September into November, and in the South beginning in Virginia in early October and running later farther south. Set the work at least two weeks before that date, because contractors book out.

Shut the supply at the stop and waste valve

Colorado State University Extension says the supply closes at the stop and waste valve, which sits in a basement, a crawlspace, or an underground valve box. Locate it before the day you need it rather than during. Switch the controller off, or to its rain setting, so a scheduled cycle cannot repressurize the line while you work.

Plan on compressed air, not the drain valves

The same guidance calls gravity draining through manual or automatic drain valves not recommended, because buried pipe shifts over the years and you cannot see inside a line to confirm it emptied. Extension describes compressed air as the only positive way to be sure the water is out.

Regulate the compressor below the limit for your pipe

Extension gives a working range of 40 to 80 psi, with 80 psi the ceiling for rigid PVC and 50 psi the ceiling for polyethylene, the flexible black pipe. Dial the regulator down before connecting anything. The guidance also carries a caution to wear proper eye protection, which matters because a nozzle or fitting can leave the ground under pressure.

Blow out every zone twice in short cycles

Run two short cycles per zone rather than one long one, and do every zone twice. A single pass leaves pockets behind at high points and in heads that did not fully retract.

Leave the backflow preventer valves half open

Once the water is out, leave all the valves on the backflow preventer half open so the body has somewhere to go if anything left inside freezes. On a pressure vacuum breaker, work both ball valves and the test cocks back and forth several times first, then leave them half open for the winter.

If you do not have a compressor that size. Rain Bird lists the same three methods and, citing potential safety risks, recommends contacting a local irrigation specialist instead of running the blow out yourself.

If the system has electric valves. Extension states those systems must be blown out with pressurized air, so drain valves alone are not an option no matter how the yard slopes.

If a line splits anyway. The steps for shutting off the water to a burst pipe apply to the house side of the same shutoff.

Sources. Preparing your sprinkler system for winter, Colorado State University Extension, How do I winterize my irrigation system, Rain Bird, When to expect the first fall freeze, NOAA NCEI.