How to winterize a sprinkler system: drain or blow out, protect the backflow preventer, and restart in spring

Why water left in the lines is expensive

Water expands by about 9% when it freezes. In a full pipe, valve body or sprinkler case there is no room for that: fittings split, valve bodies crack, heads get hairline cracks. The damage usually shows up only in spring, as a geyser in the lawn or a zone that never builds pressure.

Irrigation lines in most yards sit 6 to 12 in deep, well above the frost line, which is normal as long as they are empty for the winter. The parts most at risk are above ground or just below it: the backflow preventer, exposed copper or PVC at the point of connection, valves in shallow boxes, and the controller transformer if it lives outside.

When: before the first hard freeze

In much of the northern US the window is October to early November; in mountain areas it can be September. A light frost rarely hurts buried pipe, but one hard night can crack an above-ground backflow preventer. Because lawns need little water in late fall anyway, winterizing a week early costs nothing. In the mild South and Southwest many systems are not blown out at all; there the job is to cut run times for winter and wrap exposed pipe and the backflow device before the rare cold snap.

Step 1: shut off, and set the controller to rain or off

Close the irrigation shutoff valve, usually inside the house or in a box near the meter. Put the controller in rain or off mode: the programs stay in memory and no zone opens on a dry system. If a pump start relay or a master valve is wired to the controller, disconnect it so nothing starts over winter.

Step 2a: manual or automatic drain

Systems built with manual drain valves at the low points drain by gravity: open each zone briefly to release pressure, then open the drain valves and let the lines empty. Automatic drain valves open by themselves whenever pressure drops, which is convenient but means they release a little water after every summer run and can stick or leak as sand builds up.

Gravity draining only works if the pipe really slopes to those drains. Any sag between two high points keeps water through the winter, which is why many contractors blow out systems even when drains exist.

Step 2b: compressor blowout

Most systems are blown out with compressed air. What matters is air volume, not tank size: a residential system needs a compressor that delivers roughly 10 to 25 CFM, which is more than a typical garage compressor; rental yards and irrigation contractors have suitable units. Set the regulator low: no more than 50 PSI for poly pipe and 80 PSI for PVC, and never more than the rating of the weakest component.

Connect the compressor to the blowout port or a fitting downstream of the backflow preventer, never through the backflow device itself. Open a zone first, then slowly let in air; never pressurize against closed valves. Start with the zone farthest and highest from the connection, run it until the heads put out only mist, move to the next zone, and go through all zones a second time briefly. Do not let rotors spin on dry air for minutes at a time; their gear drives are cooled by water. Open the end caps of drip lines so air can pass through.

Step 3: the backflow preventer and everything above ground

• Backflow preventer: with the supply off, open the test cocks to drain the body. A common practice for pressure vacuum breakers is to leave the ball valves half open (about 45°) for the winter so no water is trapped in them. Insulate with a bag or cover made for the purpose, without blocking vents or relief ports. Follow any utility rule about removal or spring testing.
• Exposed supply pipe: drain and insulate the section between the house and the backflow device.
• Outdoor controller: leave it powered in off or rain mode, or bring battery models inside; protect the box from moisture.
• Pumps from wells or tanks: drain them and store frost-free if they are above ground.

Spring startup without cracked fittings

After the last hard freeze: close manual drains and test cocks, put the backflow valves back in their operating position, and then open the irrigation shutoff slowly, over 30 seconds to a minute. A sudden surge of water into empty pipe slams into the first elbow and the backflow device. Let air escape through the heads of the first zone.

Run each zone by itself for a few minutes and walk it: every head should pop up, turn and retract, and wet spots or a sinkhole in the lawn point to a leak. Heads that do not retract or spray sideways often have a freeze crack. Then check the clock, programs and rain sensor, and start with short spring run times: /enHow long to run sprinklers: minutes per cycle by soil and head type, with the math behind them.

Do it yourself or hire it out

With working drain valves, winterizing takes half an hour. With a compressor of the right size it is about an hour of work. Without the right compressor, or with a large system, a fall blowout service from a local irrigation contractor is inexpensive insurance, and in some places the annual backflow test can be scheduled at the same visit. If you are building a new system, low-point drains and a blowout port next to the valves cost very little during installation and make every winter easier: /enHow to bury irrigation pipes: depth and techniques.

Frequently asked questions