How to Winterize a Sprinkler System (Blow-Out Method)

In any climate that freezes, water left in your sprinkler pipes, valves, and backflow can freeze, expand, and crack them — a repair that costs far more than an afternoon of winterizing. The compressed‑air "blow‑out" is the most thorough way to get that water out.

Why winterizing matters

Water expands roughly 9% when it freezes. Trapped inside a rigid pipe, a valve body, or a backflow assembly, that expansion generates enormous force — enough to split PVC, crack valve housings, and rupture the delicate internals of a backflow preventer. Backflow assemblies are especially vulnerable because they sit above ground and hold water in tight passages.

If you live where the ground and pipes reliably freeze in winter, you need to remove standing water before the first hard freeze. If you're in a mild, no‑freeze region (much of the Deep South, coastal California, the desert Southwest at low elevation), you may not need to winterize at all — though a hard cold snap can still surprise you, so know your local climate.

The three drain methods

There are three common ways to clear the system, and they aren't equally thorough:

• Manual drain. If your system was built with manual drain valves at the low points, you shut off the water and open those valves to let gravity empty the mains. Simple, but it only works if the pipe was pitched to drain and valves were installed.
• Automatic drain. Some systems have drain valves that open automatically when pressure drops below a threshold. Convenient, but water can still sit in heads and in the section between the shutoff and the backflow.
• Compressed‑air blow‑out. You push air through each zone to force water out of the pipes and heads. This is the most complete method and the one most pros use, but it carries real risk if done carelessly.

Because the blow‑out is the most thorough — and the one people most often ask about — the rest of this guide walks through it step by step.

The blow-out method, step by step

The idea is straightforward: shut off the water, connect an air compressor downstream of the backflow, and run one zone at a time until only a fine mist comes out of the heads.

Set your compressor pressure limit first

This is the part people get wrong. You are not trying to blast water out with high pressure — you're using volume (CFM) of air at low pressure. Too much pressure will damage fittings, heads, and seals. Set the compressor's regulator before you connect, and keep it there.

If you're not sure whether your yard runs PVC or poly, treat it as poly and stay under 50 PSI — the lower limit is always the safe choice. Not sure which you have? See PVC vs. poly pipe.

The steps

1. Shut off the water supply to the irrigation system at the main isolation valve. Then turn off the controller (or set it to the "rain"/off position so valves don't cycle unexpectedly).
2. Relieve pressure. Open a manual drain or run a zone briefly to bleed off the trapped pressure before you connect anything.
3. Connect the compressor downstream of the backflow preventer, typically at a blow‑out port or hose‑bib adapter after the backflow. Never blow air backward through the backflow assembly.
4. Set the regulator to the pressure limit from the table above before opening the air valve.
5. Open one zone at a time at the controller — start with the zone farthest from the compressor, or the highest. Slowly introduce air.
6. Run each zone briefly until the heads spit water and then throw only a light mist. As soon as it's mist, stop that zone. This usually takes only a minute or two per zone.
7. Never run a dry zone for long. Air moving through empty pipe builds heat with no water to carry it away, which can melt or warp heads and gears. Cycle zones and let the compressor catch up rather than force one zone.
8. Repeat down the line, then do a second quick pass on each zone — a little water often re‑collects.
9. Protect the backflow. Open its test cocks a quarter turn to drain and relieve any trapped water, and insulate or cover the assembly. Some homeowners remove and store exposed backflow components per the manufacturer's guidance.
10. Leave the controller off (or in rain mode) for the season and disconnect the compressor.

Common mistakes to avoid

• Using a small pancake compressor and cranking the pressure. Home compressors often lack the air volume (CFM) to clear a zone, and people compensate with pressure — exactly the wrong move. Low pressure, adequate volume, and patience win.
• Blowing out multiple zones at once. Air follows the path of least resistance; running one zone at a time ensures each actually clears.
• Forgetting the backflow and any above‑ground piping. These freeze first because they're exposed.
• Winterizing too late. Do it before the first hard freeze, not after you've already had one.

After winter

Come spring, don't just slam the water back on. Open the main slowly to avoid a pressure surge ("water hammer"), reopen the backflow test cocks, then run each zone and check for cracked heads or leaks. If something isn't spraying right, our troubleshooting guide covers the usual culprits, and your watering schedule can be reset for the season.

Winterizing is cheap insurance for an expensive system. If you're still planning or expanding your setup and want it built to drain well from the start, design your system from a satellite photo and lay it out with winter in mind.