How to Winterize an In-Ground Sprinkler System Before the First Hard Freeze

An in-ground sprinkler system stays full of water unless someone takes it out: in the pipes, the zone valves, the heads, and the backflow preventer, the brass assembly that usually stands above ground by the house. When that water freezes, it expands and cracks whatever holds it. Nothing shows until spring, when the first run becomes a geyser in the lawn, a soggy patch over a split pipe, or a backflow preventer spraying the siding.

This guide covers the fall shutdown: the supply valve, the controller, the three ways a system is drained, the backflow preventer, and the outdoor faucets. One part, the compressed-air blowout, we recommend leaving to an irrigation professional, and the section on it explains why. The fall checklist times all of this to the first hard freeze.

Before You Start

Know which drain method your system uses

Hunter Industries describes three ways systems in freezing climates are built to be emptied: manual drain valves, automatic drain valves, or a compressed-air blowout. Which one applies depends on how the system was installed; the installer or your irrigation company can tell you. Small valve boxes at the low ends of the lines usually mean manual drains. Hunter also notes that a system relying on check valves in the heads, which stop water draining out of low heads after each run, generally cannot be emptied with drain valves and needs a blowout.

What you need

• Safety glasses. Rain Bird warns that manual drain valves and drain caps can be under pressure and cause injury if opened before the pressure is relieved.
• A flat screwdriver for the small test cocks on the backflow preventer.
• A flashlight for the irrigation shutoff, often in a basement corner or a valve box.
• Foam pipe insulation, if the shutoff or any pipe above ground is somewhere that freezes.
• An insulated cover for the backflow preventer and covers for the outdoor faucets.

Step-by-Step

1. Pick the day. Before the first hard freeze, when the temperature drops to around 28°F for several hours. If a pro is doing the blowout, book it now and ask which of these steps they handle.
2. Shut off the irrigation supply at its main valve. In cold climates it is usually in the basement or crawl space where the irrigation line branches off the house supply; elsewhere, often in a valve box near the water meter or the backflow preventer. Texas A&M's School of Irrigation notes that most systems have an isolation valve for this purpose. If you cannot find it, start where the house supply comes in; our main shutoff guide shows where.
3. Protect that valve from freezing. A valve in a heated basement needs nothing. One in a crawl space, garage, or outdoor valve box gets foam pipe insulation around the valve and the pipe on both sides; with house pressure behind it, a cracked shutoff leaks the moment it thaws.
4. Turn the controller to off or rain mode. Either stops the schedule and keeps the clock and programs for spring. If you unplug it instead, check the manual first; not every controller keeps its program without power. If a pro is coming, leave it powered, since they use it to open each zone.
5. Drain the lines by your system's method, described below.
6. Drain and cover the backflow preventer once the lines are empty. If the system is being blown out, its ball valves stay closed during the blowout so no air passes through it, and it is drained afterward.
7. Finish with the outdoor faucets and hose bibs on the house, which freeze on the same nights.

Method 1: Manual drain valves

Manual drain valves sit at the low points and ends of the lines, usually in small valve boxes. With the supply off, Hunter's procedure is to open every one and let the lines empty; wear safety glasses and keep your face back as you open each. Once the main line has drained, open the drain cap or small drain valve by the irrigation shutoff to empty the pipe between the shutoff and the backflow preventer. If some heads have check valves, Hunter says to pull up on each so the water in its body drains out.

Method 2: Automatic drain valves

Automatic drain valves are spring-loaded valves at the low points that open when the pressure drops, so they drain a little after every run. To winterize, shut off the supply and run one zone briefly from the controller to release the pressure, so the drains open and empty the lines. Drain the pipe between the shutoff and the backflow preventer as in Method 1, and lift any heads with check valves. Low spots between drains can still hold water, and Colorado State University Extension advises not relying on manual or automatic drain valves alone; it recommends a blowout. Where the ground freezes hard, ask your irrigation company whether yours needs one.

Method 3: Compressed-air blowout

A blowout pushes the water out of each zone with a compressor connected downstream of the backflow preventer, on the lawn side. It is the most thorough method and the one most likely to do damage. Pros size the compressor to the system's flow (Hunter and Colorado State Extension divide the largest zone's gallons per minute by 7.5 to get the cubic feet per minute of air needed); a small home compressor usually falls short, and the temptation is to make up for it with pressure. These are the rules a pro follows, and the ones to insist on if anyone blows out your system:

• Follow your system's pressure limit, from the installer or the equipment manufacturer. Hunter's limits are no more than 50 psi for polyethylene pipe and 80 psi for PVC. Those are ceilings, not targets.
• Never blow air through the backflow preventer or a pump. Drain them separately afterward.
• Open a zone before the air goes in, then open the compressor's valve slowly. Rain Bird warns that running the compressor with no zone valve open risks overpressurizing the system.
• Wear eye protection (Hunter specifies ANSI-approved safety eyewear), and never stand over pipes, heads, or valves while air is flowing.
• Never leave the compressor running unattended.
• Stop when a zone runs dry, when the heads give only a fine mist. Hunter warns that air moving through dry pipe heats it by friction and can damage it.
• Do not leave manual drain valves open after a blowout, per Hunter.

The backflow preventer

The backflow preventer keeps lawn water from being drawn back into your drinking water. It is the most exposed part of the system, and its body cracks when water freezes inside. With the supply off and the lines empty, open its test cocks, the small slotted valves on its side, a quarter turn with a screwdriver. Then turn the handles of its two ball valves to 45 degrees, halfway between open and closed. Watts and FEBCO instructions for their pressure vacuum breakers call for this so the water drains out of the ball valves and cannot crack the casting, and Colorado State Extension also advises leaving the valves half open over winter. If your device's instructions differ, follow them, and do not take it apart. Many water utilities require an annual test by a certified tester; our glossary explains why.

Then cover it. An insulated cover protects the device from wind and slows heat loss, but it makes no heat, so it goes on top of draining, not instead of it.

Measure your backflow preventer first: this cover is 16 inches wide and 20 inches tall, and a larger assembly needs a bigger size.

Outdoor faucets and hose bibs

Disconnect every hose, close the interior shutoff to each outdoor faucet, drain the line, and cover the spigot. A frost-free hose bib still bursts if a hose is left on it. Our outdoor faucet guide walks through it.

What Normal Looks Like

Normal is a short gush from each drain that slows to nothing, a little water from the backflow preventer's test cocks, and, during a blowout, heads that pop up, spray, and sputter to a mist. Not normal: water that keeps flowing from a drain after the supply is closed (the shutoff is not holding), water dripping from the backflow preventer with the supply off, or a valve box that refills after draining. Each needs a repair before the freeze. If a pipe near the house freezes anyway, see frozen pipes: what to do.

How Often

Every fall before the first hard freeze. In spring, after the last hard freeze, reverse it: drains and test cocks closed, the backflow preventer's ball valves set as its instructions say, and the supply opened slowly. Then run each zone and look for geysers and soggy spots; the spring checklist covers the startup.

Frequently Asked Questions

Do I need to blow out my sprinkler system, or is draining enough?

It depends on how the system was built. Hunter describes systems designed for manual drains, automatic drains, or a blowout, and systems relying on check valves in the heads generally need a blowout. Colorado State University Extension advises not relying on drain valves alone. Ask your irrigation company.

What PSI should I use to blow out sprinkler lines?

Whatever your installer or equipment manufacturer specifies; Hunter's maximums are 50 psi for polyethylene pipe and 80 psi for PVC. Air volume matters more than pressure, which is one reason to hire a pro.

Is it enough to just shut off the water?

No. Closing the supply stops new water coming in, but the water already in the pipes, valves, heads, and backflow preventer is still there to freeze.

Do I need to winterize sprinklers in a mild climate?

Yes, if you get hard freezes at all. Buried pipe is less exposed where the ground rarely freezes, but the backflow preventer and above-ground pipe are not. Texas A&M's School of Irrigation recommends shutting off the supply, draining above-ground parts such as the backflow preventer, and insulating exposed pipe.

The Bottom Line

Close the irrigation shutoff and insulate it if it can freeze, set the controller to off or rain mode, and empty the lines by the method the system was built for: manual drains, automatic drains, or a compressed-air blowout we recommend leaving to an irrigation pro. Drain the backflow preventer with its test cocks open and ball valves at 45 degrees, cover it, and do the outdoor faucets the same afternoon. Whoever does the blowout follows the system's pressure limits, wears eye protection, never stands over components, and never leaves the compressor running unattended.

Sources