How often should you winterize your sprinkler system?

Short answer

Winterize the sprinkler system once a year, before the first hard freeze. In a climate that doesn't freeze, skip it. The job is to get the water out of the pipes, the heads and the backflow preventer, and the way that works is a blow-out: compressed air pushed through each zone until the heads spit mist instead of water. A sprinkler company charges $50 to $150 for it. Doing it yourself takes 30 to 60 minutes and $0 to $50 for a compressor rental, if you have the right kind.

Why it matters

Sprinkler pipes run just under the surface, well within reach of frost, and they're full of water from the last run of the season. When that water freezes, the pipes split, the heads crack, and the backflow preventer, the brass assembly on the supply line that keeps lawn water out of the house's drinking water, breaks inside its body. The backflow preventer is the expensive part, and it's the part that fails first, because it sits above ground.

None of it shows until spring. The first start-up sends water into the ground through the split pipe, a geyser from the cracked head, and a stream out of the backflow preventer. Then it's a repair bill, a week without water for the lawn, and a hunt for underground breaks. A blow-out in the fall costs less than a single broken head and takes less time than finding one.

When to do it more often

The index gives one modifier, and it removes the job: in a climate that doesn't freeze, there's nothing to winterize. Everywhere else, once a year, and the timing is the whole trick. Do it after the last watering you need and before the first hard freeze, which in most places means a window of a few weeks in fall. If an early freeze is forecast before you've done it, shut off the supply, open the drains and the test cocks on the backflow preventer, and get the blow-out done the next day.

What you need

An air compressor with enough volume to move air through a zone, which is more than a small tire inflator can manage
A blow-out adapter that connects the compressor's hose to the system's blow-out port or the backflow preventer's test cock

Steps at a glance

Shut off the water supply to the sprinkler system at its valve inside the house or at the meter.
Open the drain valves and the test cocks on the backflow preventer, and let the water run out.
Connect the compressor to the blow-out port, set its pressure to what the pipe can take, and don't exceed it.
From the controller, open one zone, then start the air. The heads will spray water, then a mist, then air.
Shut off the air before the heads run dry for long, since dry air heats and damages the heads, and move to the next zone.
Repeat for every zone, then leave the drains and test cocks open and the controller set to off for the winter.

You're done when every zone has blown clear mist and then air, and the drains, test cocks and controller are set for winter.

Signs it's overdue

Frost in the forecast with the system still full of water.
Cracked heads found in spring.
Geysers at the spring start-up, from pipes that split over winter.

DIY or pro

This is a pro-recommended job, and the reason is the compressor. A blow-out needs air volume, not just pressure, and the compressor in most garages can't move enough air to clear a zone. Too much pressure, on the other hand, blows the heads apart or damages the valves. A rental compressor of the right size solves the volume problem, and a gauge and some care solve the other, but it's a job that goes wrong in ways that cost money.

A sprinkler company charges $50 to $150 and brings a tow-behind compressor that clears a zone in a minute. They also close the system properly and often note anything they saw that needs fixing in spring. For most homes, that's the sensible price for the job, and the DIY route makes sense mainly for someone who already has a large compressor.

Safety

Pressure: compressed air in a pipe system is dangerous in a way water isn't. Keep the pressure within the limit for the pipe, keep clear of the heads while a zone is being blown, and don't stand over the backflow preventer with the test cocks open. Wear eye protection. Shut the air off between zones rather than letting it run against closed valves.

FAQ

When should I winterize my sprinklers?

After the last watering of the season and before the first hard freeze. In most climates that's a window of a few weeks in fall.

Can I winterize sprinklers without a compressor?

Only if the system was built with manual or automatic drains at every low point, and even then water stays in the heads and the backflow preventer. A blow-out is the method that works.

What happens if you don't winterize sprinklers?

Water in the pipes freezes and splits them, the heads crack, and the backflow preventer breaks. It shows up as leaks and geysers at the spring start-up.

How much does it cost to winterize a sprinkler system?

A sprinkler company charges $50 to $150. A rental compressor for a DIY blow-out runs $0 to $50 for the day.