Blowing Out Your Irrigation System Before the Freeze

Every spring, irrigation techs get the same calls: a backflow preventer split open, a valve body cracked, a lateral line that sprays like a fountain. Almost all of it happened in one night last winter, when water sitting in the system froze. On a golf course, winterizing irrigation was a two-week job on the calendar every fall. At home it's an afternoon, and it's worth doing right.

When to do it

Blow out the system before the first hard freeze, when overnight lows are forecast to drop into the mid-20s or lower for several hours. A light frost won't freeze buried lines, but the exposed parts (the backflow preventer, the main shutoff, above-ground piping) freeze first. Across most of the northern US that means October into early November. Earlier up north and in the mountains.

If you water late into fall to help the lawn and trees, that's fine. Just keep an eye on the forecast and blow out before the first hard freeze.

Three ways systems get winterized

1. Manual drain: some systems have valves at low points that let water drain out. Common in some older systems; doesn't always get every line.
2. Automatic drain: valves that open when pressure drops. Same limitations.
3. Blowout with compressed air: the most thorough and the most common method in the northern US. This is what most pros do.

Blowout step by step

You need an air compressor with enough volume, measured in CFM (cubic feet per minute), not just pressure. Small pancake compressors run out of air quickly and can't push water out of long zones. Many homeowners rent a tow-behind compressor or hire this out.

1. Shut off the water to the irrigation system at the main shutoff valve.
2. Turn off the backflow preventer's valves per its design, and open its test cocks halfway so water can escape.
3. Connect the compressor to the blowout port (or a fitting after the backflow preventer) with the compressor's valve closed.
4. Set the pressure low. A common guideline is no more than about 50 psi for flexible poly pipe and up to around 80 psi for rigid PVC. Check your system and fittings; more pressure is not better.
5. Run one zone at a time from the controller, starting with the zone farthest from the compressor. Open the air valve slowly.
6. Blow each zone until only mist comes out of the heads, usually a couple of minutes, then move on. Go through the zones twice.
7. Never run the compressor with every valve closed. Air with nowhere to go builds heat and pressure that can damage parts.
8. Disconnect, leave the backflow test cocks open at 45 degrees, and set the controller to off or rain mode for the winter.

Safety

Compressed air can turn a loose fitting or a sprinkler head into a projectile. Wear safety glasses, stand away from heads while a zone is running, and never stand over the backflow preventer while the system is under air.

When to call a pro

Call an irrigation company if you don't have the right compressor, the system is large or you don't know where every valve is, or if your backflow preventer needs an annual test (many towns require one). The cost of a blowout is small compared with a cracked backflow preventer.

In spring

Open the main shutoff slowly so you don't send a water hammer through the lines, close the test cocks, and run each zone to check for leaks. And keep an eye on deep, infrequent watering once the season starts.

Photo: Impact Sprinkler Mechanism 2 by JJ Harrison, CC BY-SA 3.0.

Related: When to Stop Mowing for the Season (and How Low the Last Cut Should Be)