How to winterize your irrigation system

Why winterizing is essential

Ice takes up about 9% more space than liquid water. That expansion creates internal pressure that splits polyethylene pipe, plastic sprinkler bodies and solenoid valves. The damage is often invisible until the first spring startup, when water leaks from joints and fittings that the winter cracked open.

A mid-size system (6–10 zones) with freeze damage runs $200–800 in repairs, not counting digging up the yard to reach buried pipe. One hour of work in fall avoids all of it.

It is not only a cold-climate concern: even in milder regions a late frost in March or April can catch a system that has been turned back on but still faces below-freezing nights.

When to start

Drain the system when night temperatures drop consistently below about 40°F (4–5°C). In much of the northern US the typical window is October–November; in mountain areas, as early as September.

Do not wait for the first hard freeze: one night at 28°F (-2°C) with water still in shallow buried pipe (less than 12 inches / 30 cm deep) is enough to cause damage — the soil does not insulate quickly enough.

If you have a smart Wi-Fi controller (Rachio, Hunter), some models send freeze alerts and pause the schedule. Even so, still do the physical drain before winter.

Manual draining: step by step

Shut the system's main supply valve. Manually run each zone from the controller for 30–60 seconds: this releases leftover pressure and pushes water out of the heads. Open every manual drain valve at the low points of the circuits.

Remove and clean the mesh filters ahead of the solenoid valves — limescale and grit come off easily now, but would be crusted on by spring. Set the controller to OFF (or "rain mode") to prevent accidental startups.

If your system has a backflow preventer — the safety device that stops irrigation water from being siphoned back into your drinking water — drain it too: most have small bleed screws on the side. Leave the main valve cap slightly loose so air can enter and prevent a vacuum in the pipes.

Compressor blow-out: the correct technique

Blowing the lines out with an air compressor is the most thorough method, especially for systems with no built-in drain valves. Connect the compressor to the system's blow-out fitting — never directly to a sprinkler head.

Recommended pressure: 35–50 PSI (2.4–3.5 bar) for poly pipe. Do not exceed 50 PSI: higher pressure wrecks the internal seals of heads and valves. Run each zone separately for 1–2 minutes — you will see water first, then only air.

Repeat 2–3 cycles per zone, pausing 30 seconds between them so parts do not overheat. Start with the zones farthest from the main valve and work toward the nearest. Wear eye protection: debris can shoot out of the heads with force.

Protecting the above-ground parts

The controller and any solenoid valves in an outdoor box need protection from moisture and freezing. A foam enclosure or an insulated cover ($10–20) is enough for most climates.

Wrap exposed pipe — the connectors between valves and the first buried line — with pipe-insulation tape or spray foam. Pay special attention to the supply connection at the meter, which is often left exposed above ground.

Spring startup checklist

Before you turn the system back on, follow these steps in order. Reinstall the cleaned filters. Visually inspect exposed pipe for cracks or bulges. Reconnect the controller and set the date, time and schedule.

Open the main valve slowly, over 30–60 seconds — never all at once. A fast opening creates water hammer (a pressure shock wave when fast-moving water is stopped suddenly) that can damage fittings and valves. Run each zone for 1–2 minutes and check that every head rises, rotates and retracts properly.

Check the throw distances: if some heads seem weaker than last year, their internal filters may be clogged. Cleaning takes about 5 minutes per head with a small brush and running water.

Common winterizing mistakes

Relying on manual drain valves alone in a cold climate. Gravity draining leaves water trapped in low spots, around heads and inside valves. Where hard freezes are normal, a compressor blow-out is the only method that clears those pockets — manual draining is a supplement, not a substitute.

Using too much air pressure. It is tempting to crank the compressor to clear the lines faster, but anything over 50 PSI can blow out the rubber seals inside heads and valves, turning a free chore into a repair bill. Keep it at 35–50 PSI and let several short cycles do the work.

Forgetting the backflow preventer and the controller. The brass backflow assembly above ground is the part most exposed to frost, so drain its bleed screws and insulate it. And set the controller to its off or rain mode so it does not try to run a dry system on a freezing night, which can burn out a valve solenoid.

Opening the water back up too fast in spring. After months sitting empty the pipes are full of air, and a sudden surge slams that air against the first closed valve and shocks every fitting. Crack the main valve open over a full minute and let the system fill gently.