Winterizing an irrigation system, step by step
19 August 2026 · 10 min read
Water expands by about 9% when it freezes. That is enough to split a solenoid valve body, crack an elbow on a PE pipe or burst a sprinkler riser — and the repair bill can exceed the cost of winterizing for five seasons running. This guide walks the whole job step by step.
When to winterize
There is no date in the calendar, only a rule: before the first sustained frosts. In practice that means acting once night-time lows start approaching freezing regularly, not once they arrive. If the forecast shows two consecutive nights below −3 °C, you are already late.
A single −1 °C dawn frost will not damage a buried pipe — soil at 30–40 cm responds slowly. The danger is everything that sits shallow or above ground: solenoid valves in the box, sprinkler risers, the filter, the pressure regulator and the backflow preventer. Those freeze first and those are what split.
Three methods — and why only one is sufficient
| Method | How it works | Enough on its own? |
|---|---|---|
| Gravity draining | You open drain valves at the low points and the water runs out | No. Pipe is laid to fall, but water always stays in risers, elbows and any counter-slope run |
| Automatic drain valves | Valves open by themselves once pressure drops below roughly 0.5 bar | Not alone. They only work where they are fitted, and only for water that has somewhere to go |
| Compressed-air blow-out | A compressor pushes water out of the pipes and sprinklers zone by zone | Yes. This is the standard, and the only method that gives certainty on a system with pop-up sprinklers |
Step 1. Switch the controller off
Set the controller to OFF, or to rain mode / 0% seasonal adjust. Do not cut its power unless you have to — most controllers hold the schedule in non-volatile memory, but the clock backup battery drains faster when fully disconnected, and you start spring by resetting the time.
Controllers mounted outdoors in an unheated enclosure are worth removing and storing indoors, unless the manufacturer explicitly rates them for sub-zero operation.
Step 2. Close the supply and release the pressure
Shut the isolating valve that feeds the irrigation system — usually in the meter chamber, or wherever it branches off the house plumbing. Then bleed the pressure: open the lowest drain valve, or run one zone manually until the stream dies away.
This matters for safety too. Introducing compressed air into a system that still holds water pressure ends, at best, in a soaked jacket.
Step 3. Drain whatever runs out by itself
Open the drain valves at the low points and wait a few minutes. You will collect a good share of the volume and shorten the blow-out that follows. If your system has no drain valves, nothing is lost — move on, the compressor will do the work.
Step 4. Blow the system out with compressed air
This is the actual winterizing. Air is connected downstream of the isolating valve, at a fitting provided for the purpose. If the system does not have one, a quick-connect coupling is fitted on the run before the valve box.
What compressor you need
Against intuition, volume matters, not pressure. A small 24-litre hobby compressor will reach 8 bar but cannot move enough air to push water out of a zone — its pressure collapses after a few seconds and you spend the day waiting for the tank to refill. For a domestic garden aim for a compressor delivering roughly 150–300 l/min. Anything sized for bicycle tyres is useless here.
How to blow out
- Open one zone — from the controller in manual mode, or with the bleed screw on the solenoid. Never blow into a closed system.
- Start with the zone furthest from the compressor connection and work back towards the source.
- Keep each pass short. One pass is about a minute — until the heads emit mist rather than water. Then close the zone, let the compressor refill, and repeat once.
- Move to the next zone. Treat each one separately: try to blow them all at once and the air takes the shortest path, leaving the furthest zone full of water.
Do not overdo it. Rotary sprinklers use a water-lubricated gear drive — a few minutes running dry destroys it more effectively than frost ever would. Two short passes per zone beat one long one.
Step 5. Protect the valve box
- After the blow-out, leave the solenoid valves slightly open (manual bleed in a mid position) — the diaphragm is not held under tension for six months, and any remaining water has somewhere to escape.
- Unscrew and take indoors the mesh filter, pressure regulator and backflow preventer if they are mounted in the box or above ground. These are the parts that crack most often in the whole system.
- Bail out any water in the bottom of the box and cover it — rigid foam, a mat, or a layer of bark. A dry, covered box is a different proposition from an open chamber full of leaves.
Step 6. Drip line and above-ground parts
Drip lines freeze just as readily, and are often laid shallow or straight onto the soil under bark mulch.
- Open the flush ends at the end of each line and leave them open for winter.
- Blow drip zones out at lower pressure — pressure-compensating emitters work within a few bar and do not tolerate exceeding the upper limit.
- Insulate or dismantle above-ground runs, fittings and garden draw-off valves.
The most common mistakes
| Mistake | What happens | How to do it right |
|---|---|---|
| Relying on gravity draining alone | Water stays in sprinkler risers and elbows — the first hard frost splits them | Always finish with a compressed-air blow-out |
| Blowing all zones at once | Air takes the shortest path; the furthest zone stays full of water | Zone by zone, furthest first, working back |
| Too much pressure | Ejected nozzles, damaged seals and solenoid diaphragms | Up to 4 bar on sprays and drip, up to 5 bar on rotary heads |
| Blowing one zone for too long | Rotary sprinkler gear drives run dry and seize | Two passes of about a minute, with a pause between |
| Leaving the filter and regulator in the box | They split first, sitting above the drainage level | Unscrew them and store them somewhere warm |
| Waiting because it has not frozen yet | One unforecast −5 °C night is all it takes | Go by the season, not by the current week's weather |
Do it yourself or hire it out?
The decision usually comes down to the compressor. A unit with enough delivery to clear a zone properly is a real purchase, and it only pays back over several seasons — and mostly if neighbours have irrigation too and you can share it. Hiring an installer avoids that outlay entirely, and rates typically scale with the number of zones.
If this is your first time, a sensible compromise: run the first season with an installer and watch what they do and in what order, then take it on yourself.
What you need to do it quickly
The blow-out goes fast on one condition: you know how many zones you have, which sprinklers belong to which, and where the valve box is. On a system built a few years ago by eye, that is exactly what is missing — and half the time goes on guesswork.
If you are still planning an installation, it is worth having this documented from the start. In IrriForge the zone split, the sprinkler assignment and the valve box position all land in a PDF report that stays with you for future seasons — and the rules the zones are built from are covered in how to design garden irrigation step by step. You can also draw your garden in the editor right away and see what a correct zone split would look like.
What comes next — spring start-up
In spring the order reverses and matters just as much: refill the system slowly, with the main valve only partly open. Filling empty pipe fast causes water hammer, which can do precisely what frost does. Refit the filter and regulator, flush the zones with the nozzles removed, and only then fit the nozzles and set the controller.