Sprinkler winterization: when to blow out the lines
Almost every guide gives you a month. The thing that actually breaks your system is the ground freezing with water still in it, and that does not happen on a date.
Check your region first
We read soil temperature daily and open the winterization window when the ground starts cooling ahead of frost. See whether it is time where you live.
What actually goes wrong
Water expands when it freezes, and irrigation components are rigid. Whatever holds the water when it turns to ice is what splits: buried pipe, fittings, valve bodies, sprinkler heads, the pump, and the backflow preventer — which is usually the priciest part of the whole system. None of that shows up until spring, when you turn the water on and find a geyser in the lawn.
When to do it
Before the ground freezes, with enough margin that a cold snap does not beat you to it. That is weeks apart across the country and it shifts every year, which is exactly why we publish it against a daily reading instead of a month name. Our regional pages mark the window open when soil temperature has dropped ahead of frost.
If a hard freeze is forecast and you have not done it, that is the deadline — not the weekend you had in mind.
The three ways it gets done
Manual drain
Systems built with manual drain valves at the low points: shut off the water, open the valves, let gravity do it, and drain what sits between the shutoff and the backflow device. Simple, but water trapped above a valve or inside a sprinkler body stays put.
Automatic drain
Drain valves open on their own once pressure drops below a few psi. You still have to relieve pressure and deal with the backflow assembly by hand.
Compressed air blowout
The method used in hard-freeze climates, because it clears water that gravity cannot reach. It is also the one with real risk attached, and the numbers below matter.
- Compressor — connect at the blowout port; 40–80 psi working range
- Backflow preventer — drain by hand; do not blow air through it
- Each zone — two short cycles, not one long blow
The published numbers
| What | Published figure |
|---|---|
| Working air pressure | 40–80 psi |
| Maximum for PVC pipe | 80 psi |
| Maximum for polyethylene pipe | 50 psi |
| Cycles per zone | two short ones, not one long one |
| Maximum per circuit, per pass | 2 minutes |
Those pressure and cycle figures come from Colorado State University Extension; the two-minute limit per circuit is Rain Bird's. Two details behind them are worth understanding rather than memorizing:
- Volume clears a line, not pressure. A small high-pressure compressor pushes the rating of your pipe without moving enough air to carry the water out.
- An empty pipe is the one that gets hurt. CSU warns specifically against re-blowing a zone that is already dry: with no water to carry the heat away, friction and hot compressed air work on the plastic directly.
The backflow preventer is its own job
Do not send compressed air through a pressure vacuum breaker. CSU's guidance is to work the ball valves and test cocks open and closed several times and leave them part-open for the winter; reduced-pressure assemblies get drained by hand through their check valves. Assemblies also tend to be the part your municipality requires a certified tester to touch, which is worth checking before you take a wrench to it.
Where to stop and call someone
This is the honest part. Compressed air in an irrigation system is not a gentle process: CSU's own instruction sheet says to wear eye protection and warns against standing over components while a zone is being blown, because a sprinkler body can come apart under air. Hand it to a professional if:
- You do not have a compressor that moves real volume — renting a bigger one is cheaper than replacing a manifold, and a shop compressor usually is not it.
- Your system has a pump, a reduced-pressure assembly, or anything you cannot identify.
- Your local code requires a certified tester on the backflow device.
- You cannot find the drain points or the zone valves, which usually means the as-built drawing is gone and guessing gets expensive.
A blowout from a service company is routine work priced per zone. A split manifold in April is not. The cost guide walks through how quotes scale with zone count and CSU Extension's compressor-sizing formula — plug in the numbers your town actually charges.
The same frost is the deadline to shut off and drain outdoor faucets. That is a five-minute job through the wall, not a compressor job, and skipping it puts ice in a line that feeds the house.
During the growing season, run times come from measurement, not from the factory default on the controller. The sprinkler run time calculator walks through the catch-can test extension services publish and scales it to the depth you want per zone.
Spring is the reverse job: turn the system back on after the frost window, pressurize slowly, and walk each zone before you trust the timer.
Questions people actually ask
When should I winterize my sprinkler system?
Before the ground freezes, not on a fixed date. That is weeks apart between the Upper Midwest and the Mid-Atlantic, and it moves year to year. Our regional pages track soil temperature daily and flag the window as it opens.
What air pressure should I use to blow out sprinkler lines?
Colorado State University Extension puts the working range at 40–80 psi, capped at 80 psi for PVC pipe and 50 psi for polyethylene. High pressure is not what clears a line — volume of air is.
How long do I run each zone?
Short. CSU Extension recommends two short cycles per zone rather than one long one, and warns against re-blowing a zone that is already dry, because friction and heat in an empty pipe can damage it. Rain Bird's guide says not to blow any circuit for more than two minutes at a time.
Can I skip it if I just drain the system?
It depends on how your system was built. Systems with manual or automatic drain valves at the low points can be drained, but any water trapped above a valve or in a sprinkler body still freezes. In hard-freeze climates the blowout is what removes that.
What happens if I do not winterize?
Water expands as it freezes and splits whatever contains it: pipes, fittings, valves, sprinkler bodies, backflow preventers and pumps. The backflow assembly is usually the most expensive single casualty.
Sources
- Colorado State University Extension — Home Sprinkler Systems: Preparing Your Sprinkler System for Winter: pressure range and pipe limits, two short cycles per zone, the warning against re-blowing dry zones, backflow preventer handling, eye protection.
- Rain Bird — How do I winterize my irrigation system?: two-minute limit per circuit and manufacturer guidance on compressor use.
General information, not advice for your specific system. Local code and your equipment change the answer.