Irrigation · Before the first hard freeze
How to Winterize Your Sprinkler System
Winterizing an irrigation system is one idea repeated in four places: get the water out of everything that can freeze, before it freezes. Water expands about 9% when it turns to ice, and buried poly lateral lines, valve bodies, sprinkler head cans and — above all — the backflow preventer are sealed containers that cannot accommodate that. The job itself takes an afternoon. The part almost every guide gets wrong is the timing: "do it in October" is six weeks wrong across the country, so the date below comes from your own location's freeze normals rather than from a calendar.
First: when do you actually need to do this?
The deadline is your own first hard freeze, and it moves by more than two months across the country. Enter your ZIP for the window where you live — then work the checklist below inside it.
Free · no signup · NOAA 1991–2020 normals
The whole job, step by step
- Step 1
Start from your first freeze date, not from a month
The deadline is your area's average first fall freeze, and it moves by more than two months between the northern plains and the Gulf coast. A normal early year runs about 12 days ahead of that average, so treat the early edge — not the average itself — as the date to be finished by, and open the window about four weeks out.
- Step 2
Shut the irrigation main off and stop the controller
Close the dedicated irrigation shut-off — usually in a basement, a crawlspace, or a below-grade box near the meter — so nothing refills the lines behind you. Then put the controller in the off/rain position rather than unplugging it, so it keeps its program and its clock for spring. If it has a backup battery, this is the moment to replace it.
- Step 3
Drain the backflow preventer — the expensive part
This is the single priciest common casualty of a missed winterization, and it sits above ground where it freezes first. How you drain it depends on which device you have: a PVB, an RPZ, a double check, or an atmospheric vacuum breaker. The universal rule is that the isolation ball valves AND the test cocks all go to about 45° — half open — and stay there, so nothing on the device is left sealed with water in it.
Never leave the isolation valves fully closed for winter. A sealed valve body is exactly the container that splits.
- Step 4
Clear the lines: blow out, or drain if your system was built for it
Three methods exist and only one of them suits most systems. A blow-out pushes the water out with compressed air and works on any layout — it is the standard for a reason. Manual drain valves work only if the installer put them at every low point and sloped the pipe to them. Automatic drain valves open themselves when zone pressure drops below about 10 PSI, which handles the laterals but leaves water between the shut-off and the zone valves for you to bleed off by hand. If you do not know which your system has, assume it needs a blow-out.
- Step 5
If you are blowing it out, do it zone by zone at low pressure
Connect at the blow-out port downstream of the backflow — never through the backflow device itself. Open one zone before you introduce any air, regulate to 50 PSI or less, and run that zone until the heads throw only mist, then move on. Air volume rather than pressure does the work — clearing a zone wants roughly 30–80 CFM depending on pipe size — so a small compressor means several short cycles per zone, not a higher regulator setting. Two or three shorter passes per zone beat one long one; continuous air with no water moving through it heats the pipe and can melt fittings and gears.
Compressed air in closed pipe is genuinely dangerous. Never run air with every zone valve shut, stay at or below 50 PSI, keep people away from heads while a zone is blowing, and wear eye protection.
- Step 6
Finish with the small stuff that also freezes
Drop and drain any hose-end timers, disconnect and drain every garden hose (a hose left connected keeps the sill cock full and splits it from inside the wall), shut off and drain frost-free hose bibs, and pull drip-system filters and pressure regulators — those hold water and crack as readily as anything else on the property.
- Step 7
Record what you did and how
Which method you used, the date, what it cost, and anything that looked marginal. Next spring's startup is a freeze-damage inspection, and it is far more useful when you can compare against what the system looked like the day you shut it down.
Why the valves go to 45°, not closed
Every device below repeats this instruction, and it is the one people talk themselves out of, because closed feels more shut off than half open. Looking down through the valve is what settles it — the handle reports where the bore is pointing, and a bore turned across the pipe is a sealed container with water in it.
Open — the summer position
Handle along the pipe, bore lined up with it. This is how the valve spends the irrigation season.
45° — the winter position
Half open is not a compromise between the other two — it is the only position that leaves the inside of the ball connected to somewhere the water can go.
Closed — what splits the valve
Closed looks tidier and is the most common winterization mistake on a backflow device. The bore is now a sealed container, and water needs about 9% more room as it freezes than the brass around it can give.
Max blow-out pressure
50 PSI
Safe for both rigid PVC and flexible poly, and you rarely know which is buried.
Professional blow-out
$50–250
Driven mostly by zone count and region.
Typical freeze repair
$170–$2,500+
The backflow preventer is the priciest common casualty.
Which backflow preventer do you have?
This is the step where generic instructions go wrong, because the four devices found on residential systems are winterized differently. Identify yours, then follow its own sequence.
PVB — Pressure Vacuum Breaker
A single brass body on a riser about a foot above the highest sprinkler head, with a domed bonnet on top, two ball valves below it, and two small test cocks on the side. The most common device on residential irrigation in freeze country.
Above ground — freezes first.
RPZ — Reduced Pressure Zone assembly
The big one: two check valves in line with a relief valve hanging underneath that occasionally dumps water on the ground. Required where the system carries fertilizer injection or a higher hazard rating, and usually the most expensive device on the property.
Above ground — freezes first.
DC — Double Check valve assembly
Two check valves in a single body with four test cocks and no relief valve underneath. Often installed below grade in a valve box, which is what makes it the least freeze-prone of the four.
Usually below grade — slower to freeze, not immune.
AVB — Atmospheric Vacuum Breaker
The small, simple one — a single brass or plastic body downstream of each valve, with no test cocks and no isolation valves. Usually one per zone rather than one for the system.
Above ground — freezes first.
Blow-out windows by USDA zone
A rough national reference. Zones 8 and warmer are absent on purpose — they do not get the sustained hard freezes that crack irrigation pipe, so there is no deadline to publish.
| USDA zone | Winterize between | Dates for your area |
|---|---|---|
| Zone 3 | August 18 – September 3 | Zone 3 page → |
| Zone 4 | August 28 – September 13 | Zone 4 page → |
| Zone 5 | September 7–23 | Zone 5 page → |
| Zone 6 | September 22 – October 8 | Zone 6 page → |
| Zone 7 | October 4–20 | Zone 7 page → |
| Zone 8 | October 18 – November 3 | Zone 8 page → |
Zone-level approximations. A hardiness zone measures winter cold rather than frost dates, so look up your ZIP or find your city for a closer answer.
The parts people actually get stuck on
The steps above are the whole job. These go deeper on the four questions that stop people halfway through it — no compressor, an unfamiliar backflow device, what it costs, and whether it genuinely matters where you live — plus the spring half of the same job.
How to Winterize Sprinklers Without an Air Compressor
Three real options, and the honest answer about the pancake compressor in your garage.
Read the guide →How to Drain a Backflow Preventer for Winter (PVB, RPZ, DC & AVB)
Four devices, four procedures — and the one mistake that splits all of them.
Read the guide →How Much Does It Cost to Winterize a Sprinkler System?
What a blow-out runs, what DIY really costs, and where the break-even actually sits.
Read the guide →What Happens If You Don't Winterize Your Sprinkler System
What actually breaks, in what order, and why you won't find out until spring.
Read the guide →And the spring half of the job
Spring Sprinkler Startup Checklist
Wake the system up slowly — most startup damage is self-inflicted.
Read the guide →How to Tell If Your Sprinkler System Has Freeze Damage
The symptoms, where to look, and what's a cheap part versus a real repair.
Read the guide →Frequently asked questions
- When should I winterize my sprinkler system?
- Open the window about four weeks before your area's average first fall freeze and be finished about 12 days before it — that early edge is where a normal early year freezes, and it is the date that actually matters. That is August in the northern plains and November on the Gulf coast, which is why a fixed month is the wrong answer. Enter your ZIP in the frost-date tool for the window where you live.
- Can one light frost damage my sprinklers?
- A brief dip to 32°F usually will not — it is a sustained hard freeze, several hours below about 28°F, that reaches buried pipe and the water inside above-ground devices. The catch is that the backflow preventer is the exception: it is above ground, exposed, and full, and it can be damaged by the first real freeze of the season.
- Do I have to blow it out, or is draining enough?
- Draining is enough only if the system was designed for it — manual drain valves at every low point with the pipe sloped to them, or automatic drains plus a hand-bled main. Most systems in hard-freeze climates were not built that way, and any low spot that holds water is a split waiting to happen. If you are unsure, blow it out.
- What pressure should I use to blow out sprinklers?
- No more than 50 PSI. Rigid PVC tolerates more and flexible poly does not, and most homeowners cannot be certain which is buried in their yard, so 50 is the ceiling that is safe for both. Pressure is not what clears the line anyway — volume is.
- What does it cost to have it done?
- A professional blow-out typically runs $50–250 depending on zone count and region. Freeze repairs typically run $170–$2,500+, and the backflow preventer alone can account for most of that, which is the whole economic argument for doing it.
- What if I already had a freeze and never winterized?
- Shut the system off, drain what you can, and leave it depressurised until spring. Damage from a freeze usually does not show itself until the system is pressurised again, so the worst thing you can do now is turn it back on to check. Plan a careful spring startup with a freeze-damage inspection instead.