Irrigation winterization and spring startup field guide

Winterization, and why an empty pipe survives the cold

Winterization is clearing the standing water out of an irrigation system before the ground freezes, so the water left in the pipes, valves, heads, and backflow has nowhere to expand when it turns to ice. An empty pipe does not care how cold it gets. A pipe with a half inch of trapped water in a low spot splits the first night it drops below freezing for long enough.

Two jobs make up the full cycle, and a system needs both. In the fall you get the water out before the freeze. In the spring you bring the pressure back, check every zone, and put the controller back to work. Skip the fall side and you spend the spring digging up cracked laterals and rebuilding a backflow. Rush the spring side and you blow a fitting with water hammer on the first fill.

This guide assumes a system that was designed and zoned to begin with. If you are sizing zones, picking heads, or setting the pressure regulation, that work lives in the sprinkler system design guide. If you are setting the controller programs and seasonal adjust, the controller programming guide covers that. Here the focus is the water in the pipe, getting it out for winter and back in for spring.

What freezing water actually does to the system

Water expands about 9 percent when it freezes, and it does not ask permission. That expansion is the whole problem. Trapped in a closed pipe or a valve body, the ice has nowhere to go, so it pushes the walls apart until something gives. PVC mainline splits along its length. Poly laterals balloon and crack. Brass and plastic valve bodies fracture at the bonnet. Spray and rotor heads crack at the cap and the riser, and the gear-drive bodies split where water sat in the case.

The backflow preventer is the part that pays first and pays the most. It sits above grade in most climates, full of water, with check valves and a relief valve and thin brass or plastic sections that crack before the buried pipe ever feels the cold. A new RPZ or PVB and the labor to swap it runs well past what the whole winterization would have cost.

How hard you have to work at this scales with the climate, so hedge to where the system actually is. A few light frosts in a mild zone may only call for draining and protecting the backflow. A region that freezes hard and stays frozen for months needs a full blow-out, because any water you leave behind will freeze solid and stay that way. The colder and longer the freeze, the less you can leave to chance.

The three winterizing methods, and which one to trust

There are three ways to get the water out: manual drain, automatic drain, and blow-out with compressed air. They are not equal, and the right one depends on how the system was plumbed and how cold it gets.

Manual drain works on systems built with drain valves at the low points. You shut the supply, open the drain valves, and let gravity pull the water out of the mainline and laterals, then drain the backflow and any check valves above the drains. It is simple and needs no equipment, but it only clears water that can run downhill to a drain, and it does nothing for water trapped above a check valve or in a head.

Automatic drain valves open on their own once system pressure drops below a set point, usually a few psi, and let the lines drain after every cycle. They help, but they are not a complete winterization by themselves in a hard-freeze climate, because they leave water above the valves and in the heads.

Blow-out pushes compressed air through the lines and forces the water out through the heads, including the water that gravity and auto-drains leave behind. It is the most thorough of the three and the standard for any system in a real freeze climate. The rest of this guide is mostly about doing the blow-out right, because that is the method that goes wrong and breaks things when it is done wrong.

The blow-out method, start to finish

Blow-out connects a compressor downstream of the backflow preventer and pushes air through each zone in turn, driving the water out through the heads. You connect after the backflow, never through it, because forcing air backward through the check valves and relief can damage the assembly. Most systems have a blow-out port, a hose bib, or a union just downstream of the backflow for exactly this.

The sequence is steady and unglamorous. Shut off the water supply to the system. Connect the compressor to the blow-out point with the air off. Set the controller or the manual bleed to open one zone, the farthest or the highest first depending on the layout, before you let air in. Open the air slowly until the heads in that zone pop up and start throwing water. Hold it until the stream thins to a fine mist and the heads run dry, then close that zone and move to the next.

Mist is the signal you are watching for. A head throwing a solid stream still has water behind it. A head throwing a fine mist is nearly clear, and once it stops misting that zone is done. Work through every zone, then go back and give the longest or lowest zones a second short pass, because a single long pass leaves water in the low spots and overheats the heads. Short repeated passes clear more water and are easier on the system than one long blast.

Finish at the backflow and the controller, both covered in their own sections below. Air in the lines is only most of the job. The assembly above grade and the brain on the wall still need attention.

What pressure do you use to blow out sprinklers?

Keep the air pressure low. A common ceiling is about 50 psi for systems with flexible black poly pipe and up to about 80 psi for rigid PVC, but treat those as upper limits and stay under them, not targets to hit. Many techs run 40 to 50 psi on residential systems and let the air volume do the work. Confirm the pressure against the pipe and head manufacturer's guidance, because the limit is set by the weakest component in the zone, not by the compressor.

Too much pressure is how a blow-out turns into a repair list. Over-pressurized air cracks fittings, splits the spray bodies, blows the caps off heads, and can warp or melt the internals of a gear-drive head running dry under high pressure. The damage often does not show until spring, when the zone that was over-pressurized in October leaks at three fittings in April.

The rule that keeps you out of trouble is simple: set the regulator before you open the air, and never open the air against a closed zone. Air with nowhere to flow builds pressure fast and finds the weakest fitting. Open the zone first, then bring the air up slowly. If a head is whistling and shaking instead of misting, the pressure is too high, so back it down.

Why CFM matters more than pressure

Volume moves the water, not pressure. CFM, cubic feet per minute, is the measure of how much air the compressor delivers, and that volume is what pushes a column of water down a lateral and out the heads. Pressure only tells you how hard the air can push against a restriction. You want enough volume to sweep the line and low enough pressure that you do not break anything doing it.

This is why a small pancake or hot-dog compressor does not work for a real system. A pancake unit might hit 120 psi, which sounds like plenty, but it delivers only a couple of CFM, so it cannot move the volume of water sitting in the zones. It empties its tank in seconds, the heads barely lift, and you walk away thinking the zone is clear when it still holds water that freezes and splits the pipe. High pressure with no volume is the classic way to leave water in and crack the lines anyway.

For anything beyond a tiny system, get real volume. Many techs look for a compressor in the range of 20 to 50 CFM at the low pressure they plan to run. A rough sizing rule some crews use is the GPM of the largest zone divided by about 7.5 to get the CFM you want at the connection. For larger and commercial systems, that means a tow-behind or skid compressor, not a portable. Rent the volume if you do not own it. The cost of the rental is a fraction of one cracked mainline.

One zone at a time, and don't cook the heads

Blow out one zone at a time, never several at once. The air follows the flow, and a single open zone gives all the volume one path to push the water out. Open two or three zones together and the air splits, the velocity drops in each, and none of them clear properly. One zone open, the rest closed, every time.

The opposite mistake is running a single zone too long. A sprinkler head, especially a gear-drive rotor, relies on water moving through it to keep the internals cool. Run it dry under air for too long and the friction heats the gears, and the plastic can warp or melt. You hear it before you see it: a head that was misting starts to chatter and squeal, and that squeal is the signal to shut the zone and move on.

The technique that respects both rules is short passes. Open the zone, run it until the heads mist and run dry, which is usually a minute or two, then close it. Come back for a second short pass on the zones with low spots or long laterals rather than holding the first pass longer. You clear more water and you never cook a head. Two short passes beat one long one on every count.

Do you have to winterize the backflow preventer?

Yes, and it is the part most likely to break if you skip it. The backflow preventer, whether a reduced pressure zone assembly (RPZ) or a pressure vacuum breaker (PVB), sits above grade full of water with thin brass and plastic sections that freeze and crack before the buried pipe ever feels the cold. It is the number one freeze casualty in irrigation, and it is the most expensive single part to replace.

Drain it and relieve it. After the lines are blown out, shut the inlet and outlet isolation valves, then open the test cocks to let the trapped water drain and air in. Back the ball-valve handles to about 45 degrees, halfway between open and closed, so any water left in the valve body has room to expand without splitting the valve. The 45-degree position is the one step people forget, and a fully closed ball valve with water behind it cracks just like a pipe.

Then protect or remove it. In milder freeze zones, an insulated, breathable cover over the drained assembly is often enough. In hard-freeze climates, many techs remove the assembly entirely and store it indoors, or insulate heavily and accept the risk. Confirm the procedure against the device manufacturer and your local plumbing code, because the assembly is a code-required device and how it is handled is governed, not optional. Whatever you do, do not leave it full and exposed.

Setting the controller down for winter

Put the controller in its rain, off, or no-water mode rather than pulling the power. Most modern controllers hold the programs and the time in memory either way, but the rain or off setting keeps the clock and the schedule intact so spring startup is a single switch back to run, not a full reprogram. The controller programming guide covers building those schedules in the first place; here you are just parking them.

If the system has a pump or a master valve, make sure they are off and will stay off through the winter. A controller left in run mode that energizes a master valve or kicks a pump against a drained, depressurized system is asking for trouble. Some techs pull the common or the pump-start wire as a belt-and-suspenders step so nothing can drive the system while it is down.

Leave a note in the controller box or on the account record that the system was winterized and on what date. The next person to open that box, including you in March, should be able to see at a glance that the water is out and why the controller is dark.

Pump and well systems

A system fed by a pump or a well needs the pump winterized too, and the steps depend on the equipment. Shut off the power to the pump so it cannot dry-start, then drain the pump body, the pressure tank, and the suction and discharge lines that sit above the frost line. Any exposed piping, the pump head, and the pressure switch hold water that freezes and cracks the same as a lateral.

Follow the pump manufacturer's winterizing steps for the specific unit, because drain points and protection vary by model, and a submersible in a deep well is a different job from an above-grade jet pump. For a well, the part that freezes is whatever sits above the frost line, so that is what you drain and protect. Get this wrong and you replace a pump, not a fitting, in the spring.

Drip and micro-irrigation zones

Drip zones winterize like the rest of the system but with a few wrinkles, because they run at low pressure and carry filters and emitters that clog and crack. Blow them out at the low end of your pressure range, not the high end, since drip tubing and fittings take far less pressure than buried PVC and will pop apart under a hard blast. Some crews drain drip zones by gravity and air rather than driving them hard at all.

Pull and clean the filters before you store them, and check that the pressure regulator and emitters are clear. Water left in an emitter or a filter bowl freezes and splits it, and a cracked filter housing in spring leaks until you find it. The drip design and install guide covers the components themselves; for winter, the short version is low pressure, flush, and clear the filters and regulators so nothing holds water over the winter.

Compressed-air safety at the heads

Compressed air in an irrigation system is stored energy, and it does not behave like water. A head under air can launch its cap or internal parts, and a fitting that lets go throws debris. Wear eye protection the entire time the compressor is connected, and keep it on while you walk the zones, not just at the compressor.

Do not stand over a head while a zone is charging or running. A spray or rotor that pops up under air pressure comes up hard and can throw a cap or a nozzle straight up. Walk the zone to confirm the heads are misting, but stand to the side, not over the top of a head, and keep anyone else clear of the active zone.

Bleed the pressure before you disconnect anything. Air left in the lines or the hose stays under pressure after you shut the compressor, so open a zone or a bleed to release it before you break a connection. A hose under pressure that whips loose at the fitting is the kind of injury that ends a workday. Treat the air side with the same respect you would give any pressurized system.

How do you start up a sprinkler system in spring?

Spring startup is the fall process run in reverse, and the one rule that governs all of it is slow. You bring the water back gradually, walk every zone, reset the controller, and get the backflow tested. Rushing the first fill is how you undo a clean winterization with a single burst of water hammer.

Work in order. Confirm the freeze risk has passed for the season before you charge the system, because a system filled ahead of a late hard frost is right back where it started. Reopen and reset the backflow assembly, turning the ball valves back to full open and closing the test cocks. Open the main supply slowly so the lines fill without slamming. Then run each zone one at a time, watching for broken heads, leaks, low spots, and coverage gaps that the winter or the blow-out may have left. Reset the controller out of rain or off mode back to its run schedule, adjusted for the season. Finally, get the backflow tested if your jurisdiction requires it.

The walk-through after the fill is where you catch the winter's damage. A head sheared by a plow, a lateral that cracked despite the blow-out, a wiper seal that froze and now weeps: you find these by running each zone and watching, not by turning the whole system on and assuming it is fine.

Why the spring recharge has to be slow

Open the main slowly on the first fill of the year. A dry, empty system has air in every line, and when you crack the supply wide open, the incoming water races down the empty pipe and slams to a stop at the first closed valve or capped end. That hydraulic shock is water hammer, and it can split a fitting, blow a head, or crack the very mainline you protected all winter.

Filling slowly lets the water push the air ahead of it and out through the heads or a cracked-open zone, instead of compressing it into a hammer. Open the supply valve part way, let the system fill and the air work out, then open it the rest of the way. On a large system, fill zone by zone if you can, so no single line takes the full shock at once.

The mistake is treating the spring turn-on as flipping a switch. The faster you fill, the harder the hammer, and the damage shows up immediately or as a slow leak that takes weeks to find. A two-minute slow fill is cheaper than the fitting it saves.

The spring backflow test

Most jurisdictions require the backflow preventer to be tested annually by a certified tester, and spring startup is when that test happens for an irrigation system. The assembly protects the potable supply from cross-contamination, so the test is a public-health requirement, not a formality, and missed tests can bring fines or a water shutoff in many areas.

Schedule the test after the assembly has come through the winter and been recharged, since a freeze can damage a check or the relief in a way that only a test catches. If the assembly cracked over the winter, the test fails and the repair happens before the system goes into service. Confirm the testing interval and who is allowed to perform it against your local code and water authority, because both vary by jurisdiction.

When to winterize and when to start up

Winterize before the first hard freeze of the fall, and start up after the freeze risk has passed in the spring. The exact dates depend entirely on the climate, so hedge to the local frost calendar rather than a number on a page. In a cold region that means the back half of the fall, ahead of the first night that drops well below freezing and stays there. In a milder zone the window is later and shorter.

The cost of being early is small and the cost of being late is a cracked system, so lean early in the fall. A system blown out a few weeks before it strictly needed to be is fine. A system caught full by an early hard freeze is a repair list. Watch the forecast in the fall and book the work ahead of the first real cold snap, not after it.

In the spring the bias runs the other way: lean late. A late frost after you have charged the system puts the water right back in the lines for the freeze you were trying to avoid. Wait until the hard freezes are done for the season, then start up. The local frost-free date is the guide, and a few days late costs nothing while a few days early can cost a backflow.

Commercial, campus, and large-site systems

Large commercial irrigation runs on the same physics but a different scale, and the equipment has to match. A campus, an HOA, a sports complex, or a commercial property can have dozens or hundreds of zones across long mainlines, and a portable compressor cannot move that volume. These systems get a tow-behind or skid-mounted compressor with the CFM to sweep large-diameter mains, and the blow-out still goes zone by zone, just with the volume to do it.

Plan the sequence on a large site so you are not walking back and forth across acres. Work the zones in a logical order off the controller, group them by mainline and by the compressor connection points, and confirm each zone clears before moving on. Large systems often have multiple points of connection and multiple backflow assemblies, and every one of them has to be drained and protected, not just the one nearest the truck.

Data-center and commercial-campus landscapes are worth a specific note, because the irrigation often shares the site with critical infrastructure and the grounds still have to look maintained. The winterization itself is the same, but the documentation and scheduling matter more, since a missed backflow test or a cracked main on a high-visibility site is a bigger problem than on a back lawn. Treat these as accounts where the record is part of the deliverable.

What to document, per account

Document the winterization and the startup for every account, every year. The record is what tells you and the customer that the system was protected, on what date, by whom, and in what condition. It is also what protects you when a system that was never on your winterization list cracks and the owner asks why.

Log the date, the method, the pressure used, which zones cleared and which gave trouble, the condition of the backflow, and any damage found and fixed. In the spring, log the startup date, the slow fill, the zone walk-through findings, the controller reset, and the backflow test result. A per-property record built up year over year shows the pattern: the zone that always holds water, the head that breaks every winter, the backflow that is getting tired.

A field tool like FieldOS keeps that record tied to the property and the visit, with photos of the backflow and the problem heads, so the history travels with the account instead of living on a clipboard that gets lost. The next visit starts from what the last one found, not from scratch. The table below is the short version of what belongs on the record.

Common mistakes

• Using a small pancake compressor with no CFM, which cannot move the water and leaves it in the lines to freeze and split the pipe.
• Over-pressurizing the blow-out and cracking fittings, splitting spray bodies, or blowing the caps off heads.
• Skipping the backflow preventer, the number one freeze break, by leaving it full and exposed instead of draining and protecting it.
• Recharging the system too fast in spring and slamming a fitting or mainline with water hammer.
• Running a single zone dry too long under air and overheating the gear-drive heads until the plastic warps.
• Opening the air against a closed zone, so pressure spikes and finds the weakest fitting.
• Working the heads with no eye protection and standing over a head while a zone is charging.
• Starting up in spring before the late freezes are done, putting water back in the lines ahead of a frost.

Field checklist

Standards and references

Winterization is governed less by one code and more by the manufacturer's limits and the local climate, so cite the right source for each call. The blow-out pressure ceiling comes from the pipe and head manufacturers, who publish a maximum air pressure for their products, and the safe number is the lowest among the components in the zone. Treat the 50 psi poly and 80 psi PVC figures as common upper limits to stay under, not targets, and confirm them against the products you actually installed.

The backflow preventer is the part with real code behind it. Backflow assemblies are required by the plumbing code, IPC or UPC depending on the jurisdiction, and the annual test is a requirement of the local water authority, with the device standards set by bodies like ASSE and the USC Foundation for Cross-Connection Control. How the assembly is winterized, tested, and who is allowed to test it is set by the adopted code and the local water purveyor, so confirm both.

For practice and timing, the Irrigation Association (IA) and local water-authority guidance are the references the trade leans on, and the frost calendar for the specific region drives the dates. Three points carry across all of it, and they are the ones to hold to: volume clears the water, so size for CFM and not pressure; the backflow is the part that breaks, so drain and protect it every time; and the spring fill has to be slow to avoid water hammer. Hedge the exact pressures and dates to the manufacturer and the climate, never to a single number.

Units and terms

Winterization spans a few units and a handful of terms that show up across compressor specs, device sheets, and the trade's own shorthand.

Air pressure is in psi (pounds per square inch), sometimes bar or kPa on imported gear. Air volume is in CFM (cubic feet per minute), the number that actually matters for a blow-out. Pipe is poly (flexible black polyethylene) or PVC (rigid white pipe), each with its own pressure limit. The backflow preventer is the RPZ (reduced pressure zone assembly) or PVB (pressure vacuum breaker). The blow-out itself is also called winterizing or fall shutdown, and spring startup is also called the spring turn-on or charge-up.

CFM

Cubic feet per minute, the volume of air a compressor delivers, which is what moves the water out of the lines

psi

Pounds per square inch of air pressure, kept low so the air does not crack pipe or heads

Blow-out

Pushing compressed air through each zone to force the water out through the heads before the freeze

Backflow preventer (RPZ / PVB)

The above-grade assembly that protects the potable supply, and the part most likely to crack if not winterized

Water hammer

The hydraulic shock from filling a system too fast, which can split a fitting or mainline at spring startup

Test cocks

The small ports on a backflow assembly, opened to drain and relieve it for winter and used to test it

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