How to Blow out Underground Sprinkler System? Safe Guide

Introduction

Maintaining an underground sprinkler system is one of the best investments you can make for your lawn and garden. However, like any irrigation system, it requires regular upkeep to function properly throughout the year. One of the most critical maintenance tasks is learning how to blow out underground sprinkler system lines before winter sets in. Failing to remove trapped water from your sprinkler pipes can lead to frozen, cracked components, costly repairs, and a non-functional system when spring arrives.

Blowing out your underground sprinkler system uses compressed air to push residual water out of pipes, heads, and valves, preventing freeze-related damage. Whether you are a seasoned homeowner or a complete beginner, this safe guide will walk you through every step of the process, from gathering the right equipment to troubleshooting common issues.

Table of Contents

• What Is Blowing Out an Underground Sprinkler System?
• Why You Need to Blow Out Your Sprinkler System
• When to Blow Out Your Sprinkler System
• Tools and Equipment You Will Need
• Step-by-Step Guide to Blowing Out Your Underground Sprinkler System
• Compressed Air Blowout vs Manual Drain Method
• Tips and Best Practices for a Safe Blowout
• Common Mistakes to Avoid When Blowing Out Sprinklers
• Signs Your Underground Sprinkler System Needs a Blowout
• Frequently Asked Questions
• Conclusion

What Is Blowing Out an Underground Sprinkler System?

Blowing out an underground sprinkler system is the process of forcing compressed air through your irrigation pipes to expel standing water. During normal operation, water remains inside the pipes, valves, and sprinkler heads. When temperatures drop below freezing, that water expands as it freezes, which can rupture PVC pipes, crack brass fittings, and destroy sprinkler heads.

The blowout process typically involves connecting an air compressor to the sprinkler system’s main water supply line, then opening each zone valve to allow the air to push water out through the heads and drain points. The result is a dry system that can withstand freezing temperatures without internal damage.

Understanding how to blow out underground sprinkler system components is essential for any homeowner who wants to protect their irrigation investment. It is a straightforward procedure, but it does require the correct equipment and a careful approach to avoid damaging the system.

Why You Need to Blow Out Your Sprinkler System

The primary reason to blow out your sprinkler system is freeze protection. Water expands approximately 9% when it freezes, and that expansion exerts enormous pressure on pipe walls. PVC and polyethylene pipes, which are commonly used in residential irrigation, are particularly vulnerable to freeze damage.

Here are the key consequences of skipping the blowout process:

• Cracked pipes: Frozen water can split PVC pipes, leading to leaks when the system is turned on next season.
• Damaged sprinkler heads: Water trapped inside sprinkler head housings can crack the plastic components or corrode metal springs.
• Broken valves: Solenoid valves and manual valves can split when water inside them freezes, requiring expensive replacements.
• Costly repairs: Repairing freeze-damaged irrigation systems can cost hundreds or even thousands of dollars, far exceeding the cost of a professional blowout.
• System failure: A damaged system may not function at all in spring, leaving your lawn without irrigation during the critical growing season.

Blowing out your system each year is a small effort that prevents these significant problems and extends the lifespan of your entire irrigation network.

When to Blow Out Your Sprinkler System

Timing is crucial when it comes to blowing out your sprinkler system. You should perform this task after your final watering of the season and before the first hard freeze in your area. A hard freeze is generally defined as temperatures consistently dropping below 32°F (0°C) for several hours.

In most temperate climates, this means performing the blowout in late fall, typically between October and November. However, the exact timing depends on your local climate zone:

• Cold climates (Zones 3–5): Blow out as early as late September or early October, as freezing temperatures can arrive suddenly.
• Moderate climates (Zones 6–7): November is usually a safe timeframe, but monitor weather forecasts closely.
• Warm climates (Zones 8–10): You may not need a full blowout every year, but it is still wise to drain vulnerable components if a rare freeze is forecast.

Do not wait until the first freeze has already occurred. If water is already inside the pipes when temperatures drop, damage can begin within hours.

Tools and Equipment You Will Need

Having the right tools on hand before you start is one of the most important steps in how to blow out underground sprinkler system lines safely. Using improper equipment can damage your system or create safety hazards.

Here is a complete list of what you will need:

• Air compressor: A reciprocating piston-type compressor rated for at least 80–100 PSI is ideal. Avoid using a small pancake compressor, as it may overheat or lack sufficient capacity. For larger systems, a 5–7.5 HP compressor works well.
• Pressure regulator: This attaches between the compressor and the sprinkler system to control airflow. Most irrigation components can withstand up to 80–100 PSI, so the regulator should be set no higher than 80 PSI for residential systems.
• Hose adapter or quick-connect fitting: You need a way to connect the compressor’s air line to your system’s irrigation fitting, typically a 3/4-inch or 1-inch male threaded fitting.
• Safety glasses and hearing protection: Compressed air and running compressors are loud and can pose eye hazards from flying debris.
• Rags or towels: Useful for wiping down connections and catching any water that escapes during the process.
• Notebook or phone: To record which zones you have completed and note any issues you discover.
• Lubricant (optional): Some compressor setups benefit from a light application of pneumatic tool oil on fittings, but this is not always necessary.

Before starting, inspect all your equipment to ensure nothing is damaged and all connections are secure. A poorly fitted adapter can blow off under pressure, creating a dangerous situation.

Step-by-Step Guide to Blowing Out Your Underground Sprinkler System

Now that you have your equipment ready, follow these steps carefully to complete a safe and effective sprinkler blowout.

Step 1: Shut Off the Water Supply

Locate the main shut-off valve for your sprinkler system and turn it off completely. This prevents any new water from entering the system while you are blowing it out. The main shut-off valve is typically found near the water meter, in a basement, or in an irrigation valve box near the property line.

Step 2: Run the System to Drain Gravity

After shutting off the water, turn on all zone valves manually or through the controller to allow as much water as possible to drain out by gravity. This step significantly reduces the amount of water the compressor needs to push out. Open any drain valves or blowout drains installed in low spots of the system. Let the system sit for several minutes to allow gravity to do most of the initial work.

Step 3: Connect the Air Compressor

Attach the pressure regulator to the air compressor outlet, then connect your hose adapter to the regulator. Thread the adapter onto the sprinkler system’s main water inlet fitting, which is usually located near the backflow preventer or at the point where the irrigation line enters the property. Make sure all connections are tight and secure. A loose connection under pressure can cause the fitting to fly off, potentially injuring someone nearby.

Step 4: Set the Compressor Pressure

Turn on the compressor and adjust the pressure regulator to 80 PSI or lower. For PVC pipe systems, never exceed 80 PSI, as higher pressures can crack fittings and joints. For systems with poly tubing, keep the pressure even lower, around 50–60 PSI. Start at a lower pressure and increase gradually if needed.

Step 5: Blow Out Each Zone

Open one zone valve at a time and allow the compressed air to push water out through the sprinkler heads. Each zone typically needs 1 to 3 minutes of airflow, depending on the length of the pipe run and the amount of residual water. Watch the sprinkler heads as they expel water and air. When the spray changes from water to a mist or dry air, the zone is sufficiently cleared.

Work through every zone systematically, and keep track of which ones you have completed. Do not run all zones simultaneously, as this reduces the airflow pressure to each individual zone and may leave water behind.

Step 6: Blow Out the Valves and Backflow Preventer

After clearing all zones, direct airflow through the valves and the backflow preventer assembly. These components often hold the most residual water and are among the most vulnerable to freeze damage. Hold the air line near each valve body and allow the air to push out any trapped water for 30 to 60 seconds per valve.

Step 7: Disconnect and Stow Equipment

Once all zones, valves, and backflow components have been blown out, turn off the compressor and carefully disconnect the air line. Cap or close the fitting to prevent debris from entering the system. Stow your equipment properly for the winter.

Following these steps ensures that your system is thoroughly drained and protected against freezing. Taking your time with each step is far better than rushing and leaving water behind.

Compressed Air Blowout vs Manual Drain Method

There are two primary approaches to removing water from underground sprinkler systems: compressed air blowout and manual draining. Each method has its advantages and limitations. Understanding the differences helps you choose the right approach for your system.

For most homeowners in areas with freezing winters, the compressed air blowout method is the recommended approach because it provides the most thorough water removal. In milder climates, manual draining combined with leaving a few faucets slightly open may be sufficient.

Tips and Best Practices for a Safe Blowout

Even if you understand how to blow out underground sprinkler system lines, these best practices will help you do the job more safely and effectively:

• Always use a pressure regulator. Never connect the compressor directly to the sprinkler system without a regulator. Uncontrolled pressure can destroy fittings and joints instantly.
• Start with the zone farthest from the compressor. This ensures that the air pushes water through the entire length of each zone rather than losing pressure at closer points.
• Never exceed 80 PSI for PVC systems. PVC pipes and fittings have specific pressure ratings, and exceeding them, even briefly, can cause catastrophic failure.
• Wear safety glasses. Water and debris can be expelled forcefully from sprinkler heads during the blowout process.
• Keep your hands away from sprinkler heads while they are pressurized. Heads may pop off or eject small parts under air pressure.
• Blow out the backflow preventer thoroughly. This component has multiple chambers and check valves that trap water easily. A thorough blowout here is one of the most important preventive measures you can take.
• Document your process. Take notes or photos of each zone and any issues you find. This helps with spring startup and any future repairs.
• Consider hiring a professional if you are unsure. If you do not own an air compressor or feel uncomfortable managing high-pressure air, hiring a licensed irrigation professional is a wise investment.

Common Mistakes to Avoid When Blowing Out Sprinklers

Even experienced homeowners can make errors when performing a sprinkler blowout. Here are the most common mistakes and how to avoid them:

• Using too much pressure: This is the number one cause of damage during blowouts. Always start low and increase only if necessary. Stick to 80 PSI maximum for PVC and 50–60 PSI for poly tubing.
• Skipping the gravity drain step: Trying to blow out a system full of water makes the process longer and less effective. Always drain as much water as possible by gravity first.
• Blowing out all zones simultaneously: This divides the airflow among all zones, reducing effectiveness. One zone at a time is the correct approach.
• Forgetting the backflow preventer: Many homeowners focus only on the pipes and heads, neglecting the backflow preventer, which holds significant water in its internal chambers.
• Not blowing out the valves: Solenoid valves contain small cavities that collect water. Direct airflow at each valve body for at least 30 seconds.
• Leaving the system uncapped after blowout: Open fittings can allow debris, dirt, and pests to enter the system during winter, causing blockages and damage in spring.
• Ignoring underground leaks: If you notice soggy areas in your yard near sprinkler lines, have a professional inspect for leaks before winter. Water trapped in leaking sections is more prone to freeze damage.

Avoiding these mistakes will protect your system and make the blowout process smoother and more effective.

Signs Your Underground Sprinkler System Needs a Blowout

Sometimes your system will show warning signs that it has water trapped inside and needs immediate attention. Watch for these indicators:

• Puddles or soggy areas near sprinkler lines: This suggests underground leaks where water has collected and may freeze during cold snaps.
• Cracked or bulging pipes: Visible damage to exposed pipe sections indicates that freeze damage may have already begun.
• Sprinkler heads that do not pop up fully: Water trapped inside the head housing can freeze and prevent the head from operating correctly.
• Water leaking from valves: Leaking valve bodies often mean water has been sitting inside and may have caused corrosion or cracking.
• Unusual sounds from the system: Gurgling or hissing sounds when the system is running can indicate trapped air or water in the lines.
• Previous year freeze damage: If your system suffered freeze damage in a prior year and was repaired but not fully blown out, residual water may still be present in other sections.

If you notice any of these signs, perform a blowout as soon as possible or consult a professional to assess the situation before freezing temperatures arrive.

Frequently Asked Questions

Can I Use a Shop Vac Instead of an Air Compressor to Blow Out My Sprinkler System?

A shop vac is not recommended for blowing out sprinkler systems. While a shop vac can generate some airflow, it is not designed for sustained pneumatic use and can overheat quickly. Additionally, most shop vacs lack the cubic feet per minute (CFM) output needed to effectively push water through underground pipes. An air compressor specifically designed for pneumatic tools is the safer and more effective choice for how to blow out underground sprinkler system components.

How Often Should I Blow Out My Underground Sprinkler System?

You should blow out your sprinkler system at least once per year, ideally before the first hard freeze in your area. In regions with particularly harsh winters, some professionals recommend performing the blowout twice, once in late fall and a mid-winter check if temperatures drop significantly below normal. In warmer climates, an annual or biennial blowout is usually sufficient.

How Much Does It Cost to Have a Professional Blow Out a Sprinkler System?

Professional sprinkler blowout services typically cost between $100 and $300, depending on the size of your system, the number of zones, and your local labor rates. Some companies charge per zone, while others offer flat-rate pricing. Investing in a professional blowout is far less expensive than repairing freeze-damaged pipes, valves, and heads.

What Happens If I Do Not Blow Out My Sprinkler System Before Winter?

If water remains inside your sprinkler pipes and components when temperatures drop below freezing, the water expands and can crack PVC pipes, split fittings, damage sprinkler heads, and break valves. The resulting damage can range from minor leaks to complete system failure. Repair costs can easily reach several hundred to several thousand dollars, making the relatively inexpensive and simple blowout process a critical seasonal task.

Can I Blow Out a System with Poly Tubing Instead of PVC?

Yes, but you must use lower pressure, typically between 50 and 60 PSI. Poly tubing is more flexible than PVC but can burst if too much pressure is applied. Always check the manufacturer’s pressure rating for your specific tubing and reduce the compressor pressure accordingly. Using a pressure regulator is especially important when working with poly irrigation systems.

Do I Need to Flush the System Again in Spring?

It is a good practice to run each zone briefly in early spring after the winter shutdown to check for any damage that may have occurred and to flush out any sediment or debris that may have entered during the blowout or winter months. This spring startup check helps ensure your system is operating correctly before the active growing season begins.

Conclusion

Learning how to blow out underground sprinkler system components is a vital seasonal task that protects your lawn, your wallet, and your peace of mind. Frozen water inside irrigation pipes and fittings causes thousands of dollars in damage every year, and the good news is that a proper blowout prevents nearly all of this damage when performed correctly.

By following the step-by-step guide in this article, using the right equipment, avoiding common mistakes, and adhering to best practices, you can confidently blow out your sprinkler system each fall. If you are uncertain about any part of the process, do not hesitate to hire a qualified irrigation professional who can complete the job safely and efficiently.

Taking the time to properly maintain your underground sprinkler system now will reward you with reliable, trouble-free performance when spring arrives. Protect your investment, prevent costly repairs, and enjoy a lush, healthy lawn year after year by making the annual blowout a non-negotiable part of your seasonal maintenance routine.