How to Winterize a Sprinkler System with a Backflow Preventer Using an Air Compressor

Failing to properly winterize your sprinkler system before freezing temperatures arrive can result in cracked pipes, damaged valves, and a destroyed backflow preventer — repairs that can easily cost hundreds or even thousands of dollars. The blowout method using an air compressor is the most reliable way to remove all residual water from your irrigation lines, and when done correctly zone by zone, it protects every component from freeze damage. This step-by-step guide walks you through the entire process, including the correct PSI settings, how to handle your backflow preventer, and the proper sequence for blowing out each zone safely.

What You Will Need

Air compressor capable of producing 50–80 PSI with a minimum of 20 CFM (cubic feet per minute) for residential systems- Compressor-to-irrigation adapter fitting (typically a quick-connect coupler to garden hose thread)- Flathead screwdriver or irrigation valve key- Safety glasses- Work gloves

Understanding PSI Settings for Sprinkler Blowout

Using the correct air pressure is critical. Too much pressure will crack PVC pipes or damage polyethylene tubing, while too little will leave water behind that can freeze and expand.

Pipe Material	Recommended PSI	Maximum PSI
Rigid PVC pipe	50–60 PSI	80 PSI
Flexible polyethylene pipe	40–50 PSI	50 PSI
Drip irrigation lines	30–40 PSI	45 PSI

**Important:** CFM (volume of air) matters more than PSI alone. A small pancake compressor at 80 PSI will not effectively clear lines because it lacks sufficient airflow. You need at least 20 CFM for most residential systems. For larger properties with rotor heads, 50+ CFM is recommended.

Step-by-Step Zone-by-Zone Blowout Process

Step 1: Shut Off the Water Supply

Locate the main irrigation shut-off valve, which is typically found in your basement, crawl space, or near the water meter. Turn the valve to the fully closed position. This prevents any new water from entering the system during the blowout process.

Step 2: Drain the Backflow Preventer

Your backflow preventer (pressure vacuum breaker, double-check valve, or reduced-pressure zone device) must be drained and protected before blowing out the system. Follow these sub-steps:

Close both the upstream and downstream shut-off valves on the backflow preventer.- Open the test cocks (small petcocks on the side of the device) by turning them to a 45-degree angle. This allows trapped water to drain out.- If your backflow preventer has a blow-out port or drain cap, open it and let the water fully drain.- Leave the test cocks in the open (45-degree) position throughout the winter so any residual moisture can escape and the internal components are not under pressure during freeze-thaw cycles.

- ### Step 3: Connect the Air Compressor Attach your compressor hose to the blowout port downstream of the backflow preventer. Most systems have a dedicated fitting for this purpose. If yours does not, you can connect to the hose bib or a convenient access point downstream of the backflow device. **Never force compressed air through the backflow preventer itself** — this can damage the internal seals and check valves. - ### Step 4: Set Your Compressor to the Correct PSI Adjust the regulator on your compressor to the appropriate pressure for your pipe material (refer to the table above). Start at the lower end of the recommended range and only increase if needed. Ensure the compressor is off or the air line is not pressurized when you make the connection. - ### Step 5: Begin with the Zone Farthest from the Compressor Activate the farthest zone using your irrigation controller or by manually opening the zone valve. Then turn on the compressor and allow air to flow through the zone. Starting with the farthest zone ensures you push water away from the compressor connection point and out through the sprinkler heads. - ### Step 6: Monitor the Sprinkler Heads Watch the heads as the air pushes through. Initially, you will see a strong mist of water. Continue blowing until only a thin, dry vapor or no moisture is visible exiting the heads. This typically takes 2–3 minutes per zone, but never exceed 3–4 minutes of continuous air per zone to avoid overheating components. - ### Step 7: Shut Off the Air and Close the Zone Turn off the compressor first, then close the zone valve. This order prevents a sudden pressure spike in the lines. Allow the compressor tank to recharge fully before moving on to the next zone. - ### Step 8: Repeat for Each Remaining Zone Work your way from the farthest zone back toward the compressor connection point. Repeat the blowout process for every zone in your system. Do not skip any zones — even zones you rarely use can hold enough water to cause damage. - ### Step 9: Blow Out the Main Line After all zones are complete, open the manual drain valve on the main supply line (if equipped) and give it a brief blast of air to clear any remaining water between the shut-off valve and the zone valves. - ### Step 10: Final Steps and Controller Settings Disconnect the air compressor. Close all manual drain valves. Set your irrigation controller to "rain mode" or "off" for the winter — rain mode keeps your programming intact while preventing any scheduled watering. Verify that the backflow preventer test cocks are at a 45-degree open position for the winter.

Common Mistakes to Avoid

Running air through the backflow preventer: Always connect downstream. Compressed air damages the rubber seals and check assemblies inside backflow devices.- Using too much pressure: Exceeding 80 PSI on PVC or 50 PSI on poly pipe can cause invisible micro-cracks that fail during the next season.- Blowing a zone for too long: Running compressed air through dry lines generates heat through friction, which can melt pipe fittings and warp valve diaphragms. Limit each pass to 3 minutes.- Forgetting to drain the backflow preventer: The backflow device is the most expensive and exposed component. Frozen water inside will crack the body and require full replacement.- Skipping the compressor recharge: Blowing a zone with a half-empty tank delivers insufficient volume and leaves water behind.

Frequently Asked Questions

What size air compressor do I need to winterize my sprinkler system?

For most residential irrigation systems, you need a compressor rated at a minimum of 20 CFM at 50 PSI. Small portable compressors (such as 6-gallon pancake models) produce only 2–4 CFM and are inadequate for blowouts. You can rent a suitable compressor from most equipment rental stores for approximately $40–$75 per day. For large properties with rotary heads or systems with more than 8 zones, a compressor producing 50–80 CFM is recommended.

Can I winterize my sprinkler system without an air compressor?

Some systems are designed with manual or automatic drain valves that allow gravity drainage without compressed air. However, the blowout method is far more thorough and is the industry standard in any region where ground temperatures drop below 32°F (0°C). Gravity drains often leave water in low spots, fittings, and valve bodies that can freeze and cause damage. If you live in a climate with hard freezes, the air compressor blowout method is strongly recommended.

When should I winterize my sprinkler system?

Winterize your system before the first hard freeze, which is defined as temperatures at or below 28°F (−2°C) for four or more consecutive hours. In most northern U.S. climates, this means completing the process in October or early November. Check your local frost date forecasts and aim to complete the blowout at least one week before the expected first hard freeze. It is always safer to winterize early rather than risk waiting too long.

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