Quick Answer

Many homeowners attempt to winterize their sprinkler systems by simply draining the main lines, but often overlook the crucial step of properly clearing all water from lateral lines and sprinkler heads. This oversight is the single biggest cause of freeze damage, leading to cracked PVC, shattered manifold valves, and broken sprinkler heads that can cost hundreds or even thousands of dollars to repair. The correct method involves using an air compressor to blow out the system, ensuring every drop of water is expelled before freezing temperatures arrive.

The Problem

As temperatures plummet below 32°F (0°C), water trapped within your underground sprinkler system expands when it freezes. This expansion creates immense pressure—far more than any PVC pipe or brass valve is designed to withstand. The result? Burst pipes, fractured manifold valves, damaged backflow preventers, and broken sprinkler heads. These issues often remain hidden until spring, when you reactivate your system only to find geysers erupting from your lawn and a massive water bill. The cost of repairing a damaged irrigation system due to improper winterization can easily range from a few hundred dollars for a minor repair to several thousand for a complete system overhaul, not including potential landscape damage from extensive digging.

How It Works

An automatic sprinkler system consists of several key components: a main water supply line, a backflow preventer to prevent irrigation water from re-entering your potable water supply, a manifold of zone valves that control water flow to different areas, and a network of lateral lines leading to individual sprinkler heads. Water is pushed through this system by your home's water pressure. When winterizing, the goal is to remove all water from every part of this system. While some systems have manual drain valves, these often only clear the main lines and some of the lower lateral lines—they rarely clear the water from higher elevation pipes or the sprinkler heads themselves, especially in systems with varying terrain. Furthermore, residual water can cling to pipe walls, even in drained sections. The blow-out method, using an air compressor, forces compressed air through the system, creating a turbulent flow that pushes nearly all remaining water out through the sprinkler heads. This ensures even the smallest pockets of water are eliminated, protecting the entire system from freeze damage.

Step-by-Step Fix

Safety Note: Always wear eye protection when working with compressed air. Never stand directly over a sprinkler head during the blowout process. Ensure the maximum pressure of your air compressor does not exceed 80 PSI for PVC pipes or 50 PSI for poly pipes, as excessive pressure can damage components.

1. Shut Off Water Supply – Locate and close the main shut-off valve for your sprinkler system. This valve is typically found near your water meter or where the sprinkler line branches off from your main home water supply. Often, there's a separate ball valve specifically for the irrigation system. Do not skip this step, as failing to do so can cause water to continue flowing into the system during the blowout.
   • If you can't find the valve: Consult your home's original blueprints or call your irrigation system installer.
2. Drain the Main Line – Open any manual drain valves located on the main supply line or backflow preventer. Allow as much water as possible to exit by gravity. Some backflow preventers (e.g., Pressure Vacuum Breaker (PVB) or Reduced Pressure Zone (RPZ) devices) have small test cocks or ball valves that can be partially opened to aid in drainage.
   • Tools needed: Adjustable wrench or screwdriver for test cocks.
3. Bypass the Backflow Preventer (If Applicable) – Consult your backflow preventer's manual to determine if it requires specific winterization steps or if it can be bypassed for the blowout. Some backflow devices are designed to be removed and stored indoors, while others are rated for freeze protection themselves (though blowout is still recommended for system lines). If you have an anti-siphon valve, ensure it is open.
   • Important: Never apply compressed air directly through a backflow preventer designed for static water pressure; connect your air compressor union after it.
4. Connect the Air Compressor – Locate the compressor connection point, which is usually a dedicated plug on the main line downstream of the backflow preventer, or sometimes a hose bib. Use the appropriate adapter (typically a quick-connect or threaded fitting) to firmly attach your air compressor hose. Ensure a tight seal to prevent air leakage.
   • Tools needed: Air compressor (minimum 5 CFM at 90 PSI for most residential systems), air hose, quick-connect fitting or threaded adapter matching your system's connection point.
   • Set pressure: Start your compressor and set the regulator to a low pressure, around 20-30 PSI. You will gradually increase it, but never exceed 50-80 PSI.
5. Open the Farthest Zone Valve – Activate the zone valve for the sprinkler circuit that is physically farthest from your compressor connection. You can do this manually at the valve manifold or by using your irrigation controller in manual mode. Opening the farthest zone first helps push water through the maximum length of pipe.
   • Tip: If your controller doesn't have a manual zone activation, you may need to open valves physically.
6. Blow Out the First Zone – Allow compressed air to flow into the opened zone. You will see water and mist blowing out of the sprinkler heads. Let the air run until only mist or air emerges, indicating most of the water has been expelled. This typically takes 1-3 minutes per zone, depending on its length and the compressor's output.
   • Caution: Do not run the compressor for too long on a dry line, as it can generate heat and damage plastic components. Once you see only air, move to the next step.
7. Continue with Remaining Zones – Close the first zone valve and open the next zone in sequence (working your way back towards the compressor). Repeat the blowout process until each zone has been cleared of water. Ensure you cycle through all zones, including any drip irrigation lines or specialty zones.
   • If a zone seems stubborn: Briefly close and then re-open the zone valve to create a surge of air that might dislodge remaining water.
8. Clear the Backflow Preventer – Carefully open the test cocks or ball valves on your backflow preventer after all zones are blown out. Allow any residual water in the backflow device to drain. Depending on the type, you may need specific instructions for this component. Leave these valves open at a 45-degree angle for winter, or according to manufacturer instructions. For PVB and RPZ devices, rotate the handles to a 45-degree open/closed position to relieve pressure and allow any residual water to drain.
9. Disconnect and Protect – Turn off your air compressor and disconnect the air hose. Replace the cap on your compressor connection point. Ensure all manual drain valves and backflow preventer test cocks are left open to the atmosphere (or in the recommended winter position) to prevent pressure buildup from any remaining moisture or expansion.
   • Final step: Turn off your irrigation controller completely or set it to

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