Quick Answer

To prevent costly freeze damage, the most critical step in winterizing your sprinkler system is to ensure all water is completely removed from the irrigation lines. This is best achieved using an air compressor to blow out each zone individually, starting with the highest pressure-rated components. Overlooking even small pockets of water, particularly in low-lying sections of the system, can lead to burst pipes, sprinkler heads, or backflow preventers when temperatures drop below freezing. A systematic approach, coupled with proper safety precautions, will protect your investment.

The Problem

As colder weather approaches, any water remaining in your underground sprinkler system, including the pipes, valves, and sprinkler heads, becomes a ticking time bomb. When water freezes, it expands with immense force, easily rupturing plastic pipes, cracking brass fittings, and destroying delicate internal components of sprinkler heads and backflow preventers. The cost of repairing a freeze-damaged system can range from a few hundred dollars for a single burst pipe to several thousand for extensive damage involving the main lines or the backflow assembly. For many homeowners, the thought of this annual chore is daunting, and often, critical steps are missed or shortcuts are taken, leading to expensive repairs come spring. The core problem isn't just emptying the system; it's ensuring complete emptiness, even in the system's most vulnerable parts.

How It Works

An irrigation system is designed to deliver water efficiently to your lawn and garden. It typically consists of a main water supply line, a backflow prevention device (to stop irrigation water from re-entering your potable water supply), a series of zone valves, buried PVC or polyethylene pipes, and various sprinkler heads or drip emitters. When you activate a zone, water flows from the main supply, through the backflow preventer, through the open zone valve, and out to the designated sprinkler heads. The system is pressurized, meaning water constantly fills the lines up to the zone valves. Gravity plays a significant role in how water drains, or doesn't drain, from the system. Even after shutting off the main water supply and opening drain valves (if your system has them), residual water will invariably remain in the lines, especially in dips, low spots, and within the sprinkler heads themselves. This is where compressed air comes in. By introducing high-volume, low-pressure air into the system, you force this remaining water out through the sprinkler heads, effectively 'drying' the lines. The key is to use enough air volume to push the water, but not so much pressure that you damage components.

Step-by-Step Fix

1. Shut Off Water Supply – Turn off the main water valve to your irrigation system.

Locate the main shut-off valve for your sprinkler system. This is usually near your home's main water meter or where the irrigation line branches off from your house water supply. Turn it completely off. If your system has a separate shut-off valve for the backflow preventer, close that as well. Disconnect the main irrigation supply line from the backflow preventer if your manufacturer recommends it, or if you have a valve specifically for winterizing the backflow preventer. This ensures no new water enters the system during the blowout process.

2. Drain Backflow Preventer – Carefully open the test cocks on your backflow device.

Your backflow preventer (e.g., PVB, RPZ) is particularly susceptible to freeze damage due to its internal components and brass construction. Locate the test cocks (small, usually slotted or square-headed valves) on the backflow preventer. Slowly open these cocks, one at a time, to allow any standing water to drain. Be prepared for a small amount of water to come out. Leave these cocks open at a 45-degree angle or fully open (depending on manufacturer instructions) for the duration of the blowout and through the winter. Some systems have a main drain valve on the supply side of the backflow preventer; open this if available. Safety Note: If your backflow preventer is connected with unions, you might also consider slightly loosening them, but only after all pressure has been released. Consult your specific backflow preventer's manual.

3. Connect Air Compressor – Attach your air compressor to the main irrigation line.

Locate the compressor connection point, usually a quick-connect fitting or a threaded cap, on the main line past the backflow preventer (on the irrigation system side). Connect your air compressor hose to this fitting. Ensure the connection is secure. Set your air compressor regulator to 30-50 PSI for PVC systems and 50-80 PSI for polyethylene (black flexible pipe) systems. Never exceed 80 PSI for any irrigation system. Start with a lower PSI and increase only if absolutely necessary to clear the lines. Use an air compressor with a CFM (Cubic Feet per Minute) rating of at least 5-10 CFM for smaller systems, and 10-20 CFM for larger systems with more zones or longer runs. A higher CFM ensures sufficient volume to push water effectively.

4. Open Furthest Zone Valve – Manually open the irrigation zone valve furthest from the compressor connection.

Begin the blowout process with the zone valve located furthest from your compressor connection. This allows the air to push water through the longest run of pipe first. Manually open this zone valve at the controller. If your system allows for manual operation at each individual valve, open it there. Do not open more than one zone at a time during the blowout. Activating multiple zones simultaneously can reduce air pressure and prevent thorough water removal.

5. Blow Out the Zone – Gradually introduce compressed air until only mist emerges from sprinkler heads.

Slowly open the air compressor's main valve to introduce air into the system. Watch the sprinkler heads in the activated zone. They will pop up, and water will begin to spray out, followed by a mixture of water and air, and eventually, a fine mist or just air. Allow the compressor to run for 1-2 minutes after only mist is visible from all heads in that zone. It’s crucial that you see mist from every head, indicating the line is clear. Repeat this process 2-3 times per zone, allowing the compressor's tank to refill completely between cycles. This on-and-off cycling helps to create surges of air that can dislodge stubborn water pockets. Safety Note: Stand clear of sprinkler heads during the blowout; debris can be ejected.

6. Repeat for All Zones – Move systematically through each remaining zone, blowing them out.

Once the furthest zone is completely dry, close its valve. Then move to the next furthest zone and repeat step 5. Continue this methodical process until every irrigation zone has been blown out thoroughly. Always ensure only one zone is open at any given time. Pay extra attention to any zones that have extremely low-lying heads or significant elevation changes, as water can easily collect there. If you have drip irrigation lines, treat them as a separate zone and ensure they are also cleared of water.

7. Disconnect Compressor and Relieve Pressure – Shut off air, disconnect hose, and release any residual system pressure.

After all zones are dry, turn off your air compressor and disconnect the hose from the irrigation system's connection point. Before completely packing up, ensure any remaining air pressure in the irrigation line is released. You can do this by momentarily opening one of the zone valves or by slowly loosening the compressor connection point. This prevents any trapped air from potentially causing issues.

8. Open System Drain Valves (If Present) – Open any manual drain valves at low points in your system.

Some older or custom-installed sprinkler systems may have manual drain valves located at the lowest points of the main line or specific zone lines. If your system includes these, open them now, if you haven't already. While the air blowout is the primary method, these drains can help ensure any final drips of water escape.

9. Turn Off Controller – Switch off your irrigation controller.

Finally, turn your irrigation controller to the

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