How To Winterize A Sprinkler System With Backflow Preventer: A Professional Guide

Winterizing an irrigation system with a backflow preventer requires a systematic evacuation of all pressurized water from the piping, valves, and mechanical assembly to prevent catastrophic freeze damage. The process hinges on shutting off the main water supply, manually draining the backflow assembly, and utilizing a high-volume air compressor to purge lateral lines until only a light mist remains.

Foundational Requirements and Equipment Specifications

Successful winterization is predicated on total water removal. Any residual moisture trapped in the copper or brass housing of a backflow preventer can expand by approximately 9% upon freezing, leading to cracked bonnet assemblies, ruined internal poppets, and expensive component replacement. Before beginning, ensure the site is prepped with the necessary hardware.

• Essential Equipment:
  
  
  • High-volume air compressor (minimum 10-20 CFM at 50-80 PSI for residential systems).
  • Quick-connect air hose adapter (fitting the specific blowout port on your system).
  • Adjustable crescent wrench and flathead screwdriver.
  • Thermal insulation tape or foam covers for external pipe segments.
  • Safety goggles to protect against debris ejected during the blowout.
• Prerequisite Standards:
  
  
  • The main irrigation shut-off valve must be located inside the structure, typically in a basement or crawlspace, to remain protected from the frost line.
  • Compressed air pressure must never exceed the operating PSI of the PVC lateral lines, typically capped at 80 PSI to avoid joint rupture.
  • Professional systems often require two separate blowouts: one for the backflow device and one for the lateral zones.

The Systematic Blowout and Drainage Procedure

Step 1: Isolation and Main Water Shut-off

Locate the main irrigation shut-off valve inside your home or at the point of connection. Rotate the handle to the perpendicular closed position. Once the supply is terminated, return to the irrigation controller and activate a zone to release residual pressure remaining in the lines. If the system has an internal drain valve located between the indoor shut-off and the outdoor backflow device, open this valve now to gravity-drain the vertical riser pipe.

Step 2: Draining the Backflow Preventer Assembly

The backflow preventer, usually situated above ground, is the most vulnerable component. Locate the test cocks on the side of the device—these are the small, slotted screw heads. Use a flathead screwdriver to turn these cocks to a 45-degree angle (half-open). If the device features ball valves, rotate these to a 45-degree position as well. This prevents trapped water from being "bottled up" between the ball valve and the internal components. Leave these cocks in the 45-degree position throughout the winter to ensure any condensation can escape.

Step 3: Integrating the Air Compressor

Connect your air compressor hose to the blowout port, which is generally located downstream of the backflow preventer. Ensure the connection is secure. If you attempt to blow air through the backflow device itself, you risk damaging the internal seals; always introduce air into the irrigation side of the system, not the potable water supply side.

Warning: Never turn on the compressor before the hose is connected, and never exceed 80 PSI. High-pressure air can cause plastic PVC pipes to shatter, turning them into hazardous projectiles.

Step 4: Sequential Zone Purging

Begin with the zone farthest from the blowout port. Set your irrigation controller to manual operation for that specific zone. Start the compressor and slowly introduce air. You should observe water spraying from the sprinkler heads. Continue until the spray transitions into a light, airy mist with no visible water droplets. Do not exceed two minutes of continuous air flow per zone to prevent friction-based heat buildup in the piping. Repeat this for every zone, moving from the most distant to the closest.

Step 5: Final System Decommissioning

Once all zones have been cleared, turn off the compressor, disconnect the hose, and close the blowout port. Return the sprinkler controller to the "Off" or "Rain" setting. Wrap the entire backflow assembly in insulating foam or a thermal blanket if local temperatures regularly drop below 20 degrees Fahrenheit for extended periods.

Irrigation Component Comparison and Operational Limits

Common Failure Scenarios and Field Corrections

• Incomplete Drainage: If water remains in the low points of the yard, it indicates the air volume was insufficient.
  
  
  • Root Cause: Compressor CFM too low.
  • Actionable Fix: Rent a professional-grade tow-behind compressor with at least 50+ CFM to properly displace water in long runs.
• Leaking Backflow Test Cocks: The cocks may drip after winterization, causing ice buildup.
  
  
  • Root Cause: Worn O-rings or seals within the test cock.
  • Actionable Fix: Replace the test cock screws or, if the leak persists, the entire backflow assembly housing.
• Sprinkler Head "Pop-ups" Remain Up: Heads stay elevated after the blowout, risking damage from lawnmowers or foot traffic.
  
  
  • Root Cause: Vacuum seal preventing the spring from retracting the head.
  • Actionable Fix: Manually depress the head once the system is fully depressurized to break the vacuum seal and allow the spring to retract.

Frequently Asked Questions

Is it safe to use a small tire inflator to winterize my sprinkler system?

No. Tire inflators lack the necessary cubic feet per minute (CFM) to move a column of water through underground piping. Using a low-volume device will leave water standing in the lines, which will freeze and potentially rupture PVC fittings.

Do I need to remove the backflow preventer entirely?

In regions with extreme, sustained sub-zero temperatures, many professionals recommend removing the backflow preventer and storing it indoors. However, for most moderate climates, simply draining the device and leaving the valves at a 45-degree angle is sufficient.

How do I know if I have successfully cleared all the water?

You have successfully cleared the system when the sprinkler heads emit a fine, dry mist rather than water droplets. If you see water "spitting" or hear heavy gurgling, continue the air cycle until the output is consistent, dry air.

Should I leave the irrigation controller plugged in during winter?

It is standard practice to turn the irrigation controller to the "Off" or "Winter" position. While leaving it plugged in is safe, removing the backup battery is recommended to prevent battery acid leakage within the unit.

Protect your landscape investment by ensuring your irrigation infrastructure is fully purged before the first hard frost. If you encounter complex plumbing configurations, consult a local certified irrigation technician to verify your system integrity.