The use of compressed air to flush a sprinkler system, commonly called a “blow-out,” is the most reliable method for winterizing irrigation lines. This process involves forcing all water from the underground pipes and components before freezing temperatures arrive. Water expands by about nine percent when it solidifies into ice, generating significant pressure that can easily crack pipes, valves, and sprinkler heads. Removing the residual water eliminates the primary cause of freeze damage, protecting the entire irrigation system from costly repairs and ensuring proper function when reactivated in the spring.
Pre-Flushing System Preparation
Before connecting the air compressor, the water supply to the irrigation system must be completely isolated from the main household line. Locating the primary shut-off valve, often found in a basement, utility room, or outside before the backflow preventer, is the first step. Turning off this valve protects the sprinkler system from a continuous flow of water during the blow-out process.
If the system includes a backflow prevention device, its isolation valves should also be closed to protect the internal components from the initial surge of compressed air. After securing the main water supply, relieve the built-up pressure in the mainline, typically by briefly opening a hose bib or a drain valve located downstream of the shut-off. This preliminary draining reduces the volume of water the air compressor must displace.
Gathering the correct equipment prevents interruptions and ensures safety. You will need an air compressor equipped with a pressure regulator, a specialized blow-out adapter to connect the air hose to the system’s port, and safety gear. The adapter usually connects to a dedicated blow-out port near the backflow device or directly to a drain valve.
Connecting the Compressor and Clearing Zones
With the system prepared, attach the air compressor hose to the blow-out connection using the specialized adapter. Clearing the zones must be managed through the system’s controller, operating only one section at a time. Activating individual zones ensures the air velocity is adequate to push the water out effectively.
The flushing process should begin with the zone located farthest from the air compressor connection point. This strategy ensures the longest pipe runs are cleared first, minimizing the chance of water settling in low spots. Once the furthest zone is active, open the compressor valve slowly and gradually introduce air into the system.
The compressed air forces water out of the sprinkler heads. Allow the air to run until the spray transitions from a heavy stream to a fine mist or fog. Running the zone for short periods (one to two minutes) is better than a continuous blast, which can cause excessive heat and friction within the pipes. Advance the controller to the next zone and repeat the process systematically until every zone is thoroughly cleared of water.
Critical Air Pressure and Safety Guidelines
Controlling the air pressure is the most important safety factor, as excessive force can cause catastrophic damage to the system. The pressure setting must never exceed the maximum rating of the system’s piping; therefore, a compressor with an adjustable pressure regulator is mandatory. For most residential systems using rigid PVC piping, the pressure should not exceed 80 PSI. For more flexible polyethylene piping, the limit is often lower, around 50 PSI.
The goal is to use a large volume of air, measured in cubic feet per minute (CFM), at a low, controlled pressure to push the water, rather than using high pressure to blast it. High pressure can ruin internal components like sealing gaskets or cause sprinkler heads to fly off. Always wear safety glasses and avoid standing directly over any irrigation components during the operation, as flying debris or components under pressure pose an injury risk.
Post-Flush System Shutdown
After all zones have been flushed and only a fine mist is visible, safely shut down and secure the system for the winter. Turn off the air compressor and disconnect it from the blow-out port. Before disconnecting the air hose, leave a zone valve open to allow any remaining air pressure to safely dissipate from the mainline.
The system’s controller should be manually deactivated or set to an “off” or “rain mode.” This prevents the system from running during the winter and ensures the dry valves do not cycle. Any residual water that may have settled in the backflow preventer must be drained by carefully opening its test cocks, often requiring a flat-head screwdriver.
The mainline isolation valve should remain closed for the entire winter. In climates with severe freezing, exposed above-ground components like the backflow preventer may need insulation or removal, depending on local codes. A final inspection confirms all valves are secured and the system is protected against the upcoming cold.