Winterizing an irrigation system is necessary in cold climates to prevent extensive damage caused by freezing water. The “blow out” method uses compressed air to force out residual water that remains in the underground pipes after the main supply is shut off. Even small amounts of trapped water can exert enough pressure to rupture pipes, valves, and sprinkler heads when frozen. This guide offers a detailed overview for homeowners preparing their systems for the winter season.
Essential Equipment and Safety Precautions
The correct air compressor is the most important piece of equipment for safely and effectively clearing the lines. Air volume, measured in Cubic Feet per Minute (CFM), is more important than pressure (PSI), because volume moves the water out of the system. A small residential system typically requires a compressor capable of delivering at least 20 CFM. The needed CFM can be estimated by dividing the Gallons Per Minute (GPM) of the largest zone by 7.5.
Safety is the highest priority when working with high-pressure air, which can cause severe injury or system failure if misused. Wear safety glasses or goggles to protect your eyes from debris or pressurized water that may eject from the sprinkler heads. The air compressor must be equipped with a functional pressure regulator to manage the output into the system.
The maximum pressure allowed depends on the pipe material, and exceeding these limits can shatter components. PVC pipe can generally handle up to 80 PSI, but polyethylene (poly) pipe should not exceed 50 PSI. To minimize risk to internal sprinkler components, regulate the compressor to an initial pressure between 30 and 50 PSI. Never stand directly over any irrigation component while the air is flowing, as components could fail or eject under pressure.
Step-by-Step Procedure for Clearing the Lines
The procedure begins by completely shutting off the main water supply valve that feeds the irrigation system, often located near the meter or in a basement. Once the water is off, open any manual drain valves or briefly run a zone from the controller to release the initial static pressure from the main line.
Connect the air compressor hose to the designated “blow out” port, which is typically a quick-connect fitting located downstream of the backflow preventer. The compressor’s regulator should already be set to the maximum safe PSI for your system, but start the air flow at the lower end of the recommended 30–50 PSI range.
Activate the first irrigation zone using the system controller. Start with the zone that is highest in elevation or furthest from the compressor connection point, as this usually requires the most pressure to clear. Allow the compressed air to flow for one to two minutes until only a fine mist or clear air is exiting the sprinkler heads.
After the first zone is clear, advance the controller to the next zone and repeat the process. Do not run the air in any single zone for more than a few minutes continuously, as the friction of compressed air can generate heat and damage pipe materials. Once all zones have been individually cleared, repeat the entire cycle two or three times. Allow the compressor tank to fully refill between each zone to maintain consistent volume and pressure. This cyclical process ensures the removal of any residual water that may have pooled in low spots.
Finalizing the System and Troubleshooting
Once the final cycle is complete and only air is escaping from the heads, release any remaining pressure in the main line before disconnecting the air compressor. This is typically done by opening a bleed valve or the isolation valve near the compressor connection point.
The backflow preventer, if present, requires specific attention to prevent freezing damage. After the main supply is off, open the test cocks on the backflow device to drain any remaining water from its chambers. Then, position the ball valves on either side of the backflow preventer at a 45-degree angle. This allows for moisture expansion without cracking the valve body.
If a zone fails to clear and continues to spray significant water after two minutes, it may indicate a blockage or a failure in the zone valve itself. Persistent water flow from the main line connection suggests a leak at the main shutoff valve, meaning the water supply is not fully isolated. The main water supply must be verified as completely off before proceeding, or a professional should be consulted.