I arrived at a property in late October. The system had never been blown out in fifteen years. The homeowner said he drained the pipes manually every fall. He just opened the drain valves. He let gravity do the work. I explained that gravity drainage leaves standing water. It stays in low spots, lateral lines, and head risers. I connected my air compressor. I set the regulator to 50 PSI. I blew each zone for three minutes. Ice fragments shot out of the farthest head on the third zone. Those were solid frozen chunks. They would have cracked that head. They would have cracked the PVC pipe behind it. It would have happened within the first hard freeze. That homeowner has scheduled an annual blowout every October since that day. I have winterized over five hundred systems. They are across freeze zones from Zone 6 through Zone 8. I can tell you that a proper blowout is the most important task. It matters for any irrigation system in a freezing climate.
When should I blow out my sprinkler system for winter?
I schedule sprinkler blowouts between two and four weeks before the first expected hard freeze. For Zone 6 that means mid-October. For Zone 7 it is late October. For Zone 8 it is mid-November. I check the 10-day forecast. I look for overnight temperatures below 28 degrees Fahrenheit.
I do not wait until the first freeze warning appears on the news. By then, the soil temperature has dropped enough. Standing water in shallow lateral pipes turns into ice. Ice expanding inside a 3/4-inch PVC pipe creates roughly 2,000 PSI of force. That is ten times the normal operating pressure. That force cracks pipe joints. It also splits the pipe wall. I track the historical freeze date for each property’s zip code. I use National Weather Service data. The average first freeze hits November 1 in some areas. I schedule blowouts between October 15 and October 25. This window leaves time for unexpected early cold snaps. The soil is still warm enough. The ground has not frozen solid. I also winterize any above-ground backflow preventers. I wrap them in foam insulation tape. I cover them with an insulated plastic bag. I secure the bag with zip ties. Backflow preventers have brass parts. Those parts crack when trapped water freezes and expands. A cracked backflow preventer costs $80 to $200 to replace. I explain freeze protection and system care in my sprinkler system components guide. That guide covers pipe materials and freeze tolerance.
What size air compressor do I need to blow out sprinkler lines?
I recommend a tow-behind air compressor. It should deliver 10 to 15 CFM at 50 PSI. This works for residential systems with up to 8 zones. Smaller pancake compressors are rated 2 to 5 CFM. They do not move enough air volume. They cannot push water out of long lateral runs over 150 feet.
The key spec for sprinkler blowouts is cubic feet per minute. It is not the maximum PSI. High pressure without enough volume produces a thin air stream. It moves very fast. It vaporizes water instead of pushing it as a liquid column. I set my compressor regulator to 50 PSI maximum. I never go above 80 PSI even if the compressor can deliver more. Pressures over 80 PSI blow sprinkler heads off their risers. They also turn water into mist. That mist stays inside the pipe. I connect the compressor to the system through a quick-connect blowout fitting. It is installed at the main shutoff valve. It can also go at a boiler drain spigot near the backflow preventer. I use a 3/4-inch brass blowout fitting. It has a male hose thread. It threads directly onto the spigot. I connect the air hose from the compressor to the fitting. I always install a shutoff valve between the compressor and the irrigation system. A ball valve rated for 150 PSI lets me control air flow. I open the ball valve slowly. I do not snap it open. This prevents a sudden pressure surge. Surges can damage the valve diaphragms. I discuss pressure settings and equipment choices in my irrigation valve repair guide. I explain how air pressure affects valve parts during winterization.
How do I blow out each irrigation zone step by step?
I blow out each zone by first shutting off the water supply at the main valve. I open the manual bleed screw on one valve. This relieves system pressure. Then I close the bleed screw. I activate the first zone from the controller. Then I connect compressed air to the blowout fitting.
I select Zone 1 on the controller. I start the irrigation cycle for that zone. This opens the solenoid valve. It allows water to flow through the zone pipes. I connect the air compressor hose to the blowout fitting. I slowly open the ball valve on the air line. Air enters the system. It pushes water out through the zone’s sprinkler heads. I watch the heads for the change from water spray to mist to clear air. This transition takes 1 to 3 minutes per zone. It depends on pipe length and diameter. Once clear air comes out of all heads on the zone, I close the ball valve. I repeat the process for Zone 2. I blow zones one after the other from the controller. I do not disconnect the air hose between zones. I pay close attention to the farthest head on each zone. Air might flow from the closest heads. But the farthest head might still spray water. That means the pipe has a low spot. Water pools there. It needs an extra 30 to 60 seconds of air flow to clear. I blow each zone for at least 2 minutes. That is true even if the heads clear after 30 seconds. Trapped water in lateral branches takes longer to drain. After I finish all zones, I open the drain valves at the main shutoff. I also open any low-point drains. This lets any leftover condensation drain by gravity. I leave all zone valves open for 24 hours after blowout. This lets remaining moisture evaporate. Then I close the valve box. I provide the full seasonal maintenance schedule in my lawn irrigation 101 guide. That guide covers year-round system care.
Can I winterize my sprinkler system without an air compressor?
I do not recommend winterizing without an air compressor. This applies if ground freezing reaches below 6 inches. Gravity drainage through manual drain valves leaves 10 to 20 percent of the water trapped. It stays in low spots, lateral head branches, and valve bodies. That trapped water expands into ice. It damages components.
Manual drainage works only in special systems. They must have automatic drain valves at every low point. The system must have perfect slope from the highest head to the lowest drain. There must be zero dips or sags in the pipe runs. I have inspected dozens of systems that the homeowner said were drained. I still found water leaking from a cracked head riser in January. If no air compressor is available, I drain every manual drain valve. I remove the solenoid from each valve. This lets the valve body drain. I tilt each sprinkler head downward. This helps water run out of the stem. I also remove the drain cap at the bottom of the backflow preventer. That lets that chamber empty completely. I place bags of silica gel desiccant inside each valve box. It absorbs moisture that forms during winter temperature swings. This method without a compressor lowers the risk. But it does not remove it. I still recommend hiring a pro with a proper compressor. Do a full blowout at least every other year. Systems with drip irrigation zones need special care. Drip tubing traps water in every emitter location. It does not drain well under gravity. I cover freeze protection for drip systems in my drip irrigation types guide. I explain winter tips for different emitter styles.
References
Irrigation Association — Freeze Protection Resources
University of Minnesota Extension — Winterizing Irrigation Systems