A late-October inspection can reveal a system that has been manually drained for years but never properly blown out. Manual draining alone often leaves water trapped in low spots, lateral lines, and head risers.
He just opened the drain valves. He let gravity do the work. Gravity drainage can still leave standing water in low spots and lateral lines.
Water stays in low spots, lateral lines, and head risers. Connect the air compressor and set the regulator to 50 PSI.
Run each zone for roughly three minutes. If solid ice fragments or water continue to exit the farthest head, repeat a short cycle after a pause rather than raising pressure.
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. They are across freeze zones from Zone 6 through Zone 8. It matters for any irrigation system in a freezing climate.
When should I blow out my sprinkler system for winter?
Schedule sprinkler blowouts two to four weeks before the first expected hard freeze. For USDA Zone 6 that often means mid-October; for Zone 7, late October is common.
For Zone 8, mid-November is often appropriate. Check the 10-day forecast for overnight temperatures below 28°F.
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.
Use National Weather Service data and the property ZIP code to check the historical freeze date. In some areas the average first freeze arrives around November 1.
In many cold-winter regions, a blowout window from October 15 to October 25 leaves time for an unexpected early cold snap while the soil is still workable.
The ground has not frozen solid yet. Winterize any above-ground backflow preventer as part of the same shutdown, using foam insulation only after its body is drained.
Cover the drained assembly with an insulated plastic bag and secure it with zip ties. Backflow preventers contain brass parts that still need protection from direct weather exposure.
Those parts crack when trapped water freezes and expands. A cracked backflow preventer costs $80 to $200 to replace. support/sprinkler-systems-complete-guide/). That guide covers pipe materials and freeze tolerance.
What size air compressor do I need to blow out sprinkler lines?
Consider 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.
Compressed air moves very quickly and vaporizes water rather than pushing it as a liquid column. Set the compressor regulator to a maximum of 50 PSI for a typical residential sprinkler system.
Do not exceed 80 PSI even if the compressor can deliver more. Higher pressure can blow sprinkler heads off their risers and turn remaining water into mist.
That mist can remain inside the pipe. Connect the compressor through a quick-connect blowout fitting installed at the main shutoff valve.
It can also go at a boiler drain spigot near the backflow preventer. Use a 3/4-inch brass blowout fitting. It has a male hose thread.
The fitting threads directly onto the spigot. Connect the compressor air hose to it and keep a shutoff valve between the compressor and the irrigation system.
A ball valve rated for 150 PSI provides air-flow control. Open it slowly to prevent a sudden pressure surge.
Surges can damage the valve diaphragms. The related guide explains pressure settings and equipment choices in the related irrigation valve repair guide. The related guide explains how air pressure affects valve parts during winterization.
How do I blow out each irrigation zone step by step?
For each zone, first shut off the water supply at the main valve and open the manual bleed screw on one valve to relieve system pressure.
Close the bleed screw, activate the first zone from the controller, and then connect compressed air to the blowout fitting.
Select Zone 1 on the controller and start its irrigation cycle. This opens the solenoid valve.
The open valve allows air to move through the zone pipes. Connect the compressor hose to the blowout fitting and slowly open the ball valve on the air line.
Air enters the system. It pushes water out through the zone’s sprinkler heads. 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.
Repeat the process for Zone 2 and then each remaining zone from the controller. Watch the farthest head on every zone for continuing water discharge.
Air might flow from the closest heads. But the farthest head might still spray water. That means the pipe has a low spot.
Water often pools there and may need an extra 30 to 60 seconds of air flow to clear. Run each zone for at least two minutes.
That is true even if the heads clear after 30 seconds. Trapped water in lateral branches takes longer to drain.
Open any low-point drains so remaining condensation can drain by gravity. Leave zone valves open for about 24 hours after the blowout.
This lets remaining moisture evaporate. support/lawn-irrigation-101/). That guide covers year-round system care.
Can I winterize my sprinkler system 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.
Pipe runs should have no dips or sags that trap water; a cracked head riser may otherwise show up only after a January freeze. If no compressor is available, drain every manual drain valve.
Remove the solenoid from each valve so the valve body can drain. Tilt each sprinkler head downward.
This helps water run out of the stem. Remove the drain cap at the bottom of the backflow preventer so its chamber can empty completely.
Place silica-gel desiccant bags inside each valve box to absorb moisture from winter temperature swings. This no-compressor method lowers risk but is not equivalent to a full blowout.
It does not remove all trapped water. When a proper compressor is unavailable, use a qualified irrigation professional for 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.
The related guide covers freeze protection for drip systems in the related drip irrigation types guide. The related guide explains winter tips for different emitter styles.
References
- EPA WaterSense (U.S. Environmental Protection Agency)
- Irrigation Association (Irrigation Association)
- Penn State Extension – Drip Irrigation (Penn State University)