How to Winterize a Sprinkler System

A sprinkler head watering the lawn.
What You'll Need
Eye protection
Sprinkler system manufacturer's guide
Air compressor
Metal fitting for compressor air line
Stop and waste valve key (if needed)
Insulated blanketing

Winterizing a sprinkler system requires a homeowner to know the type of system installed. Proper care and maintenance in the fall prevents heartache over cracked pipes, ruined seals, and broken valves in the spring. Damage to an underground irrigation system is costly to fix and may require a large portion of the yard to be dug out to locate the problem.

Most systems have a built-in manual or automatic drain valve that uses gravity to move water from the pipes. Yet the thin tubing used in irrigation systems has a tendency to shift and sink over time. This movement allows water to become pooled within the pipes. Any water left behind has the potential to destroy the plastic and metal within the system as it expands in freezing temperatures.

Winterizing is fairly easy to learn and most do-it-yourselfers already own the necessary tools for the job. To completely remove water from the system, a purge must be completed using compressed air. Follow these steps to "blow out" an underground sprinkler system.

Step 1 - Shut Off the Water Supply

All underground sprinkler systems have a main shut off point. For safety, it is usually located in a basement, a crawlspace, or buried underground in a valve box. Valve boxes are buried up to 5 feet underground and may require a long key to shut off. If the original key cannot be located, contact the contractor that installed the system or the manufacturer to obtain a replacement. The shut-off valve should turn easily — do not force it. Simply turn the valve to the "off" position. If the system operates with a pump, this will need to be drained, disconnected, and stored indoors for the winter. If it is not a removable variety, wrap it in insulated blanketing for the winter. This can be purchased at the hardware store and is similar to insulation used for hot water heaters.

Step 2 - Purge the Backflow Device

Sprinkler heads in a lawn.

Systems that run from community water usually have one of two kinds of backflow devices: an Atmospheric Vacuum Breaker (AVB) or a Pressure Vacuum Breaker (PVB). Sometimes a PVB is also called a Reduced Pressure Device. If the system is supplied with privately owned water, check the installation plan. If no backflow device was installed, then the homeowner can skip this step.

AVBs are installed below each valve and are purged with the system. PVBs should not be purged with compressed air because the heat may cause them to soften and melt. After the system is purged, return to the PVB and rotate the ball valves and test cocks back and forth several times. Then, leave them in a half-open position to prevent moisture from building up on either side of the valves. Draining a Reduced Pressure Device is much more complicated, and a homeowner should refer to their manufacturers' instructions to find the correct procedure for their device.

Step 3 - Drain the System

Compressed air is a great way to ensure all of the water is removed from the pipes and valves in the downstream, a portion of the system. Both pressure and air volume must reach a critical point to completely purge the system. A range of 40-80 pounds per square inch (psi) is ideal, with 80 psi being the maximum for rigid PVC tubing and 50 psi for a polyethylene pipe. To find the appropriate volume, use this simple formula: gallons per minute (GPM) divided by 7.5 equals the cubic feet per minute of compressed air necessary.

Most sprinkler systems are given a GPM rating per zone at the time of installation. Since each system has a unique design, the GPM is marked on the original design plan. If needed, this can also be calculated based on the information provided in the manufacturer's instructions or on their website. Essentially, the zone GPM is calculated by adding up the GPM for each nozzle within the zone. Find the GPM rating for the different nozzle models on the manufacturers' website.

Use a compressor providing the proper CFM of air within the correct psi range. If a smaller compressor is used it will take much longer for a complete purge and will overwork the compressor. An oversized compressor can still be used as long as it is fitted with a regulator, without an air pressure regulator the system will "blow out" too fast causing friction as the hot air passes through dry pipes. This will result in melted pipes and blown valves — a costly mistake.

The backflow device must have a blowout fitting downstream. If not, compressed air cannot be used to purge the system, but one can be added any time after installation. While a metal fitting for the air compressor line is not required, it will reduce the amount of friction and heat passed into the system and will prevent unnecessary damage to the system.

Attach the compressor line to the blowout valve. Be sure the isolation ball cock downstream of the backflow device is closed. During this step, you should wear eyewear protection. Any loose fittings or nozzles may pop off unexpectedly and can cause injury. Turn on one or more of the zones at the irrigation controller and set the compressor to the correct psi and start it up.

Use two short cycles of compressed air to completely blow out each zone. Once a zone is blown out do not run it through again. This will cause undue heat to build up and may cause damage to the pipes underground that will not be visible until the system is turned on in the spring.

Step 4 - Program the Controller

Four sprinkler heads in a row.

After disconnecting the air compressor, make sure to turn all valves on the backflow prevention device to the half-open position for the winter. Also, leave the controller plugged in since the small amount of heat it gives off will prevent condensation from building up inside the device. Program the controller to run at a minimum cycle once a week to keep the solenoid plungers from becoming stuck in one position.

In the spring, remember to turn all the test cocks and ball valves to the closed position again before pressurizing the system. When the water supply is re-established, slowly reopen the ball valves to prevent damage.

Protect Your Pets

Domestic animals, both dogs and cats, are especially sensitive to sounds outside the range of human hearing. The good news is you will know that the purging process is working if the neighborhood dogs begin howling. The bad news is any pet in the yard will try to run in order to get away from the painful sound. Remember to protect your pet's sensitive ears by keeping them inside for this project or even sending them to doggie daycare for the day.