How to Blow Out Sprinklers

Blowing out your sprinkler system with compressed air is the best way to prepare it for winter. This is a potentially hazardous procedure and one for which you need the right compressor. If you aren't confident that you can do it yourself, don't hesitate to call a service pro, who will likely contract with you to return in the spring and start your system up correctly.

To blow out your pipes properly, you need a compressor that can supply the proper volume of air -- anything less than the proper volume, and water may be left in the pipes to freeze. You can compute the size of the compressor you need if you know the flow rate in gallons per minute of each zone. Just divide that number by 7.5. If you don't know your system's flow rate, just rent a 10 cubic-feet-per-minute compressor -- it can handle every system. Don't rely on your small shop compressor -- it probably doesn't supply enough air for this job.

The compressor should ideally supply a pressure between 40 and 80 pounds per square inch -- do not exceed 80 psi for PVC pipes and 50 psi for polybutylene ones, or the air could blow off the sprinkler nozzles and cause other damage. If your system is connected to a timer system, opening the valves using the timers is recommended because that keeps you out of harm's way. Whether or not you have to open the valves manually, wear eye protection throughout the procedure.

Turn off the water supply. If your system is connected to domestic water, it must have a back-flow preventer, and if so, there is a stop and waste valve between the back-flow device and the water supply. Turn that off, too.

Activate all the valves in the system, using the timer. Finish with the valve that controls the zone farthest from the water supply. If your system includes drain valves, open the one for each zone and let water drain.

Connect the compressor, turn it on and set the pressure regulating valve to 50 psi for PB systems and 80 psi for PVC systems. Open the zone valves nearest the water supply, using the timer, and turn open the compressor valve, gradually increasing pressure until all the sprinkler heads in that zone pop up. Let air flow until water stops spurting from the sprinklers, then close the compressor valve.

Air driving the sprinkler gears quickly heats them up in the absence of water. To prevent damage, do not let air flow for longer than two minutes. If necessary, blow out the system twice, waiting a few minutes after doing it once before doing it again.

Open the valve for the next zone, then close the valve for the zone you just purged. Repeat the blowout procedure for each zone in turn, finishing with the one farthest from the water supply. Leave the valve for that zone open until you shut off the compressor.

If your system has manual valves, the procedure is essentially the same, except for one important difference. Because of the danger of sprinklers popping off or pipes exploding, you should turn off the compressor and wait for air to drain before opening the valve for the next zone and closing the valve for the previous one.

If you have an automatic system, leave it plugged in over the winter and program it to come on once a week to prevent the parts from freezing.