Home Sprinkler Systems:Preparing Your Sprinkler System for Winter

Home Sprinkler Systems: Preparing Your Sprinkler System for Winter by M. Higgins and C. Swift 1 (1/09) Quick Facts.... Prepare your sprinkler system for winter by expelling all the water from the irrigation system and equipment. Do not trust manual or automatic drain valves. The system should be blown out with pressurized air. To determine the best sized compressor for your system, know the gallons per minute (GPM) that flow through each zone. If your irrigation system is attached to domestic water, it is required to have a backflow prevention device.

The concept of preparing your sprinkler system for winter must be done correctly to assure there are no costly repairs and replacements to make in the spring at system start-up. The process consists of expelling all the water from the irrigation system and equipment. This is necessary because water freezing in the irrigation system will break pipes, fittings, valves, sprinklers, pumps, and other system components.

Most substances contract as they get cold; however, when water cools, it contracts only until it reaches a temperature of 39 degrees Fahrenheit. Upon further cooling to 32 degrees Fahrenheit, water expands as it turns to ice. Water expands and increases in volume by one-eleventh, so 11 cubic feet of water will form approximately 12 cubic feet of ice. This expansion force is sufficient to cause pipes and fittings to burst valves to crack, and sprinkler and pump cases to split open.

Draining the system may be accomplished by the use of either manual or automatic drain valves which rely on gravity to purge water from the system. These valves rely on a properly-installed system laid to grade with no humps in the pipe to trap water in low areas. This method is not recommended since sub-surface pipes have a tendency to shift with time and there is no way to visually inspect lines once draining is complete. The only positive way to be sure enough water has been expelled from the system is by using compressed air to blow the water out.

Systems with electric valves must be blown out with pressurized air. There is no other way to drain the water off the top of the diaphragm of the valve.

Air volume is as critical as air pressure. If an insufficient volume of air is used, after forcing some water out, the air will ride over the top of the water. This will result in the remaining water draining into low spots and subjecting the system to freeze damage. Ideal pressures are in the range of 40 to 80 pounds per square inch (psi) for the air compressor with 80 psi being the maximum for rigid PVC pipe and 50 psi for polyethylene pipe (flexible black pipe). Set the pressure regulator on the air compressor accordingly. If the pressure is in excess of what the nozzles are rated, the excessive pressure will blow the sprinkler nozzles off and could cause other damage. The rating of the nozzles is available at the manufacturers home web page.

Rule of thumb: If the sprinkler heads stay up after the water is blown out and compressed air continues to flow through the system, you are using the right size compressor.

The idea is to "blow" your system out using only the volume of air necessary. If you normally run one zone at a time when irrigating, the system should blow out the same way. If you try to do more, the excess velocity of flow and added friction will heat up the pipe and fittings to a point where they could possibly melt. If the pipe and fittings do not burst during this operation, they could be damaged and reduce their life.

To figure out the best sized compressor for the system you need to know the gallons per minute (GPM) that flow through each zone in the system. When the system was designed it should have had a GPM rating for each zone marked on the irrigation plan. If this information is not available, a quick estimate of GPM per zone can be made by adding up the GPM rating for each sprinkler head nozzle on the zone. GPM data is available on the manufacturers web site.

Divide GPM by 7.5 to determine the cubic feet per minute (CFM) needed. This is the ideal sized compressor to blow out your system.

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Home Sprinkler Systems:Preparing Your Sprinkler System for Winter