How do I winterize a sprinkler system in Arizona?

Colorado State University Extension explains the physics. Water contracts as it cools until it reaches 39 degrees Fahrenheit, then expands as it turns to ice at 32. It expands by about one-eleventh, so 11 cubic feet of water becomes roughly 12 cubic feet of ice. That force is enough to burst pipes and fittings, crack valves, and split sprinkler and pump cases. The University's advice is that the job is to expel all the water from the system.

Salt Lake City Public Utilities adds a point that also holds in Arizona: most sprinkler piping is not buried below frost level, so it is at risk, and even a partly drained system can break, because the water left inside expands and can crack PVC. Flexible polyethylene pipe can expand under pressure but can still rupture its walls when water is left inside. Water in the backflow preventer or its ball valves can crack the device or the valve.

Phoenix Water Services notes that exposed pipes, especially plastic ones such as PVC, are more susceptible to extreme temperatures, leaks and breaks. An above-ground backflow assembly is exactly that: exposed brass and plastic on a stand. See the backflow preventer rules for sprinklers in Arizona for what the assembly is and why your city cares about it.

Frozen pipes are rare in Phoenix, as our page on freezing explains, so a full blowout is not an annual chore there. The parts that sit in the open air are the risk: the backflow assembly, any above-ground piping, and hose-end fittings. On a forecast freeze, the practical step for the Valley is protecting the backflow assembly. Salt Lake City's guidance is to wrap the backflow preventer with towels, blankets or other insulation, and to drain it as described below. Our page on whether pipes can freeze in Phoenix shows how cold it has to get.

In the high country, plan on the full routine each fall, before the first hard freeze.

Colorado State's fact sheet lays out the sequence. Follow it in this order.

1. 1Shut off the water to the system. A stop-and-waste valve or another drain should have been installed between the water supply and the backflow device. Turn it off. It may be in a basement, a crawlspace or an underground valve box, and a stop-and-waste valve can sit about 5 feet down, so a long key may be needed. Do not force the valve while opening or closing it.
2. 2Drain the pump, if you have one. Drain it, remove it, and store it inside. If you cannot remove it, drain it and wrap it against the weather.
3. 3Winterize the backflow device. The right steps depend on the type (see the next section).
4. 4Blow out the zones. Connect the compressor to the blowout fitting on the downstream side of the backflow device, start a zone at the controller, and run the compressor at the right pressure.
5. 5Repeat each zone. The fact sheet says to blow out each zone twice, and that two short cycles per zone are better than one long one.
6. 6Set the valves for winter. Leave the backflow device's ball valves half open, as described below.

### Gravity drain or compressed air?

Colorado State says a system can be drained by manual or automatic drain valves that rely on gravity, but only if it was laid to grade with no humps that trap water. It does not recommend that method, because buried pipe shifts over time and you cannot see inside the lines to confirm they are empty. The only sure way to know enough water has left, it says, is compressed air. It adds that systems with electric valves must be blown out with pressurized air, since there is no other way to drain the water on top of the valve diaphragm.

### The air limits that matter

Air is not harmless in a plastic system. The fact sheet gives these limits:

• Volume matters as much as pressure. With too little volume, air rides over the water and leaves it in low spots.
• Pressure. Ideal compressor pressure is 40 to 80 psi, with 80 psi as the maximum for rigid PVC pipe and 50 psi for flexible polyethylene pipe.
• One zone at a time. If you normally run one zone at a time, blow out one zone at a time. Too much flow heats the pipe and fittings through friction and can melt them.
• Compressor size. Divide the zone's gallons per minute by 7.5 to get the cubic feet per minute the compressor should deliver. The example given: 20 gallons per minute divided by 7.5 is about 2.66 cubic feet per minute at 30 psi.
• Eye protection. The fact sheet's caution is to wear it.

Backflow devices are one of the most freeze-prone parts. Colorado State's backflow fact sheet says they will be damaged by freezing unless they are drained of all water, and that hose connection vacuum breakers are best removed, drained and stored for the winter.

Never blow air through a pressure vacuum breaker or a reduced pressure assembly. The University warns that the rubber seals inside can melt from the heat of the compressed air. An atmospheric vacuum breaker, which sits downstream of each valve, is winterized by the air when the zone is blown out.

Pressure vacuum breaker (PVB). After the system is blown out, exercise both ball valves and the test cocks back and forth several times, then leave them half open for the winter. Fully closed or fully open ball valves tend to trap water, which then expands. Salt Lake City puts it as a quarter turn on each test cock screw and the shutoff valves at a 45 degree angle.

Reduced pressure (RP) assembly. The two check valves must be drained separately. The University's steps involve opening test cocks, and removing the check cap, spring and disk holder on the second check so water drains through the body. This is a job for someone who knows the assembly, and the manufacturer's manual applies.

In spring, close the test cocks and ball valves before you pressurize the system, and open the ball valves slowly to avoid water hammer.

### Three examples of how this goes

These are made-up examples to show the reasoning, not real jobs.

A Phoenix house, forecast freeze, no blowout. The buried lines are fine. The homeowner shuts the irrigation supply, then wraps the above-ground backflow assembly and opens its test cocks a quarter turn. It survives the night.

A Flagstaff house, first week of November. The owner shuts the stop-and-waste, blows out each zone twice, and leaves the backflow ball valves half open. The system starts up in spring without a leak.

A DIY blowout at full pressure. A homeowner sets the compressor high and blows several zones at once. The pipe heats, a fitting cracks, and the leak shows in spring when the system turns on. Lower pressure and one zone at a time would have avoided it.

Compressed air, an underground stop-and-waste, and an RP assembly are enough reasons to bring in a pro. Call a licensed plumber or irrigation contractor if your system has an RP assembly, if you cannot find the shutoff, or if a spring startup shows a leak. For a valve that will not close or a zone that will not stop, see why a sprinkler system won't shut off. To understand how the pieces fit together, read how an in-ground sprinkler system connects to home plumbing.

HQ Plumbing offers free estimates, upfront pricing and 24/7 service, and can replace or install a backflow assembly or fix freeze-damaged supply piping.