Frozen backflow preventers and irrigation damage

This guide is part of our Montana Winter Property Guide and focuses on one component. For the whole job, see how to winterize an irrigation system in Montana. The technical details below come from Colorado State University Extension, whose sprinkler winterizing guide covers the devices closely, and the local rules come from Missoula Water. If you are on a different water system or a well, ask your provider or county what applies to you.

What does a backflow preventer actually do?

It keeps irrigation water from running backward into the drinking supply. Missoula Water's program summary explains that backflow occurs when pressure in the distribution system drops, siphoning water from a customer's system into the mains, or when a customer's system is pressurized enough to overcome the pressure in the main. Anything in contact with the water through a cross-connection can then follow.

Irrigation water is not drinking water. The same document says water in an irrigation system is deemed non-potable. Colorado State says if your irrigation system is attached to domestic water, it is required to have a backflow prevention device. That is why it cannot simply be left out of the system when it breaks.

Why do backflow preventers freeze and split?

Water expands as it freezes. Colorado State says water contracts only until it reaches 39 degrees Fahrenheit, then expands as it turns to ice at 32, so that 11 cubic feet of water becomes about 12 cubic feet of ice. That force is enough to burst pipes and fittings, crack valves and split sprinkler and pump cases. The American Red Cross likewise lists water sprinkler lines among the pipes that freeze most often, because they are exposed to severe cold.

A backflow preventer has the problem in concentrated form. It has internal chambers and check valves that hold water, which is why Colorado State gives specific draining steps for it. Our sprinkler blowout page notes the first hard freeze can come early here, often in the first half of October in the Bitterroot and sometimes sooner at elevation, so a device left full in September is on borrowed time. Missoula Water's rules make protection from freezing the water user's responsibility for reduced pressure and double check assemblies, and say a pressure vacuum breaker must be protected from freezing temperatures.

What are the common types of backflow device?

Colorado State describes three on residential irrigation. An atmospheric vacuum breaker, or AVB, is installed downstream of each valve and is winterized with compressed air when the zone is blown out. A pressure vacuum breaker, or PVB, is a single assembly with ball valves and test cocks. A reduced pressure device, or RP, has two check valves that must each be drained. Colorado State notes AVBs may not be legal in some areas.

Locally, Missoula Water's hazard table lists premises, including single family residences, with underground irrigation systems as needing an RP, with a properly installed AVB or PVB allowed in lieu of an RP with written approval from Missoula Water. Systems into which fertilizers, herbicides or pesticides can be introduced are also listed as RP. If you are not sure what you have, photograph it and ask.

Can you blow compressed air through a backflow preventer?

No, not through a PVB or RP. Colorado State is direct about this: pressure vacuum breaker or reduced pressure backflow devices must never be blown out with compressed air, because the rubber seals inside can easily melt from the heat of the air. The compressor connects to a blowout fitting on the downstream side, with the downstream isolation ball valve closed. If the device does not have a downstream blowout fitting, one needs to be added before the system can be blown out.

That is an easy way to damage one. It also shows why a compressor at the wrong pressure is risky. Colorado State gives ideal pressures in the range of 40 to 80 psi, with 80 psi being the maximum for rigid PVC and 50 psi for flexible black polyethylene pipe. If you would rather not work that out, our sprinkler blowout and winterization service handles the air and the draining as one job.

How should a backflow preventer be left for the winter?

Drained, with the valves partly open. For a PVB, Colorado State says that after the system is blown out, the ball valves and test cocks should be exercised back and forth several times, then left in a half-open position for the winter, because valves that are fully closed or fully open tend to trap water, expand and cause damage. For an RP, it says to drain both check zones and leave the isolation ball valves at about 45 degrees for the winter.

Removing the water is the point. Colorado State says to start by shutting off the water supply at the stop and waste valve or drain, which may be in a basement, crawlspace or an underground valve box, and possibly several feet down, so it may need a long key. Don't force that valve. And do not put antifreeze in the lines unless directed: the Red Cross warns it is environmentally harmful and dangerous to people, pets, wildlife and landscaping.

How do you know a backflow preventer froze?

Usually when the water comes on. Our sprinkler blowout page notes that freeze damage often does not show up until the system is started in spring. Look for water spraying or weeping from the body, bonnet, test cocks or fittings, or a visible crack or bulge in the body. A leak on the supply side of the isolation valves will keep running until you shut the supply off upstream.

Damage downstream of the device may show up differently, as a soggy patch in the lawn or a zone that will not hold pressure. Our guide on how to know your irrigation system needs repair goes through those signs. If a pipe or fitting inside the house or crawlspace has frozen rather than the irrigation line, that is a plumber's job: the Red Cross says to call a licensed plumber if you cannot locate or access the frozen section or cannot thaw it.

What should you do if it is leaking at startup?

Shut the water supply off at the stop and waste or main valve first, then look. Do not keep opening valves to see what happens. Colorado State's spring procedure is a good reminder of why: close the test cocks and ball valves before pressurizing, and open the ball valves slowly once the supply is on, because opening them slowly prevents damage caused by water hammer.

Our spring lawn and landscape startup guide gives the wider sequence. A careful first pressurization is also a good time to find leaks while you are standing next to the shutoff, rather than from an inside window a week later.

Can a frozen backflow preventer be repaired?

Sometimes, depending on what broke. Seals, a cap or a fitting may be replaceable, and a cracked body may mean replacing the assembly. A professional will need to see it, and on a public water system the replacement has to meet the system's rules. Missoula Water requires approved assemblies listed by the Foundation for Cross-Connection Control and Hydraulic Research at the University of Southern California, and it also says these requirements may be updated periodically, so check before you buy.

We diagnose and repair irrigation problems, including older systems, through our sprinkler repair work, and we can tell you what we find. Testing the assembly is a separate step, as the next section explains.

Does a backflow preventer need testing in Missoula?

If you are on Missoula Water, plan for it. The program summary says that approved assemblies currently in service must then be tested yearly by a certified backflow prevention assembly tester. Missoula Water publishes a list of certified assembly testers, along with test forms. It gives 406-552-6756 as the number for questions about specific projects.

Requirements vary with your water system, so check your own provider if you are outside the city. Please confirm the details directly: these programs change, and what we describe is what the city's document said when we checked it in November 2026. This is general information rather than legal or plumbing advice.

Can a heated box or insulated cover protect one?

Covers are sold for this, and they may help during a surprise cold snap, but they are not a substitute for draining. We did not find an extension source that tests insulated covers on a drained or undrained device, so we won't quote a temperature for them. Colorado State's recommendation is to expel the water and leave the valves half open. Missoula Water's rules say the water user must keep certain assemblies from freezing, so if yours has to stay live through winter, such as one serving something other than seasonal irrigation, ask a qualified plumber about a protection plan.

Keep the area around the device clear as well. A backflow preventer near a driveway is an easy target once it is under snow. Marking it is simple, as our guide to snow stakes and markers explains, and our guide to where to pile snow helps you keep heavy piles away from it.

What if a hard freeze comes before the blowout?

Our winterizing guide answers that in detail, including what to do when a freeze is forecast before your appointment. Booking ahead is the real fix, since the appointment window tightens as the first freeze approaches.

To book a blowout or get a repair started, call (406) 215-7319 or contact us. We also run irrigation systems installation and service across Missoula and the Bitterroot Valley, and our snow and 24-hour storm services keep the rest of the property moving after the freeze sets in.

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