Frozen Backflow Preventer: What a Freeze Does and What to Do

Updated October 28, 2026

If your backflow preventer froze, leave it alone until it thaws, then turn the water to it on slowly and look for spraying or dripping. In Northern Virginia the one that freezes is almost always the outdoor device on a lawn sprinkler line, and the yearly test that checks it after a fix is $250, normally $300, on our backflow testing page.

Picture a can of soda left in the freezer. Water gets bigger when it turns to ice, and the can has nowhere to give, so it splits. A backflow preventer left full of water on a hard freeze night does the same thing. The difference is that this one is built with a part that’s meant to give first. Here’s what a freeze does, the part that usually cracks, and what to do tonight.

Frozen backflow preventer, the short answer

Ice doesn’t hurt a backflow preventer by itself. Water trapped inside it, turning to ice, does. When that happens one of three things is true:

• Nothing broke. It thaws, you turn the water on, and it doesn’t leak. It still gets looked at at the next test.
• The bonnet cracked. That’s the cap on top. It’s a part, and it can be replaced.
• The body cracked. That’s the brass casting itself. The device gets replaced.

A backflow preventer is the one-way valve that keeps used water from running back into the pipes everyone shares. If that’s new to you, start with what backflow is in plumbing. Freeze cracks on outdoor sprinkler devices are the break we see most.

What a freeze does inside a backflow preventer

The device sits on top of the pipe, out in the cold, with little valves and chambers full of water. Once the sprinkler is done for the season, nothing moves that water. On a hard freeze night it turns to ice, gets bigger, and pushes out on whatever is weakest.

That’s usually one of these:

• The bonnet, the cap on top of the device.
• The test cocks, the little side valves the tester hooks a gauge to.
• The brass body, when the cold is long or hard enough.
• The pipe right next to it, which can split just like the device.

You often can’t tell while it’s frozen. Ice holds the crack shut. It’s when it thaws and the water comes back that it sprays, drips or just won’t hold.

The cracked bonnet on a frozen backflow preventer

On a lot of sprinkler devices, the bonnet is made to be the weak spot on purpose. The maker’s spec sheet for one common pressure vacuum breaker, the Febco 765, says its plastic bonnet is there to protect the valve body from freeze damage. Think of a soda can with one spot built to split first, so the rest of the can stays whole.

The same sheet says every part inside can be serviced in place from the top. So when the bonnet is the only thing that cracked, the fix is usually a new bonnet and whatever’s worn inside, not a whole new device.

The sheet is also clear about what it can’t do. Even with a freeze sensor added, it says the device can’t stop a freeze on its own and somebody has to act. That’s what shutting it down for winter is for.

What to do tonight if a freeze is on the way

If the sprinkler is already shut down for winter, the water’s off to the device and it’s drained, you’re set. Nothing to do.

If it isn’t, the whole shutdown is in our post on winterizing a sprinkler backflow preventer. It’s the one to follow, and it takes less time than you’d think.

If you can’t get to it before the cold comes:

• Find the sprinkler shutoff, usually in the basement where the sprinkler line leaves the main pipe, and close it. Less water headed to the device means less to freeze.
• Don’t run the sprinkler overnight.
• Call us and we’ll get the device shut down properly.

Fairfax County’s rules allow a device to come off for winter as long as it’s back on before your testing month, and Loudoun Water’s program wants a device that comes off and goes back on tested again.

If your backflow preventer is already frozen

Give it time. A frozen device usually comes back on its own once the day warms up:

1. Close the sprinkler shutoff if you can, so a crack doesn’t turn into a fountain when it thaws.
2. Let it thaw on its own. No torch, no heat gun and no pot of boiling water. Fast heat on cold brass and plastic can crack what the ice didn’t.
3. Once it’s thawed, open the shutoff slowly and watch the device for a minute.
4. Spraying, dripping or a split you can see? Close it again and call us.
5. Nothing wrong you can see? Good. Mention it at the next test, since a small crack can hide until there’s pressure on it.

Water running out of an indoor RPZ’s relief valve is a different thing, not a freeze. See why a backflow preventer leaks.

Fixing a frozen backflow preventer and testing it after

It comes down to what cracked:

• A bonnet, a test cock or a worn part inside: repaired in place, priced in writing before we start.
• The body: the device gets replaced. A new home device up to 1 inch is $800 to $1,200, installed and tested. More on that call in backflow preventer replacement cost.

Either way, Virginia’s waterworks rules want a device tested again right after a repair. Signs it broke in some other way are in what to do about a broken backflow preventer. Local details: Fairfax County, Loudoun County, Prince William County, Leesburg, Oakton and Bristow.

Why SwiftPro makes a frozen backflow preventer easy

You don’t have to figure out what cracked. That’s our job:

1. Pay $250 online, normally $300. Or call (703) 997-9757, or book a time online, whichever you like.
2. We call you to pick the day. Tell us where the device is and what you saw when it thawed. A photo helps.
3. We test it and tag it. A SwiftPro plumber hooks the gauge to the device, reads every check and writes the numbers down.
4. We send the report where your water supplier wants it. Loudoun Water, Fairfax County, Prince William Water and the town programs each take it their own way, and yours goes in the way your notice asks.
5. If it fails, you get a written quote before any repair. Nothing gets fixed until you say yes, and the device is tested again once it is.

All the details are on our backflow testing page.

Questions about a frozen backflow preventer

What happens to a frozen backflow preventer?

Water trapped inside turns to ice and gets bigger. It can crack the bonnet on top, a test cock, the brass body or the pipe next to it. Sometimes nothing breaks at all.

Can a backflow preventer freeze and not break?

Yes. Plenty thaw out fine. Turn the water back on slowly once it’s thawed and watch for spraying or dripping.

Can a cracked bonnet be replaced?

Usually, yes. On many sprinkler devices the bonnet is a part made to give first so the body doesn’t, and the parts are serviced from the top without taking the device out.

How do I thaw a frozen backflow preventer?

Let it thaw on its own. Close the sprinkler shutoff first if you can. No torch, heat gun or boiling water.

What should I do tonight before a freeze?

If the sprinkler isn’t shut down for winter, follow our winterizing post. If you can’t, close the sprinkler shutoff and call us.

Does a backflow preventer need a test after a freeze repair?

Yes. Virginia’s waterworks rules want a device tested again right after any repair, and Loudoun Water wants one that comes off and goes back on tested again.

What does it cost to replace a frozen backflow preventer?

If only the bonnet or a part cracked, it’s a repair, priced in writing first. If the body cracked, a new home device up to 1 inch is $800 to $1,200, installed and tested.

Get a frozen backflow preventer looked at

Call (703) 997-9757, book online or use the form at the top of this page. Or pay $250 online and we’ll call to pick the day. Everything is on our backflow testing page, the rest of our plumbing work is on our plumbing page, and current deals are on our offers page.

SwiftPro Heating, Cooling & Plumbing tests, repairs and replaces backflow preventers, water heaters, sump pumps and water and drain pipes, and services furnaces, heat pumps and air conditioners, across Northern Virginia from Chantilly and Ashburn, with a separate plumbing team and a dedicated HVAC team. Family owned, BBB Accredited with an A rating, 24/7 emergency service, Virginia Class A contractor license 2705192276, and rated 5.0 stars across 1,130 reviews on Google, Angi, Yelp, the BBB and Facebook. Read the reviews.