Updated October 28, 2026
A backflow preventer is a one-way door for your water. Clean water from the street is allowed in, and water from your lawn sprinkler, boiler or pool line is never allowed to slide back out into the pipes that feed your kitchen, or your neighbors’ kitchens. In Northern Virginia most homes that have one have an in-ground lawn sprinkler, and the device needs a backflow test once a year, which we do for $250, normally $300, on our backflow testing page.
Most people have never heard of it until a letter shows up with a due date. That’s normal. Here’s the simple version: where yours probably is, what’s inside it, the three kinds you’ll see on a house, who needs one, and why somebody has to check it every year.
What does a backflow preventer do, in plain words
Picture a door with a spring that only swings one way. Walk in and it opens. Try to walk back out and it slams shut behind you. A backflow preventer is that door, built for water.
Water is supposed to move one way: from the main in the street, through your meter, into the house and out your faucets. Plumbers call water moving the wrong way backflow. Virginia’s waterworks rule defines it as the “undesirable reversal of flow” of water, or water mixed with other stuff, back into the public supply.
It happens two ways, and the device stops both:
- Pulled backward. The pressure in the street drops, say a water main breaks or a crew flushes a fire hydrant, and the dip sucks water back out of houses like a straw. The real word is backsiphonage.
- Pushed backward. Something in the house pushes harder than the street does, like a boiler loop or a pump, and shoves water back toward the main. That one is backpressure.
Both come down to pressure, so we gave them their own post: how water pressure causes backflow. For the bigger picture of what backflow is, start with what backflow means in plumbing.
Where your backflow preventer is (and how to spot it)
On a Northern Virginia house it’s in one of two places.
- Outside, on the sprinkler line. A brass or bronze fitting about the size of a shoebox, sticking up out of the ground or a mulch bed near the house, with two little handles and a few small screw caps on top. In winter a lot of them wear an insulated cover or a bag.
- Inside, near where the water comes in. In the basement or utility room, on the pipe that splits off to the sprinkler, a fire sprinkler system or a boiler. It looks like a short run of brass with a valve handle at each end. If it has a funnel or drain under its belly, it’s an RPZ.
Two quick tells: there’s often a tag hanging on it with the last test date, and the letter from your water supplier usually names the device and its location. If you have no lawn sprinkler, no fire sprinkler system, no pool fill line and no boiler, you may not have one to test, and your water supplier’s notice is the tie-breaker.
$250 Yearly Backflow Test
The yearly test your water utility or county asks for, on a home's backflow preventer up to 1 inch. About backflow testing
What’s inside a backflow preventer: check valves, springs and rubber
Open one up and there’s nothing fancy in there. The parts are:
- Check valves. Little doors that only swing one way. Water pushing the right direction opens them. When it stops or tries to reverse, they slam shut.
- Springs. Each check has a spring holding it closed, so it shuts fast and tight. The state’s definitions call these “spring-loaded” check valves.
- Rubber seals. Where each door closes, a rubber disc or seat makes the seal watertight.
- A relief valve (RPZ only). Sits between the two checks. If either check leaks, it opens and dumps water out the bottom, so nothing can sneak back past. That’s why an RPZ sometimes drips, and why it needs a drain under it.
- An air inlet (PVB only). If the pressure drops, it pops open and lets air in, so the line pulls air instead of pulling water back.
- Shutoff valves and test cocks. A valve at each end and small ports on top, so a plumber can close it off and hook a gauge to every chamber.
Springs get tired and rubber gets stiff or nicked, and you can’t see either from outside. That’s the whole reason it gets tested: why backflow testing is required every year.
The three kinds of backflow preventer you’ll see on a house
The state’s rule names three testable assemblies. On homes here, these are the ones you’ll run into:
| Kind | Where it usually is | What’s inside | What it stops |
|---|---|---|---|
| PVB (pressure vacuum breaker) | Outside, sticking up from the ground on a sprinkler line | One spring-loaded check and a spring-loaded air inlet | Water pulled backward |
| Double check (double check valve assembly) | Basement, or in a box in the ground | Two spring-loaded checks in a row | Pulled or pushed backward, for lower-hazard lines |
| RPZ (reduced pressure zone) | Basement or a heated enclosure, with a drain below | Two checks with a relief valve between them | Pulled or pushed backward, for higher-hazard lines |
Those descriptions come straight from Virginia’s waterworks definitions. Which one you have depends on what’s on the other side of it. A sprinkler that sits in fertilizer and weed killer is a bigger risk than a plain hose line, so it may need the stronger device. Prince William County, for example, says a sprinkler system winterized with antifreeze can only use an RPZ.
If yours is dripping or spraying, that’s a different question: here’s what a leaking backflow preventer is telling you.
Do you need a backflow preventer at your house?
If you have an in-ground lawn sprinkler in Northern Virginia, almost certainly yes. Here’s how each water supplier puts it:
- Loudoun Water says an approved device is required for all fire suppression and irrigation systems.
- Fairfax County lists lawn irrigation systems, fire suppression systems and in-ground pools among the hookups that need one.
- Prince William County wants a backflow preventer on every lawn sprinkler system.
And Virginia rule 12VAC5-590-600 puts the yearly test on the water customer, meaning the owner of the house, not the water company. Here’s the local page for Fairfax County, Loudoun County and Prince William County, and for towns like Vienna, Reston, Sterling, Leesburg and Woodbridge.
How a backflow preventer gets checked once a year
The check is quick and it doesn’t involve digging anything up. A plumber shuts the device off, hooks a gauge to the test cocks, and reads whether each check holds tight and whether the relief valve or air inlet opens when it should. The numbers go on a report, and the report goes to your water supplier.
Every step, in order: what happens during a backflow test. Not sure if your letter wants a test or an inspection? Here is the difference. And if you’re wondering what happens when the date slips by: what happens if you skip a backflow test.
Why SwiftPro makes the yearly backflow test easy
You didn’t ask to own a one-way door for your water, so we keep the yearly part simple:
- Pay $250 online, normally $300. Or call (703) 997-9757, or book a time online, whichever you like.
- We call you to pick the day. Tell us where the device is and whether the sprinkler is shut down for winter, and keep your notice handy.
- We test it and tag it. A SwiftPro plumber hooks the gauge to the device, reads every check and writes the numbers down.
- 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.
- 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.
The price, the payment and the details are on our backflow testing page. When a device breaks instead of just failing a test, here’s what a broken backflow preventer looks like.
Backflow preventer questions homeowners ask
What does a backflow preventer do?
It lets water into your house from the street and stops it from ever flowing back out. That keeps sprinkler, boiler or pool water from getting pulled or pushed into the public water lines.
Where is my backflow preventer?
Usually outside on the sprinkler line, a brass fitting sticking up from the ground near the house, or inside in the basement where the line to the sprinkler, fire sprinkler system or boiler splits off. Your water supplier’s notice often says where it is.
What is an RPZ backflow preventer?
RPZ stands for reduced pressure zone. It has two spring-loaded check valves with a relief valve between them that dumps water out the bottom if a check leaks. It’s the strongest of the three kinds and is used on higher-hazard lines.
What’s the difference between a PVB and a double check?
A PVB has one check and an air inlet and only stops water being pulled backward; it sits outside above the ground. A double check has two checks in a row, stops water pulled or pushed backward, and is used on lower-hazard lines.
Do I need a backflow preventer if I have a lawn sprinkler?
Almost certainly. Loudoun Water requires one on all irrigation systems, Fairfax County lists lawn irrigation among the hookups that need one, and Prince William County wants one on every lawn sprinkler system.
How long does a backflow preventer last?
There’s no fixed age. The springs and rubber inside wear out first, which is what the yearly test catches, and a freeze can crack the body in one night. A repair kit often fixes a failed check; a cracked body gets replaced.
How much is a backflow test?
Our yearly backflow test is $250 for one home, normally $300, on a residential device up to 1 inch. You can pay online and we call you to pick the day.
Get your backflow preventer tested without the runaround
Call (703) 997-9757, book online, use the form at the top of this page, or pay $250 online and we’ll call to pick the day. Everything about the test 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.
