Updated October 28, 2026
An RPZ and a double check valve are both backflow preventers, and the difference comes down to one part. A double check is two one-way valves in a row. An RPZ is the same two valves with a relief valve between them that dumps water out the bottom if either one leaks, so it’s the one allowed on riskier lines. In Northern Virginia either kind gets a backflow test once a year, which we do for $250, normally $300, on our backflow testing page.
If the letter from your water supplier names a device and you’re not sure what you own, here’s the plain version: what’s inside each one, where the pressure vacuum breaker fits in, which kind a lawn sprinkler gets, and how each one gets checked. Brand new to all of this? Start with what backflow means in plumbing and what a backflow preventer does.
RPZ vs double check valve: the short answer
Picture two doors that only swing one way, one right behind the other. Water from the street walks through both. Water trying to go back toward the street hits the second door, and if that one sticks, it hits the first. That’s a double check valve assembly.
Now put a little hallway between those two doors, with a drain in the floor. If either door leaks, whatever slips past runs down the drain and out of the pipe before it can go anywhere. That’s an RPZ, short for reduced pressure zone. The hallway is the zone, and the drain is its relief valve.
Virginia’s waterworks rule spells out the split: a double check is “used for low hazard situations,” and an RPZ is used “for high or low hazard situations.” Same job. The RPZ just has a backup plan for the day a check valve wears out.
What’s inside an RPZ and a double check valve
From the outside they share the same parts: a shutoff valve at each end and a few small brass test cocks on top, so a plumber can close the device off and hook a gauge to it. Those two things are what make either one a testable “assembly” under the state’s definitions. Inside is where they split:
- Double check: “two single independently acting check valves.” Each one is a spring-loaded flap with a rubber seal. No drain and no opening to the air.
- RPZ: two spring-loaded check valves with a pressure relief valve in the space between them. The state’s definition says the pressure in that middle space stays lower than the street side. If that balance slips, the relief valve opens and spits water out the bottom.
That relief port is the easy tell. An RPZ has an opening on its belly, often with a funnel or a drain under it, because it needs somewhere for water to go. A double check doesn’t.
It’s also why an RPZ can drip or suddenly gush while a double check never does: the relief valve is doing its job because a check behind it isn’t sealing. Here’s what a leaking backflow preventer is telling you.
$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
Where the pressure vacuum breaker (PVB) fits in
The third kind you’ll see on houses here is the pressure vacuum breaker, or PVB. The state defines it as one spring-loaded check valve plus a spring-loaded air inlet, with shutoffs and test cocks like the others. When the street pressure drops, the air inlet pops open and the line pulls in air instead of pulling yard water back.
The catch is in the wording. A PVB is “designed to prevent backsiphonage,” which is water being pulled backward. The definition doesn’t cover backpressure, water being pushed backward by something on the house side that’s stronger than the street. Loudoun Water’s backflow page names boilers, booster pumps and chemical feed pumps as the kind of things that push. On a line like that, the choice is an RPZ or a double check, depending on the hazard.
You may also run into two smaller cousins. An atmospheric vacuum breaker and a dual check are backflow devices, not assemblies: no test cocks and no shutoffs, so they can’t be tested. The state says a device “is not generally designed or constructed” to control high hazards.
RPZ, double check and PVB side by side
Here are the three testable kinds next to each other:
| Kind | What’s inside | Stops water pulled back | Stops water pushed back | Hazard the state allows | Opening to the air |
|---|---|---|---|---|---|
| RPZ | Two spring-loaded checks, relief valve between them | Yes | Yes | High or low | Yes, the relief port underneath |
| Double check | Two spring-loaded checks | Yes | Yes | Low only | No |
| PVB | One spring-loaded check and a spring-loaded air inlet | Yes | No | High or low | Yes, the air inlet on top |
Every cell comes from Virginia’s waterworks definitions. The last column matters for where each one can go. The state’s rule on backflow assemblies says anything with “openings, outlets, or vents” that open during backflow can’t be installed “in areas subject to flooding or in pits.” So an RPZ or a PVB has to sit up where its opening can never end up underwater.
High hazard or low hazard: how your device gets picked
Which one you have isn’t a matter of taste. It depends on what’s on the other side of it. Virginia’s rule sorts every hookup into high or low hazard, based on what the water could carry back, whether that would hurt anyone, and whether backflow would happen by being pushed or only by being pulled.
- High hazard examples from the state’s table: “lawn irrigation systems, fire sprinkler systems with chemical additives or antifreeze, sewage,” and mixtures of water with other chemicals.
- Low hazard examples: “coffee machines, non-carbonated beverage dispensers, and residential fire sprinkler systems constructed of materials designed for potable water flow.”
Put that next to the table above and the pattern is easy to read. A double check is for low hazard lines only. An RPZ or a PVB can handle high hazard. And when a high hazard line can also be pushed backward, the RPZ is the one that covers both.
Which kind a lawn sprinkler gets around here
Lawn sprinklers sit on the state’s high hazard list, and Fairfax County says why in one line: the spray heads “are not sealed and are continuously in direct contact with the soil.” So a sprinkler line gets a high hazard device, not a double check.
Fairfax County’s cross connection page says homes here typically have an atmospheric vacuum breaker, a pressure vacuum breaker or an RPZ on the sprinkler line, and it quotes the Virginia Residential Code listing those same three for lawn irrigation. Prince William County adds a rule of its own: if the sprinkler is winterized with antifreeze, the RPZ is the only approved device.
The whole story for sprinklers is in why a sprinkler system needs a backflow preventer, and if you can’t find yours, here’s where your backflow preventer is. Local test details: Fairfax County, Loudoun County and Prince William County, plus towns like Centreville and Leesburg.
How an RPZ, a double check and a PVB get tested
All three are assemblies, so all three get tested at the same times: after they’re put in, right after a repair or a move, and once a year after that, under Virginia rule 12VAC5-590-600. Fairfax County’s requirements say the same thing and give every property its own month for it.
The test is a gauge with colored hoses hooked to the test cocks. On a double check, the plumber reads whether each check holds tight. On an RPZ, both checks, plus whether the relief valve opens when it should. On a PVB, the check, plus whether the air inlet opens. Every step is in what happens during a backflow test, and the reason it can’t skip a year is in why backflow testing is required every year.
Why SwiftPro makes the yearly RPZ or double check test easy
You didn’t ask to own a backflow preventer, 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 which kind your notice names, and keep the notice handy.
- We test it and tag it. A SwiftPro plumber hooks the gauge to the test cocks, 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.
RPZ and double check questions homeowners ask
What’s the difference between an RPZ and a double check valve?
Both have two spring-loaded check valves. An RPZ adds a relief valve between them that dumps water out the bottom if either check leaks. Virginia allows an RPZ on high or low hazard lines and a double check on low hazard lines only.
Is an RPZ better than a double check valve?
It’s stronger, not always better. The RPZ is the one allowed on high hazard lines that can be pushed or pulled backward. On a low hazard line a double check does the job, and it has no relief port to drip.
Why does my RPZ drip water?
The relief valve opens when the pressure between the two checks gets out of balance, usually because a check or its rubber seal isn’t closing tight. A drip is a sign to get it tested.
What is a pressure vacuum breaker?
A PVB is a testable assembly with one spring-loaded check and a spring-loaded air inlet. When pressure drops, the inlet lets air in so the line can’t pull water back. It stops water being pulled backward, not pushed.
Can a double check valve be used on a lawn sprinkler?
Not as the protection for an in-ground sprinkler. Virginia rates lawn irrigation high hazard, and a double check is for low hazard lines. Fairfax County lists a vacuum breaker, a PVB or an RPZ for sprinklers.
Do RPZ and double check valves both need a yearly backflow test?
Yes. RPZs, double checks and PVBs are all testable assemblies, and Virginia’s rule calls for a test after install, after a repair or a move, and every year.
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 RPZ, double check or PVB 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.
More plain answers: can you test your own backflow preventer, how long a backflow test takes, what replacing one costs and what to do about a frozen backflow preventer.
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.
