If your water provider sent a backflow test notice, the first question is usually "what do I even have?" Here's how to tell the common assemblies apart, what each one protects against, and why they're tested.
What backflow is, in one minute
Water in the street main is supposed to flow one way: into your property. Backflow is when it reverses. That can happen two ways:
- Back-siphonage: pressure in the main drops, from a main break or firefighters drawing a lot of water, and water gets pulled backward out of your system.
- Back-pressure: your side ends up at higher pressure than the main, from a pump, a boiler, or an elevated tank, and pushes water backward.
Either way, whatever is in your sprinkler lines (fertilizer, standing water, soil bacteria) or your building's equipment could end up in the public water supply. A backflow preventer stops that. Water providers require the testable ones to be checked so they know the device still works.
Quick identification
- PVB (pressure vacuum breaker): one valve body with a bonnet on top and a small shutoff on each side. Sits above ground, usually against the house. The most common residential sprinkler device on the Front Range.
- DCVA (double check valve assembly): a longer, horizontal assembly with two check valves, two shutoffs, and four small test cocks. Often in a below-ground box or vault.
- RPZ or RP (reduced pressure zone assembly): looks like a double check, plus a relief valve port on the underside between the two checks. It's built to dump water out of that port if something goes wrong. Common on commercial buildings.
- AVB (atmospheric vacuum breaker): small, no test cocks. Often on older sprinkler valves or as a hose-bib vacuum breaker. Not testable.
PVB: pressure vacuum breaker
A PVB has a spring-loaded check valve and an air inlet. If pressure drops, the air inlet opens and breaks the siphon. It protects against back-siphonage only, so it has to be installed higher than every sprinkler head and outlet it serves, typically at least 12 inches above the highest downstream point. That's why you see them sticking up beside the foundation.
PVBs are the device most likely to crack over a Colorado winter, because they sit outside holding water. Draining one properly during a sprinkler blowout is the cheapest insurance you can buy for it. A spill-resistant vacuum breaker (SVB) is a close cousin designed not to spit water when it pressurizes.
DCVA: double check valve assembly
Two independent check valves in a row, each one a backup for the other. A DCVA protects against both back-siphonage and back-pressure, but it's approved for lower-hazard situations, where backflow would be a quality problem rather than a health hazard. Because it doesn't need an air inlet, it can sit below grade, which is why many are in valve boxes or vaults, where your local rules allow it.
RPZ: reduced pressure zone assembly
The heavy-duty one. Two check valves with a relief valve between them that opens and discharges water if pressure in the zone between the checks gets too close to the supply pressure. That design makes it suitable for high-hazard connections: commercial irrigation with chemical injection, boilers, medical and dental equipment, car washes, and many commercial buildings.
An RPZ that's dripping or gushing from its relief port is telling you something: usually a fouled first check valve or a worn relief valve seat. It needs a repair and a retest, not a bucket.
What a backflow test actually is
A certified tester connects a calibrated differential pressure gauge to the test cocks and checks each part against the standard:
- PVB: the air inlet opens when it should, and the check valve holds.
- DCVA: each check valve holds tight on its own.
- RPZ: the first check holds the right pressure differential, the relief valve opens at the right point, and the second check holds.
The results go on a test report that's submitted to your water provider. If something fails, it's usually a dirty or worn part that can be cleaned or rebuilt, then retested.
How often
Colorado's drinking water rules require water providers to run cross-connection control programs. In practice, testable assemblies are tested when they're installed, after they're repaired or moved, and then once a year. Your provider usually mails or emails a notice when yours is due. Our guide covers the rules provider by provider: backflow testing requirements in the Denver metro.
Need it tested?
Singer Plumbing Co.'s owner is an ABPA-certified backflow tester. First test $95 per device for new customers. You get the completed report for your water provider, and a repair price before any work if the assembly fails. Book a backflow test →