First, it’s all about the hazard
Backflow protection is matched to how dangerous the water on your side of the connection could be:
- High health hazard: anything that could make people sick or worse, such as chemicals, fertilizers, sewage or medical waste. Requires an air gap or a reduced-pressure assembly.
- Low hazard: substances that affect taste, color or odor but aren’t a health threat. A double check may be acceptable.
It also matters whether the risk is back-siphonage (supply pressure drops) or backpressure (pressure on your side rises, like from a pump or boiler). Vacuum breakers only handle back-siphonage.
Quick comparison
| Type | Hazard | Backpressure? | Typical uses |
|---|---|---|---|
| RPBA Reduced pressure | High or low | Yes | Restaurants, medical & dental, labs, boilers with chemicals, irrigation with injectors |
| DCVA Double check | Low only | Yes | Many lawn irrigation systems, domestic services, standard fire sprinklers |
| PVBA Pressure vacuum breaker | High or low | No | Residential irrigation, installed above the highest sprinkler |
| SVBA Spill-resistant vacuum breaker | High or low | No | Indoor applications where spitting water is a problem |
| RPDA / DCDA Detector assemblies | High / low | Yes | Fire-sprinkler lines, with a bypass meter |
| Air gap | High or low | Yes | Tanks, industrial fills. Inspected, not tested |
Reduced pressure backflow assembly (RPBA)
Two independent check valves with a relief valve in between. The zone between the checks is kept at lower pressure than the supply. If either check leaks, the relief valve opens and dumps water out the bottom rather than letting it travel backward.
How to spot it: a funnel-shaped relief port on the underside, between the two check sections, plus four test cocks. RPs must be installed above ground with room to drain. Water discharging from that port means something inside needs attention.
Double check valve assembly (DCVA)
Two independent spring-loaded check valves between two shut-off valves, with four test cocks, and no relief port. Handles backpressure and back-siphonage, but only for low-hazard connections.
How to spot it: a long, fairly uniform body, often in an irrigation valve box or on a fire line, with no discharge opening on the bottom.
Pressure vacuum breaker assembly (PVBA)
A spring-loaded check valve plus an air-inlet valve under a bonnet on top. If supply pressure drops, the air inlet opens and breaks the siphon. It can’t handle backpressure, so it must sit at least 12 inches above the highest downstream outlet (typically the highest sprinkler head).
How to spot it: an above-ground, upside-down-U setup near the house with a round bonnet on top and two test cocks. This is the classic Northwest lawn-sprinkler backflow, and the one most often split by freezing.
Spill-resistant vacuum breaker (SVBA)
Works like a PVBA but is designed not to spit water when it pressurizes, which makes it a better fit indoors.
Detector assemblies (RPDA & DCDA)
Fire-sprinkler versions of the RP and double check, with a small bypass meter that records low flows to reveal leaks or unauthorized use. They’re large, usually in a riser room or an outdoor vault, and testing them is coordinated with your fire-sprinkler contractor or monitoring company.
What about atmospheric vacuum breakers?
AVBs are simple, inexpensive devices found on some older irrigation systems and hose connections. They aren’t testable and can’t be under continuous pressure, so where your purveyor requires a testable assembly, an AVB won’t satisfy it.
Which one does your purveyor require?
For premises isolation, your purveyor decides based on the hazard. For equipment inside a building, the plumbing code and your local official decide. If you’re adding irrigation, a boiler or new equipment, talk to us before you buy an assembly. Installing the wrong type is an expensive mistake.
Frequently asked questions
Is an RPZ the same as an RPBA?
Yes. RP, RPZ (reduced pressure zone) and RPBA all refer to the same type of assembly.
Why is my irrigation system’s backflow different from my neighbor’s?
Installation date, the purveyor’s requirements at the time, and whether chemicals are injected into the system all affect what’s required. Both may be perfectly compliant.
Can I replace a double check with an RP?
An RP provides a higher level of protection, so it’s generally acceptable, but it needs above-ground installation with drainage for the relief valve. We can advise on your specific situation.