The right assembly depends on the hazard. An RP is required for high-hazard connections that could carry toxic contaminants. A double check is approved for lower-hazard connections that would only affect taste or appearance. A PVB or SVB protects against backsiphonage only, so it's common on irrigation but can't be used where backpressure is possible. Your water district makes the final call.
Side-by-side comparison
| RP (RPZ) | Double check (DC) | PVB / SVB | |
|---|---|---|---|
| Hazard level | High (health hazard) | Low (non-health hazard) | High or low, backsiphonage only |
| Protects against | Backsiphonage and backpressure | Backsiphonage and backpressure | Backsiphonage only |
| How it works | Two checks with a relief valve between them | Two independent checks | One check plus an air inlet |
| Typical uses | Medical/dental, restaurants, car washes, chemical-injection irrigation, industrial | Fire sprinkler services, some commercial and residential services | Lawn and landscape irrigation |
| Installation rule | Above grade, never in a pit; relief valve needs drainage | Can be below grade where allowed, with test access | At least 12 in. above the highest downstream outlet |
| Discharges water? | Yes, when relief valve opens | No | PVB can spit at startup; SVB is designed not to |
How to tell which one you have
- RP: look for a drain opening on the underside, between the two checks. It has four small test valves.
- Double check: two check sections in a row and four test valves, but no drain opening underneath.
- PVB/SVB: one body sitting up on a pipe, with a cap on top where air gets in and two test valves.
Can I switch types?
Only with your water district's approval. Downgrading from RP to double check is rarely allowed. Upgrading is common when a property's use changes. We handle it under replacement or installation.
Testing differences
Every type must be tested annually, but the procedures differ. An RP test checks the relief valve opening point and both checks. A double check test confirms each check holds. A PVB test checks the air inlet and the check. Details are on our testing page.
Related questions
Is an RPZ the same as an RP?
Yes. RPZ (reduced pressure zone) and RP (reduced pressure principle) refer to the same type of assembly.
How often does a backflow preventer need to be tested?
In California, backflow prevention assemblies must be tested at least once a year. They also need a test right after installation, after any repair, and after being moved. Your water district sets the exact due date and usually mails a notice before it.