It comes down to what could get into the water. Anything that could make people sick calls for an RP. Lower-risk connections can use a double check. Irrigation often gets a PVB or SVB, which only protects against backsiphonage. Your district has the final say.
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?
You'll need your district's OK. Going from an RP down to a double check almost never gets approved, but moving up is common when a building changes use. We do these as a replacement or new install.
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?
Once a year at minimum. A device also needs testing right after it's installed, repaired or moved. Your water district picks the due date and normally mails a reminder ahead of time.