RP vs. DCVA: Which Backflow Preventer Does Your Property Need?
Two assemblies dominate commercial and residential backflow prevention in Southern California: the reduced pressure principle assembly (RP, often called an RPZ) and the double check valve assembly (DCVA, or DC). They look broadly similar sitting on a riser. They are not interchangeable, and in almost every case you do not get to pick which one goes on your property.
What each one actually does
Double check valve assembly (DCVA)
Two independent, spring-loaded check valves in series, with shutoff valves on either end and four test cocks. Each check is designed to hold against reverse flow on its own. Two of them in series means one can foul and the other still holds.
What a DCVA does not have is any way to relieve pressure, and no way to tell you it has failed. A fouled DCVA looks exactly like a working one from the outside. You find out at the annual test.
Reduced pressure principle assembly (RP)
Same two checks in series, plus a third component between them: a differential relief valve, and the zone between the checks that it protects.
The relief valve does not watch absolute pressure. It watches the difference between the supply pressure ahead of the first check and the pressure in the zone between the two checks. It is designed to hold that zone meaningfully below supply pressure. If the first check starts to leak backward and zone pressure climbs toward supply pressure, the relief valve opens and dumps that water to atmosphere rather than letting it move upstream.
That is the crucial distinction. An RP fails to open air, visibly, onto the ground. A DCVA fails silently.
Practical consequence: an RP needs somewhere for that water to go. It cannot sit in a pit or a below-grade vault where a discharge would submerge the relief port, and it needs adequate clearance above grade. A DCVA has no such restriction, which is one reason DCVAs still show up in vaults.
Hazard level is what decides it
The choice is not about budget or preference. It is about what would happen if contaminated water made it back into the public supply.
| Hazard | Typical assembly |
|---|---|
| Health hazard — anything that could cause illness or death. Chemicals, boilers with additives, medical and dental facilities, irrigation with fertilizer or pesticide injection, reclaimed water, auxiliary supplies and wells. | RP |
| Non-health hazard — degrades quality but is not a health risk. Many fire lines without additives, some closed loops, certain low-risk process water. | DCVA may be acceptable |
In California, that determination is made by your water purveyor and the local health jurisdiction under Title 17 and the state's cross-connection control requirements, not by the property owner and not by the plumber. Districts also differ from one another. An assembly type accepted in one city is sometimes rejected across the boundary line for the same kind of building.
If you are being told to upgrade a DCVA to an RP, that is usually a hazard reclassification, not an upsell.
Where each one typically lands
RP assemblies dominate commercial service and irrigation in most of our service area, because irrigation systems are treated as health hazards in California and irrigation is everywhere.
DCVAs appear most often on fire service lines that carry nothing but potable water, and in some containment applications where the hazard has been evaluated as non-health. Fire protection is its own conversation, since a fire line has flow and pressure-loss requirements a domestic line does not, and RPs cause more pressure loss than DCVAs.
What this means at test time
Both are tested annually, and both are $90 per device in most of Los Angeles and Orange County. The test procedures differ. A DCVA test checks each check valve independently for its required holding pressure. An RP test does that too, and adds the relief valve opening point, which has to open before the differential falls below the minimum.
An RP has more to go wrong precisely because it does more. That is not a defect. A device that tells you it has failed is worth more than one that does not.
Common questions
Can I replace an RP with a DCVA to stop the discharging?
No. If the district classified the connection as a health hazard, an RP is required, and swapping it down is a compliance failure. If it is discharging, the device needs attention, not a downgrade.
Is an RPZ the same thing as an RP?
Yes. RPZ, RPZA and RP all refer to the same reduced pressure principle assembly. RP is the term most test forms use.
Does a DCVA cost less to test?
Not with us. Testing is $90 per device in our core service area regardless of assembly type. Repairs differ, because the parts differ.
Who decides which one my property needs?
Your water purveyor, based on the hazard at your connection. If you are not sure what you have or what you are supposed to have, send us a photo and we can usually tell you from the tag and the body casting.