Reduced Pressure (RP) Backflow Preventers: How They Actually Work
There is a description of reduced pressure assemblies floating around that says the device senses a drop in supply pressure and shuts off the water to prevent backflow. That is wrong, and it is worth correcting up front, because it leads people to expect the wrong behavior from their assembly.
An RP does not shut off your water. It never closes your service. What it does is dump water on the ground.
What is in the assembly
Front to back: a shutoff valve, the first check valve, a differential pressure relief valve, the second check valve, another shutoff valve, and four test cocks. The space between the two checks is the reduced pressure zone the assembly is named after.
How it works
Supply pressure acts on a diaphragm to hold the relief valve closed. The assembly is designed so the zone between the two checks stays at a pressure meaningfully below the supply pressure — that differential is what keeps the relief shut.
Lose that differential and the relief valve opens and discharges to atmosphere. That happens if the first check starts to leak, if supply pressure falls away, or if backpressure pushes against the second check. The discharge is the protection: it drops the zone to atmospheric pressure and creates an air gap right in the middle of the assembly, so there is no continuous path back to the main.
Note what is being compared. The relief valve senses supply pressure against the zone between the two checks — not inlet against outlet. Getting that wrong is the most common error in write-ups of this device.
Why it is the highest level of mechanical protection
An RP handles both backsiphonage and backpressure, and it is the only mechanical assembly accepted on health-hazard connections — anything where backflow could introduce something that makes people sick. Chemical injection on irrigation, boilers and closed loops with additives, wells and auxiliary supplies, medical and dental equipment, most industrial process water.
A double check is fine for pollutants. Contaminants get an RP. The comparison is laid out in RPZ vs. DCVA explained.
The discharge is a feature, not a defect. When an RP dumps water, it is telling you something is wrong — while still protecting the water supply. A double check in the same condition fails silently. That visible failure is a large part of why RPs are required where the stakes are highest. If yours is discharging, see why the relief valve on an RP discharges water.
Installation, briefly
Because the relief valve has to discharge freely, an RP has real siting requirements. Horizontal orientation unless that specific model is approved otherwise. Minimum twelve inches from grade to the bottom of the assembly. Never in a pit or vault where the relief port could be submerged. The relief is never hard-piped into a sewer, storm drain, or vent — if discharge is routed anywhere, it goes through an air gap. And the site needs to handle the water, because sooner or later there will be some. More detail in can a backflow preventer be installed horizontally.
Testing and maintenance
Annual testing, performed by a certified tester with a calibrated differential gauge. The relief valve has to open at 2.0 psid or more, the first check has to hold at 5.0 psid or more, and the second check has to hold at 1.0 psid or more.
These are mechanical assemblies with rubber inside, and that rubber is a wear item. The idea that an RP does not need regular attention is another claim worth retiring — seats, discs, springs, and diaphragms all degrade, faster in water with sediment or aggressive chemistry. Most common failures are repairable with a rebuild kit rather than a full replacement. Any assembly that has been opened is out of compliance until a certified tester re-tests it and files a passing report.
Common questions
My RP is discharging constantly. Do I need a new one?
Usually not. Continuous discharge most often means debris on the first check seat or worn rubber, and a rebuild fixes it. It can also mean fluctuating supply pressure or a pressure problem elsewhere on the system, which is worth ruling out before replacing parts.
Can an RP go inside a building?
Yes, and it often has to. What it needs is a floor drain or other means of handling discharge, plus the same clearance and height requirements. An indoor RP with nowhere for the water to go is how you end up with a flooded mechanical room.
Can I replace an RP with a double check to stop the discharging?
No. The device level is set by the hazard at your connection, and that determination belongs to the water purveyor and the health jurisdiction. Downgrading is not a decision available to the property owner, and an unauthorized swap will be caught at the next test.
How long do they last?
The body commonly lasts decades. Internal rubber typically wants attention every five to ten years, sooner in hard or dirty water. Annual testing is what tells you where yours is.