What's Actually Inside a Backflow Assembly?
Most people call it a backflow preventer. On the paperwork it is a backflow prevention assembly, and that word is doing real work. An approved assembly is not just the valve body in the middle — it is the two shutoff valves, the internals, and the test cocks, all as one listed unit. That is the whole thing your tester is certifying, and it is why a perfectly healthy check valve can still produce a failed report.
Here is what each part does, and how each one fails.
The shutoff valves
Every approved assembly has one on each end. They exist so the device can be isolated for testing and repair without shutting down the building, and they are part of the listed assembly — not accessories bolted on afterward.
They are also a routine cause of trouble. A gate valve that has been sitting half open for three years mimics low water pressure perfectly. One that no longer seats means the tester cannot hold a differential, and the report comes back failed on a component that has nothing to do with backflow protection. Confirming both shutoffs are fully open costs nothing and rules out a surprising amount.
The check valves
Two of them, in series, independently acting and spring loaded. Water pushes them open in the normal direction, and the springs close them the moment flow tries to reverse. Two instead of one is deliberate redundancy — if the first fouls on a piece of scale, the second is still holding.
These are not pass or fail in a loose sense. They are measured, in pounds per square inch differential (psid), against the criteria most California purveyors work from:
| Assembly | Requirement |
|---|---|
| Double check (DC / DCVA) | Each check holds at least 1.0 psid |
| Reduced pressure (RP / RPZ) | Second check at least 1.0 psid; first check at least 5.0 psid, and no less than 3.0 psid above the relief valve opening point |
| Pressure vacuum breaker (PVB) | Check at least 1.0 psid; air inlet opens at 1.0 psid or higher |
That last RP requirement is the one people miss. The first check has to stay meaningfully stronger than the relief valve, or the relief starts discharging during normal operation.
Check valves fail from debris on the seat, worn springs, and hardened rubber. Sand and scale after a water main break will take out a check that was fine the week before.
The relief valve — RP assemblies only
This is the component that gets described wrong more than any other, including in most of what is written about it online.
An RP's relief valve is a differential relief valve. It is not there to bleed off excess pressure and it does not protect your plumbing from over-pressurization. It watches the difference between incoming supply pressure and the pressure in the zone between the two check valves. Supply pressure holds it shut. When that differential collapses — because the first check is leaking, or because supply pressure dropped — the relief valve opens and dumps that middle zone to atmosphere.
Two things happen when it does. The zone can no longer push anything upstream, because it is now open to the air. And you get an unmistakable visual signal that something is wrong, right on the outside of the device. It has to open at 2.0 psid or higher to pass.
An RP never shuts off your water service. That is not something it can do. If you want the mechanics in more detail, we wrote a whole piece on how the relief valve actually works, and another on what continuous discharge means. Short version: a steady dribble is almost always a fouled or worn first check, not a bad relief valve.
The test cocks
Small brass valves, usually four on an RP and three on a DC, numbered from the inlet side. They are the ports the differential gauge hooks to. Without them none of the numbers above can be measured, which is exactly why a device with no test cocks is not an approved assembly and cannot be certified.
They seize, they plug with scale, and the ones on outdoor devices get snapped off. Replacing them is minor work, but it has to happen before a test can be completed.
The air inlet — PVB and SVB only
A pressure vacuum breaker has one check valve and a spring loaded air inlet poppet. When supply pressure drops, the poppet opens, air comes in, and the siphon breaks.
This one is worth being precise about, because it is commonly stated backwards. A PVB is not a low hazard device. It is approved for high hazard applications — irrigation with fertilizer injection, for example. Its limitation is not hazard level, it is direction. A PVB protects against backsiphonage only and offers no protection whatsoever against backpressure. That is why it has to sit at least 12 inches above the highest downstream outlet, and why it is the wrong choice anywhere a pump or a boiler can push water back at it.
Which assembly your property needs comes down to hazard level and application, and that call belongs to your water purveyor, not to you or to us.
Body, seats, springs, and seals
Bronze, ductile iron, or stainless, depending on size and manufacturer. The wear items inside are the rubber — seat discs, o-rings, diaphragms — and the springs behind them.
Repair kits are specific to make, model, and size. A 2-inch Febco 825Y and a 2-inch Watts 909 are not interchangeable in any way that matters. This is why we photograph and record the make, model, size, and serial of every device we test. It is what lets us show up to a repair with the right kit already on the truck instead of scheduling a second trip.
What is not part of the assembly
- Strainers. Useful upstream on dirty water, but optional and not part of the listed unit.
- Enclosures and anti-theft cages. Protection for the device, not part of it.
- Air gap fittings and drain lines. Required to carry RP relief discharge safely away, but separate from the assembly. On an indoor RP, an undersized drain is how a relief event turns into a flooded mechanical room.
Why the anatomy shows up on your report
A failed test names a component. Once you can read the parts, the report stops being a verdict and starts being a repair scope. First check below 5.0 psid is a rebuild kit. Relief opening at 1.4 psid is a relief valve. Number 2 shutoff will not hold is a valve replacement. Same failed report, three very different jobs and three very different prices.
That is the part worth knowing as an owner or a property manager. You do not need to run the test yourself. You just need to be able to tell whether the number on the page justifies the number on the estimate.