Field Guide
What kind of backflow
assembly do I have?
Four common types, and they are not interchangeable. You can tell them apart from the driveway once you know what you are looking at — no tools, no opening anything.
Field Guide
Four common types, and they are not interchangeable. You can tell them apart from the driveway once you know what you are looking at — no tools, no opening anything.
Start here
Almost every assembly you will run into can be identified by two things: how many test cocks it has, and whether anything hangs off the bottom of it. Test cocks are the small valves on the body — the ports a tester screws gauge hoses onto. They are not the shutoff valves and they are not there for you to open.
A double check and an RPZ are the same shape, the same size, and both have four test cocks. The difference is underneath. An RPZ has a relief valve on the bottom that can discharge water; a double check has a flat, closed underside. That single part is why one of them can sit in an underground vault and the other one never can.
One at a time
What it looks like, where it tends to live, and what it is protecting you from.
PVB — and its close cousin the spill-resistant vacuum breaker, SVB
The tall one in the mulch bed. A vertical brass body on a riser, with a wide cap on top that houses a spring-loaded air inlet. When supply pressure drops, that inlet opens and lets air into the line, which breaks the siphon before anything can be pulled backward.
The tellIt stands straight up and it is the only common assembly with a wide cap on top. An SVB looks nearly the same but will not spill water out of the bonnet on start-up, which is why it gets used where a spill would be a nuisance.
DCVA, DC
Two independently acting spring-loaded check valves in series, inside one horizontal body, with a shutoff at each end. Redundancy is the whole idea: if one check fouls on a piece of grit, the second one is still holding. It handles both backsiphonage and backpressure, which is what separates it from a vacuum breaker.
The tellHorizontal, four test cocks, and a smooth closed bottom. If you can run your hand under it and find nothing, it is a double check.
RPZ, RP, RPZA
Two checks like a double check, plus a third component: a relief valve between them that monitors the pressure in the middle chamber. If either check starts to leak backward, the relief valve opens and dumps that water on the ground rather than letting it move toward the supply. A failure announces itself outside the pipe instead of quietly inside the drinking water.
The tellSomething hangs off the bottom. If you see a port, a funnel, a drain line or a wet spot underneath it, you are looking at an RPZ. Water coming out of that port is the assembly doing its job, not necessarily a broken part — but it does mean it needs attention.
AVB — and the small hose-bibb version screwed onto an outside spigot
The simplest device of the four and the only one on this page that is not a testable assembly. A float and a poppet sit above a vented cap; lose pressure and the float drops, air comes in, the siphon breaks. There is nothing to attach a gauge to, so it is not tested — it is inspected, and replaced when it fails.
The tellSmall, plastic or light brass, no test ports, and nothing downstream of it that can be shut off. If a valve has been added after one of these at some point, the device has been defeated and no longer protects anything.
Before you call
Every testable assembly carries an identifying plate or tag, usually stamped into the body or wired to it. Four things off that tag save a trip and let anyone quote the work accurately without standing in your yard first.
The serial number is the one people skip and the one that matters most. Water providers track assemblies by serial number, not by address, so a report filed against the wrong serial can sit unmatched in a system while the property owner believes they are current.
If the tag is corroded or awkward to reach, a clear photo of the assembly from about three feet back — plus one of the tag if it is legible — is usually enough to identify the type and size.
Close, but no
All four of these live in roughly the same place and get photographed by mistake regularly.
Pressure reducing valveA bell-shaped brass body with an adjustment screw and a locknut on top, usually right where the service enters the building. It manages pressure. It does nothing about backflow and it has no test cocks.
Water meterRound, with a register or a dial on top, often in its own box at the property line. It measures. Some meter setters have a check valve built in, which is not the same thing as a testable assembly.
Irrigation master valveUsually plastic, in a green or black valve box below grade, with a solenoid and wires running to it. It opens and closes on the controller. No test cocks, no protection.
Hose-bibb vacuum breakerThe small chrome or brass cap screwed onto an outside spigot. It is a real device and it does real work, but it is the non-testable kind — there is nothing to file a report on.
Send us a photo. Identifying the assembly type and size is the first thing that happens anyway, and it costs nothing to sort out before scheduling opens.