Backflow Prevention Devices and Assemblies

Reduced Pressure Zone Assembly (RPZ, RP, RPBA, RPPA)

• Protects againstBacksiphonage and backpressure, high (health) hazard
• PartsTwo independent spring-loaded check valves, a hydraulically operated relief valve between them, two shutoffs, four test cocks
• Where requiredAny health-hazard connection: medical/dental/vet, restaurants with chemical feed, car washes, labs, manufacturing, irrigation with fertilizer injection, private wells on city water, boilers with chemical treatment, fire lines with antifreeze or foam
• InstallationHorizontal (most models), minimum 12 in. above grade or flood level, never in a pit or vault that can flood, needs a drain that can take full relief-valve discharge, protected from freezing
• Test criteria (USC)Check #1 holds ≥ 5.0 psid; check #2 holds ≥ 1.0 psid; relief valve opens at ≥ 2.0 psid

The RPZ is the highest level of mechanical protection. Between its two checks is the zone of reduced pressure: a chamber deliberately held a few psi below supply pressure. If either check leaks, or pressure reverses, the relief valve opens and dumps water out of the zone to atmosphere — so the contaminated water goes on the ground, not into the main. That discharge is a feature, not a defect, but it is also why an RPZ can never go where the discharge could flood something.

Because it is spring-loaded and held under a pressure differential, an RPZ costs a few psi of pressure and, on a large domestic service, that matters for sizing. It also dribbles briefly during normal pressure fluctuations; a steady stream from the relief port means a fouled first check or a failed relief valve and it needs service.

Double Check Valve Assembly (DCVA, DC, DCA)

• Protects againstBacksiphonage and backpressure, non-health (pollutant) hazard only
• PartsTwo independent spring-loaded check valves in series, two shutoffs, four test cocks; no relief valve
• Where requiredIrrigation without chemical injection (most common residential/commercial DC), fire sprinkler systems without additives (as a DCDA), domestic heating systems, degreasers, buildings with large elevation differences, general commercial service isolation where no health hazard exists
• InstallationHorizontal or vertical per model listing, may be installed below grade in a vault or box (no discharge), keep accessible for testing, protect from freezing
• Test criteria (USC)Each check holds tight in reverse and closes at ≥ 1.0 psid

Two checks in series: if one sticks or leaks, the second still holds. That redundancy is why it is accepted for backpressure. What it lacks is any way to tell you it has failed — there is no relief valve, no discharge, no visual sign. A double check with both checks fouled looks exactly like a good one. That is the entire argument for the annual test, and it is why Texas will not accept a DC on a health hazard.

Most in-ground irrigation systems in North Texas have a DC in a green valve box near the meter. Common failure: debris on a check seat after a main repair or a new installation flushes grit through it.

Pressure Vacuum Breaker (PVB)

• Protects againstBacksiphonage only. Cannot stop backpressure — no pumps, boilers or elevated tanks downstream
• PartsOne spring-loaded check valve, one spring-loaded air inlet valve above it, two shutoffs, two test cocks
• Where requiredIrrigation systems (the most common residential unit in Texas), hose bib manifolds, non-pressurized process outlets. *Approved for health hazards only where the only possible backflow mechanism is backsiphonage
• InstallationVertical, at least 12 in. above the highest downstream outlet or sprinkler head, outdoors or where the air-inlet discharge can spill harmlessly; will freeze if not protected
• Test criteria (USC)Air inlet opens at ≥ 1.0 psid; check valve holds at ≥ 1.0 psid

The PVB works on a simple idea: if supply pressure ever drops, a spring pops the air inlet open and lets air into the line, which breaks the siphon before water can be pulled backward. Under normal pressure the air inlet is held shut and the check keeps the line closed. Because it lets air in it also lets a little water out when it operates, so it goes outdoors. The 12-inch height rule exists so no sprinkler head can sit higher than the air inlet and back-siphon through it.

The PVB is the unit you see on a riser next to a house with a sprinkler system. It is compact, cheap and easy to rebuild, and it is also the assembly most often destroyed by a hard freeze because it sits above ground full of water. If your irrigation system injects fertilizer, a PVB is no longer enough — Texas rules move that connection to an RPZ.

Spill-Resistant Vacuum Breaker (SVB, SRVB)

• Protects againstBacksiphonage only, same limits as a PVB
• PartsOne check, one air inlet with a bleed/vent arrangement that keeps it from spitting water on start-up, two shutoffs, one test cock plus a vent port
• Where requiredSame applications as a PVB but indoors or anywhere spillage from a standard PVB would be a problem — labs, commercial kitchens, mechanical rooms
• InstallationVertical, 12 in. above the highest downstream outlet, accessible for testing
• Test criteria (USC)Air inlet opens at ≥ 1.0 psid; check holds at ≥ 1.0 psid

A PVB spits a little water every time the line pressurizes and the air inlet snaps shut. Indoors that is a nuisance and sometimes a real problem. The SVB adds an internal design that seals the air inlet before the check opens, so it does the same job without the spill. Functionally it is a PVB for indoor use and it is tested the same way.

Atmospheric Vacuum Breaker (AVB)

• Protects againstBacksiphonage only
• PartsA single float or poppet that drops open to atmosphere when flow stops; no springs, no test cocks
• Where requiredHose bibs, shampoo sinks, laboratory faucets, some commercial kitchen equipment, older irrigation zone valves (one AVB per zone, downstream of the valve). Listed as acceptable for those items in 30 TAC 290.47(f)
• Installation6 in. above the highest downstream outlet, NO shutoff valve downstream of it, must not be under continuous pressure for more than 12 hours — it only works when the water is off
• Test criteriaNone. An AVB cannot be field tested; it is inspected visually

The simplest device: while water flows, it lifts a float that seals the vent; when flow stops, the float drops and the vent opens to air, so there is no way to siphon through it. The catch is that it only works with the water off and nothing closed downstream. Put a shutoff valve after it and hold it under pressure, and the float stays seated, the poppet can stick, and the protection is gone. That is why it is a device, not an assembly, and why cities push irrigation systems toward a PVB or DC at the point of connection instead.

Air Gap (AG)

• Protects againstEverything — there is no pipe for water to flow back through
• PartsNone. A physical, unobstructed vertical separation between the supply outlet and the flood-level rim of whatever it feeds
• Where requiredSewage lift stations and ejectors, chemical-additive cooling towers, trap primers, bulk water fill stations (30 TAC 290.44(h)(3)), tank fills, anywhere an RPZ is unacceptable
• InstallationGap of at least twice the supply pipe diameter and never less than 1 in.; the outlet must not be able to be submerged, and the gap cannot be defeated by a hose
• Test criteriaVisual inspection: is the gap still there and still the right size?

Water falls through open air into a tank, sink or drain. Nothing mechanical means nothing to wear out, freeze or foul, which is why it is the required protection for the worst hazards. Its costs are pressure (you lose all of it — the tank side needs its own pump) and space. In practice an air gap is designed into the equipment; we verify it during a hazard survey rather than “testing” it.

Fire line detector assemblies (DCDA, RPDA)

A fire sprinkler system is a cross-connection: a large volume of water that sits in black-steel pipe for years, sometimes with antifreeze or foam. The assembly on a fire service is a full-size double check or RPZ with a small bypass line carrying a water meter and its own small check valve. The bypass meter “detects” low flow — a leak or unauthorized use — without the pressure loss of metering the whole fire main. A system with plain water gets a DCDA; one with antifreeze, foam or a non-potable auxiliary supply gets an RPDA.

Texas requires the tester on a fire line to be employed by a TCEQ-approved fireline contractor. TCEQ regulatory guidance RG-345 covers the specifics. We are happy to test the domestic and irrigation assemblies at a commercial property and coordinate the fire line test with your sprinkler contractor.