Backflow preventer assembly types and how to pick one

What a backflow preventer assembly is, and why the type matters

A backflow preventer assembly is the device that keeps non-potable water from reversing out of a cross-connection and into the potable supply. It sits where drinking water meets something it must never mix with, an irrigation line, a boiler, a chemical feed, a fire system full of stagnant water, and it holds the line when the pressure tries to push or pull that water backward.

Picking the right type is the whole job, and it comes down to two facts about the connection. How bad is the substance on the other side, the degree of hazard. And which way can the water move, the backflow mechanism. Answer those two and the assembly almost selects itself. Get either one wrong and you can install a perfectly good device that protects against nothing, because it was built for a hazard or a mechanism that is not the one in front of it.

This is the selection guide. The companion basics guide covers what a cross-connection is, back-siphonage versus backpressure, and the air gap from the ground up, and the test-procedure guide covers how each assembly is field-tested every year. Here the focus is the assemblies themselves: what each one is, how it is built, and which connection it belongs on.

Backsiphonage and backpressure: the two mechanisms the type must handle

Two things cause backflow, and the assembly has to handle the one that can actually occur at the connection. Back-siphonage is the pull. The supply pressure drops below the pressure at the connection, usually from a main break, a fire department drawing hard on a hydrant, or a big draw downstream, and the negative pressure siphons water backward the way you draw soda up a straw. Backpressure is the push. The pressure downstream of the connection climbs above the supply, from a boiler, a booster pump, an overhead tank, or a closed loop that heats and expands, and it forces the contents back against the incoming flow.

This decides the type before hazard ever enters the picture. Some assemblies stop only the pull and do nothing against the push. A vacuum breaker breaks a siphon, but it has no answer for backpressure, so it offers zero protection on a boiler or a pumped system that can push back. The reduced pressure assembly and the double check handle both directions.

So the first question on any connection is the mechanism. Decide whether it can see back-siphonage, backpressure, or both, because that one answer rules out whole categories of assembly. A connection that can push back is never a job for a vacuum breaker, no matter how convenient the smaller device looks on the wall.

High hazard versus low hazard, and how it drives the type

The second fact is how dangerous backflow would be, the degree of hazard, and it sets how strong the protection has to be. A high hazard, also called a health hazard, is a connection where reversed water could cause illness or death: sewage, chemicals, pesticides, boiler treatment, process and plating fluids, reclaimed water, medical and lab systems. A low hazard, the non-health or aesthetic kind, is one where backflow would be unpleasant but not a health threat, a change in taste, odor, color, or temperature with nothing toxic behind it.

High hazard demands the most reliable protection, the air gap or the reduced pressure assembly. Low hazard can use a lighter device such as a double check. That single split is what separates the RP from the DC across most of the field, so judging the hazard right is half the selection.

When the hazard is uncertain, default up. Irrigation is the classic example, usually treated as high hazard because nobody controls what gets connected to it later and fertilizer injection turns a lawn system toxic in a season. The hazard classification on any given connection traces back to the adopted plumbing code and the water purveyor's program, so confirm the rating with the AHJ rather than guessing, and lean toward the stronger device when the call is close.

The reduced pressure zone assembly (RPZ/RPBA)

The reduced pressure zone assembly, the RP, RPZ, or RPBA, is the strongest mechanical backflow protection there is and the standard answer for high hazard. It is the testable assembly approved for a health hazard under both backflow mechanisms, so it covers the worst case: a toxic connection that can both pull and push. If you are unsure whether a connection needs an RP or something lighter, the RP is the safe direction to be wrong in, because it protects everything a DC does and more.

It earns that standing with a relief valve the lighter assemblies do not have. The RP is built to dump water to atmosphere the instant its internal protection is threatened, which is also why it has to be installed where that discharge can go somewhere safe. That relief discharge is the RP's defining feature and its main install headache, covered in its own section below.

You find RPs on boilers with chemical treatment, chemical and process feeds, reclaimed water, high-hazard irrigation, and as containment at the service of a hazardous facility. They cost more than a double check, take more room, and need a drain, and every one of those costs buys the protection a health hazard requires. The assembly is commonly built and approved to ASSE 1013; confirm the standard and edition your program adopts.

Inside the RP: two checks and the relief valve

Inside an RP are two independent spring-loaded check valves in series with a hydraulically operated relief valve in the zone between them. That middle zone is the reduced pressure zone the assembly is named for, and it is held at a pressure lower than the supply by design, commonly several psi below the inlet. The pressure difference is what makes the assembly work: as long as the zone stays below the supply, water cannot move backward through it.

The relief valve watches that zone. If the first check fouls and lets supply pressure leak into the zone, or backpressure pushes the second check and raises the zone pressure toward the supply, the relief valve opens and dumps the zone water out to atmosphere through an air-gapped port. The contamination spills to the floor or the drain instead of backward into the supply, and the visible discharge is a built-in tell that a check has failed.

Two checks plus a relief is what gives the RP its high-hazard rating. The first check holds the zone low, the second check is the backstop, and the relief is the fail-safe that opens when either check stops doing its job. The annual field test reads all three in order, the relief opening point and both checks, and that test sequence belongs to the test-procedure guide.

The double check valve assembly (DCVA/DCA)

The double check valve assembly, the DC, DCVA, or DCA, is the workhorse for low hazard. It is two independent spring-loaded check valves in series with shutoff valves and test cocks, and the one thing it does not have is a relief valve to atmosphere. The two checks back each other up: if one starts to leak, the second still holds, which is the redundancy a single check cannot give. It handles both backflow mechanisms, back-siphonage and backpressure, within the low-hazard limit.

No relief valve is exactly what keeps the DC off high-hazard connections. With nothing to dump the zone if both checks are compromised, it has no fail-safe for a toxic substance, so it is approved only where backflow would be a nuisance and not a health threat. That is the line that matters: the moment the hazard is a health hazard, the device steps up to an RP or an air gap.

The DC's advantage is that it discharges nothing in normal protection, so it fits in more places than an RP, including some below-grade vaults where the program allows it, though accessible and above grade is always better for testing. It is a testable assembly built commonly to ASSE 1015, and it carries the same install-and-annual test requirement as the RP. A DC fails quietly, with no weep and no puddle, which is the strongest argument there is for not letting its annual test slide.

What is the difference between an RP and a double check?

The difference is the relief valve and the hazard it lets each one protect. An RP has two checks plus a relief valve that dumps the zone to atmosphere if a check fails, so it is approved for high hazard and both backflow mechanisms. A DC has two checks and no relief, so it is approved for low hazard only, also under both mechanisms. Both protect against back-siphonage and backpressure; what separates them is the degree of hazard each is allowed to guard.

The practical consequences follow from that relief valve. The RP discharges water when it protects, so it must go above grade with a drain and an air gap at the relief port, and it cannot be installed where it could be submerged. The DC discharges nothing in normal service, so it fits in tighter spots and below grade where allowed. The RP costs more, takes more room, and needs a drain. The DC is cheaper and simpler but tops out at low hazard.

Pick by the hazard first. If the connection is a health hazard, it is an RP or an air gap and the DC is off the table. If it is genuinely low hazard, the DC is the right-sized device and an RP is overkill that still needs feeding with a drain. When the hazard is uncertain, the RP is the safe default.

The pressure vacuum breaker (PVB)

The pressure vacuum breaker, the PVB, protects against back-siphonage only and is the common, code-accepted choice for lawn irrigation. It has a single spring-loaded check valve and a spring-loaded air inlet, plus shutoff valves and test cocks, so it can be held under continuous pressure and it can be field-tested, which is what separates it from the simpler atmospheric breaker. When the supply pressure drops, the air inlet pops open, breaks the siphon, and air, not contaminated water, is what comes back through the assembly.

The catch is the one it shares with every vacuum breaker: it does nothing against backpressure. A PVB on a connection that can push back, a pump, a boiler, a head of water above it, is the wrong device and guards against half the problem. It works on irrigation because a typical lawn system sees only back-siphonage, the pull of a main break or a hydrant draw, with no source of backpressure.

Two install rules make or break it. The PVB mounts at least 12 in above the highest downstream outlet, every head and every drip emitter, so gravity and the air inlet can break the siphon. And it spills water out the air inlet when it operates and when it is tested, so a standard PVB belongs outdoors or over a drain, not over a finished ceiling. The assembly is commonly built to ASSE 1020; confirm the height and the standard against the local code.

The spill-resistant vacuum breaker (SVB) and the atmospheric vacuum breaker (AVB)

The spill-resistant vacuum breaker, the SVB or SPVB, is a PVB built not to spill. It does the same job, back-siphonage protection on a connection that can hold continuous pressure, with one check, an air inlet, and test cocks, but it is designed so it does not dump water across the gauge during a test or when it operates. That is what lets it go indoors, where a standard PVB would flood the room. Where you want PVB-style protection inside a building, the SVB is the version that fits, commonly built to ASSE 1056.

The atmospheric vacuum breaker, the AVB, is the simplest and cheapest backflow device, and its limits are the reason it is not a cure-all. It is a single check seat with a float or poppet that drops to admit air when flow stops, with no shutoffs and no test cocks, so it is not a testable assembly. It protects against back-siphonage only, and it carries hard restrictions a PVB does not: it cannot be under continuous pressure, it cannot have a shutoff valve downstream of it, and it cannot be installed in series. Many codes also cap its duty cycle and require it above the flood rim of the highest outlet, commonly 6 in, a height to confirm against the local code.

Those restrictions exist because the AVB's float only works if the body sees atmosphere when flow stops. Leave it under continuous pressure, or put a valve downstream that holds it pressurized, and the float can stick open and the device protects against nothing. It suits intermittent, low-duty spots, a single fixture supply, a lab faucet, a service sink, where flow comes and goes and nothing holds it charged. It is built commonly to ASSE 1001.

When to use a vacuum breaker versus an RP or DC

The dividing line is simple: vacuum breakers stop back-siphonage only, while the RP and the DC stop both back-siphonage and backpressure. So the first question is always the mechanism. If the connection can ever see backpressure, a higher downstream pressure from a pump, a boiler, an overhead tank, or a closed loop, a vacuum breaker is wrong no matter how convenient it looks. You need a DC for low hazard or an RP for high hazard.

When the connection can only see back-siphonage, the vacuum breaker becomes the lighter, cheaper option, and the hazard then picks which one. A PVB or SVB is testable and can hold continuous pressure, so it suits a system that stays charged, like irrigation on a solenoid. An AVB is non-testable and cannot hold pressure, so it suits intermittent fixture-level use only.

The mistake to never make is reaching for a vacuum breaker because it is smaller and cheaper, on a connection that can push back. That is the most common way a real backflow risk hides behind an installed device. Confirm the mechanism before the hazard, and let any chance of backpressure send you to a DC or an RP every time.

The air gap: non-mechanical, absolute protection

The air gap is the one protection with no moving parts and nothing to fail. It is a physical vertical space between the end of the potable supply and the flood rim of the receiving vessel, commonly at least twice the supply pipe diameter and never less than 1 in, so the two are never joined by a continuous column of water. Nothing travels back up through open air, so it stops both backflow mechanisms for any degree of hazard, which is why every mechanical assembly is measured against it.

Its limit is practical. An air gap only works where you can break the pipe open to atmosphere and accept that the downstream side is no longer pressurized, so it fits drains, tank fills, and the relief ports of assemblies, not a connection that has to stay piped and under pressure. The basics guide covers the air gap and its sizing in full. The point here is that when the layout allows it, the air gap is the most foolproof choice on the board, and it is exactly the protection an RP's relief port relies on.

The residential dual check

The residential dual check is the small, non-testable device you find at or near the water meter on a house, and it is not the same animal as a double check valve assembly. It is two checks in a compact body with no test cocks and no shutoffs, built for low-hazard residential service, commonly to ASSE 1024. Many water purveyors set one at the meter as basic containment for a single-family home, where the hazard is low and a full testable assembly would be more device than the connection needs.

Because it is non-testable, it is not proven with a gauge every year the way a DC or an RP is. The maintenance model instead is replacement or rebuild on a cycle, commonly around every 5 years, since the checks wear and there is no test to catch a quiet failure. Confirm the device, the application, and the replacement interval with the local program, because some jurisdictions require a testable assembly even on residential service depending on the hazard and the source water.

Which backflow preventer do I need? Selection by application

Most field decisions come down to a handful of connections, and knowing the usual answer speeds the call. Run the two questions, hazard and mechanism, on each and the device follows. Irrigation with no chemical injection and no pump is high hazard, back-siphonage only, so it gets a PVB above the highest head; add chemical injection or a pump and it sees backpressure or becomes a push-capable high hazard, so it steps up to an RP. A boiler with chemical treatment is high hazard with backpressure, which leaves exactly one answer, an RP, every time.

Fire lines split by additive. A plain wet system with no chemicals is usually low hazard and gets a double check, often a DCDA; a system with antifreeze, foam, or a non-potable source is high hazard and gets an RP, often an RPDA. Commercial food service, processing, plating, photo, mortuary, medical, and lab connections are high hazard and get an RP or an air gap. A closed heating loop that only changes water temperature, with no chemicals, may be low hazard and acceptable on a DC.

The table below is the starting map, not the final word. The water purveyor and the adopted code set the required protection for a given connection, and they push toward the stronger device when the hazard is uncertain, so confirm the call with the AHJ before you order the assembly.

Fire-line detector assemblies: DCDA and RPDA

Fire sprinkler systems are cross-connections too, because the water sits stagnant in the pipe and the system may carry additives, so they get backflow protection sized to the hazard. The wrinkle on a fire line is the detector assembly. Instead of a plain DC or RP, the fire line uses a double check detector assembly, the DCDA, or a reduced pressure detector assembly, the RPDA, which add a small metered bypass line around the main checks, itself protected by a small backflow assembly.

The bypass and its meter exist to catch small flows the large main meter would miss, a slow leak, a weeping head, or an unauthorized tap on the fire line, so the water purveyor can see water moving without slowing the full fire flow when the system trips. The detector assemblies are commonly built to ASSE 1048 for the double check version and ASSE 1047 for the reduced pressure version.

Selection on a fire line follows the same hazard logic, but the authority is doubled. The assembly has to satisfy both the cross-connection program and the fire code, and it must not restrict the required fire flow, so the type and the size get coordinated with both the water purveyor and the fire authority before anything goes in. A fire-line assembly that protects the water but starves the sprinklers has traded one failure for a worse one.

Health, medical, and large-site connections

Health hazards get the top of the board, the RP or the air gap, with no middle ground. Hospitals, clinics, dental, dialysis, labs, and mortuaries run connections where reversed water could carry pathogens, chemicals, or worse, so the protection is the reduced pressure assembly or a physical air gap, chosen high every time. Sterilizers, lab benches, processing tanks, and any equipment that injects or holds a treated fluid is a high hazard by default, and the safe call is to treat an uncertain medical connection as high hazard until the program says otherwise.

Large commercial and data-center sites are dense with high-hazard connections, and they earn an RP each. The mechanical plant alone carries chilled-water and condenser-water makeup with treatment chemicals, boiler feed, glycol loops, and cooling-tower makeup, every one of which can push treated water back toward the supply, so every one is an RP. Add campus irrigation, any reclaimed water used for tower makeup, fire protection, and generator cooling, and a single site can carry dozens of assemblies.

Reclaimed water in particular is high hazard and increasingly common for tower makeup on large sites, so its connections get the strongest protection and clear labeling to keep the two systems from ever being joined. At this scale the inventory and the annual test schedule are a real management task, so the type, the hazard, and the test date for each assembly belong in a tracked record, not in someone's memory.

Installation: orientation, clearance, and test-cock access

A backflow assembly installed wrong fails early, and a few rules carry most of the reliability. Orientation is listed, not optional. Most assemblies are approved for a specific position, many horizontal, some approved vertical, and setting one outside its listed orientation voids the approval and can keep the relief or the checks from working as designed. Read the assembly's listing before you set it, because not every model is approved every way, and the n-shaped vertical RPs have their own rules.

Clearance and access are required because every testable assembly gets tested at install and every year after for the life of the connection. Leave room around the test cocks and the shutoffs for a tester to hang a gauge and work the valves, and keep the assembly reachable instead of buried behind equipment or boxed into a chase. An assembly nobody can reach is an assembly nobody will maintain, and the inspector knows it.

Direction of flow is the rookie error. Every assembly has an arrow, and a check installed backward protects against nothing while looking perfectly normal from the outside. Confirm the flow arrow points downstream before the pipe is buttoned up, flush the line of construction debris before the assembly goes in, and set the height and clearance for the test you will be doing on it every year.

The RP relief discharge and drainage

The RP relief discharge needs a drain and an air gap, and getting this wrong is one of the most dangerous mistakes on a backflow install. An RP is designed to dump water out its relief port when it protects, sometimes a trickle, sometimes a full-bore discharge if a check fails wide open under pressure. That water has to go somewhere safe, which means an air gap at the relief port over a floor drain, a funnel drain, or a path that can carry the assembly's full relief rate.

Two things you never do. Never pipe the relief port solid into a drain line. A solid connection turns the relief discharge into its own cross-connection and can hold the port submerged, which lets the assembly be back-contaminated through the very port that is supposed to protect it. And never install an RP where it can be submerged, in a pit, a flood-prone vault, or below grade, because a flooded RP loses its air gap and its relief protection at once.

Size the drain for a real discharge, not a drip. An RP that fails open can put out far more water than people expect, and a relief that dumps onto a finished floor, into an electrical room, or into a space with no drain is a flood and a callback waiting to happen. Plan the drain before the pipe goes in the wall. This is the one detail on an RP install that earns bluntness: get the relief drainage wrong and you have built a hazard into the device that exists to prevent one.

Installing vacuum breakers above the outlets

Vacuum breakers only work if they are installed where the air inlet can do its job, and the height and the valving rules are not suggestions. A PVB or SVB mounts at least 12 in above the highest downstream outlet, every head, emitter, and hose end the system feeds, so when the supply drops, gravity and the air inlet break the siphon instead of holding a column of water that could be pulled back. Mount it below an outlet and the assembly can sit there looking installed while the siphon path runs right past it.

The AVB carries tighter rules because it has no spring on the line side and no test cocks. It cannot have any shutoff valve downstream of it, because a closed valve downstream holds the body pressurized and pins the float open, killing the anti-siphon action. It cannot be under continuous pressure for the same reason, so it is wrong on any line that stays charged. And it mounts above the flood rim of the highest outlet, commonly 6 in, a height to confirm with the local code.

The thread running through all of it: a misapplied vacuum breaker does not fail loudly, it just quietly protects against nothing. The irrigation PVB mounted below a head, the AVB with a solenoid valve downstream, the breaker put on a line that can push back, all look fine and all are open doors. Confirm the mechanism, the height, and the downstream valving before you walk away from one.

Testable versus non-testable assemblies

Testability splits the assemblies into two groups, and it changes how each one is maintained. The RP, the DC, the PVB, the SVB, and the fire-line detector assemblies are testable: they have shutoff valves and numbered test cocks, and a certified tester proves them with a calibrated gauge at install and at least annually for the life of the connection. That annual test is the only routine check these devices get, because none of them has an alarm or a display to tell you a check has quietly fouled.

The AVB and the residential dual check are non-testable. They have no test cocks, so there is no gauge reading to take, and they are maintained by replacement or rebuild on a schedule instead, commonly around every 5 years for the residential dual check, with the AVB simply swapped when it fails or sticks. Non-testable does not mean maintenance-free; it means the maintenance is a swap on a clock rather than a test.

The full test sequence, the gauge setup, the pass criteria, and the report belong to the test-procedure guide and are not repeated here. The selection point is this: choosing a testable assembly commits the owner to an annual test and a record, so factor that into the type and the location, and never hide a testable assembly somewhere a tester cannot reach it.

Freeze protection and assembly location

Freeze is the leading seasonal killer of backflow assemblies, and where you put one is part of selecting it. Water left in an above-grade assembly through a hard freeze expands as it turns to ice and cracks the body, splits the cover, distorts seats, and tears rubber. Irrigation PVBs and outdoor RPs are the classic casualties, coming up cracked in spring because nobody drained them for winter. A cracked body is a replacement, not a repair.

The prevention is location and winterizing. An outdoor assembly on a seasonal line gets drained down with the test cocks left cracked before the first freeze, or it gets an insulated, heated enclosure so it never reaches freezing. The irrigation contractor who blows the system out in fall and leaves the PVB cocks open hands the spring tester a working assembly instead of a cracked one. Confirm the winterizing approach against the assembly type and the local climate, because what holds up in a mild winter is not enough where the line freezes solid.

Indoors and in a heated space, freeze stops being the constraint and access and drainage take over. The selection lesson is to think about where the assembly lives across a full year before it goes in, because an RP that needs both a drain and freeze protection is a different install than the same RP in a heated mechanical room.

What the inspector checks

What an inspector or a cross-connection surveyor checks on an assembly is a short, predictable list, and it is the same list a crew should run before calling an install done. First, the right type for the hazard and the mechanism: an RP where the hazard is high, a device that handles backpressure where backpressure is possible, not a vacuum breaker on a line that can push back. Second, the orientation matches the listing and the flow arrow points downstream.

Then the practical items. Access and clearance around the test cocks and shutoffs for the annual test. The relief drainage on an RP, an air gap at the port over a drain sized for a real discharge, never piped solid and never submerged. The height on a vacuum breaker, above the highest outlet, with no shutoff downstream of an AVB. And a current test on every testable assembly, with the report on file with the program.

That is the QC pass, and it doubles as the punch list. Walk the assembly against it before the inspector does, because the items that fail inspection, wrong type, backward flow, no drain, no clearance, a flooded RP, are the same items that let real backflow through. The inspection is not a formality on top of the protection. It is a check that the protection is actually there.

Common mistakes

• Putting a double check on a high hazard. A DC has no relief and is low-hazard only; a health hazard needs an RP or an air gap.
• Using a vacuum breaker where backpressure can occur. PVBs and AVBs stop back-siphonage only; a pump, boiler, or overhead tank needs a DC or an RP.
• Piping an RP relief port solid into a drain, or installing the RP where it can be submerged, which lets it be back-contaminated through its own relief.
• Mounting a PVB below the highest downstream outlet, so the siphon path runs right past the air inlet.
• Installing an AVB under continuous pressure or with a shutoff valve downstream, which pins the float open and defeats it.
• Setting an assembly outside its listed orientation, or with the flow arrow backward, which voids the approval or protects against nothing.
• Skipping the annual test on a testable assembly, or hiding it where a tester cannot reach the test cocks.
• Leaving an outdoor assembly full through winter, so a freeze cracks the body and the spring test finds a replacement, not a repair.

What to document

Backflow protection is a documented program, not a one-time install, so each assembly is recorded with the hazard it guards, the mechanism it handles, and why that type was chosen. The record is what lets the next surveyor, tester, and inspector start from the reasoning instead of guessing, and what answers the question when someone later wonders why the device on the wall is the device it is. Logging each assembly and its annual test in a tool like FieldOS keeps the type, the hazard, and the test clock tied to one connection.

Field checklist

Standards and references

Backflow protection sits under overlapping authorities, and the local one usually wins. The plumbing code, the IPC or the UPC as amended, sets where protection is required and which type fits each application. The water purveyor runs the cross-connection control program, requires the assembly by hazard, and sets the annual test, with the final say at the service. The state drinking-water rules sit behind both. The standard that controls any given call is the one the AHJ has actually adopted and enforces, so confirm the required protection and the hazard rating with the purveyor and the adopted code before you commit to a type.

The assemblies themselves are built and approved to ASSE International standards, which map to the types: ASSE 1013 for the reduced pressure assembly, 1015 for the double check, 1020 for the pressure vacuum breaker, 1056 for the spill-resistant version, 1001 for the atmospheric vacuum breaker, 1024 for the residential dual check, and 1047 and 1048 for the reduced pressure and double check detector assemblies. Confirm each standard and its edition against your jurisdiction's approval list, because the standards are revised on a cycle.

Recognized references include the AWWA M14 manual, Backflow Prevention and Cross-Connection Control, and the University of Southern California Foundation for Cross-Connection Control and Hydraulic Research, whose listings and field procedures many programs adopt. Two rules carry the most weight in the field and are worth repeating: the hazard picks the type, and an RP's relief discharge must drain to an air gap, never piped solid and never submerged.

Units and terms

Backflow selection carries its own shorthand, and the same assembly goes by a few names across forms, manufacturers, and codes.

The assemblies are abbreviated by type, with the high-hazard unit and the vacuum breakers each carrying two or three common names. Heights are given in inches above the highest downstream outlet or the flood rim. The degree of hazard and the backflow mechanism are the two facts every selection turns on, so they show up on every survey form and every assembly record.

RP / RPZ / RPBA

Reduced pressure zone assembly; two checks plus a relief valve, for high hazard and both backflow mechanisms (ASSE 1013)

DC / DCVA / DCA

Double check valve assembly; two checks, no relief, for low hazard and both mechanisms (ASSE 1015)

PVB

Pressure vacuum breaker; one check and an air inlet, back-siphonage only, holds continuous pressure, testable (ASSE 1020)

SVB / SPVB

Spill-resistant vacuum breaker; a PVB built not to spill, so it can be installed indoors (ASSE 1056)

AVB

Atmospheric vacuum breaker; the simplest device, back-siphonage only, no continuous pressure, no downstream valve, non-testable (ASSE 1001)

Air gap

A physical open space between the supply and a vessel; non-mechanical, absolute protection for any hazard

DCDA / RPDA

Double check and reduced pressure detector assemblies; fire-line versions with a metered bypass (ASSE 1048 / 1047)

Degree of hazard

Whether backflow would threaten health (high) or only quality such as taste and odor (low)

Back-siphonage / backpressure

The two backflow mechanisms: a pull from negative supply pressure, or a push from higher downstream pressure

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