Backflow Prevention Reference
Choosing an assembly is two questions, in this order: which direction can backflow happen here, and how bad is what would come back. Get the direction wrong and nothing else helps — because no vacuum breaker of any kind protects against backpressure, whatever its name suggests. That one fact rules out 4 of the 10 devices below the moment a boiler, a pump, or an injector is involved.
The two directions backflow happens
Every device on this page stops backsiphonage. Only some stop backpressure, and that is the split that decides most jobs. Work out which is physically possible at the connection before looking at anything else.
Backsiphonage
Supply pressure drops below the downstream pressure and the line siphons backwards — a water main break, a hydrant flowing, or a pump starting on the same main. The classic case is a hose left lying in a full bucket.
Backpressure
The downstream system is pushed above supply pressure and forces water back — a boiler, a pressurised process line, a booster pump, or elevation head from piping above the supply main.
The mistake worth naming
A vacuum breaker works by letting air in when pressure drops. Push water at it from below and the air inlet is held shut by the very pressure it is supposed to relieve, so it passes backflow straight through. The 4 devices with "vacuum breaker" in the name — HVB, AVB, PVB, SVB — are backsiphonage-only, and no amount of mounting height changes that. Where backpressure is possible you need a double check for a low hazard, or a reduced pressure principle assembly for a high one.
Hazard class — pollution or contamination
Once you know the direction, the hazard class narrows it to one device. The IPC's own words are pollution and contamination; the trade says low and high hazard and means the same thing.
Low hazard
Code term: PollutionA substance that would affect the taste, odour, or colour of the water but is not a health risk — a fire sprinkler line with nothing but potable water in it, or a plain irrigation zone.
High hazard
Code term: ContaminationA substance that could cause illness or death — fertiliser or pesticide injection, boiler treatment chemicals, sewage, or any process water. Anything you are unsure about is treated as high hazard.
When the two are combined, only Air gap, RPZ / RPBA, RPDA cover a high hazard under backpressure — and of those, the air gap is not an in-line device at all. That is why the RPZ ends up on so many commercial jobs: for a genuine high-hazard connection under pressure, there is nothing else.
The assemblies, least to most protective
Read the two middle columns first. "Continuous pressure" means the device may sit under supply pressure for more than twelve hours at a time — the column that catches people out, because HVB and AVB may not, and a shutoff valve downstream of one is the classic installation fault.
| Assembly | Standard | Backsiphonage | Backpressure | Continuous pressure | Max hazard |
|---|---|---|---|---|---|
| Air gap Air gap | ASME A112.1.2 · IPC 608.15.1 | Yes | Yes | Yes | High |
| HVB Hose connection vacuum breaker | ASSE 1011 | Yes | No | No | High |
| AVB Atmospheric vacuum breaker | ASSE 1001 | Yes | No | No | High |
| PVB Pressure vacuum breaker assembly | ASSE 1020 | Yes | No | Yes | High |
| SVB Spill-resistant vacuum breaker assembly | ASSE 1056 | Yes | No | Yes | High |
| DC / DCVA Double check valve assembly | ASSE 1015 | Yes | Yes | Yes | Low |
| DCDA Double check detector assembly | ASSE 1048 | Yes | Yes | Yes | Low |
| ASSE 1012 Backflow preventer with intermediate atmospheric vent | ASSE 1012 | Yes | Yes | Yes | Low |
| RPZ / RPBA Reduced pressure principle backflow preventer | ASSE 1013 | Yes | Yes | Yes | High |
| RPDA Reduced pressure detector assembly | ASSE 1047 | Yes | Yes | Yes | High |
Air gap
Air gapASME A112.1.2 · IPC 608.15.1
The only method that cannot fail mechanically, because there is no mechanism — the pipe simply stops above the water. It also destroys the pressure in the line, which is why it is used at fixtures and tanks rather than mid-run.
Installation: A physical vertical separation between the outlet and the flood level rim, at least twice the effective opening.
HVB
Hose connection vacuum breakerASSE 1011
The cheapest device in plumbing and the one that prevents the single most common household cross connection: a hose left in a pool, a bucket, or a chemical sprayer.
Installation: Screwed onto the hose thread of a sillcock or service sink faucet.
AVB
Atmospheric vacuum breakerASSE 1001
May not be under continuous pressure — more than 12 hours in any 24 and the float can stick shut. A valve downstream of it is the most common installation error, because it holds the device under pressure permanently.
Installation: At least 6 inches above the flood level rim of the fixture, with no shutoff valve downstream.
PVB
Pressure vacuum breaker assemblyASSE 1020
The AVB's problem solved with a spring-loaded air inlet, so downstream valves are permitted. Still no backpressure protection at all — height above the system is what makes it work.
Installation: At least 12 inches above the highest downstream outlet or head.
SVB
Spill-resistant vacuum breaker assemblyASSE 1056
A PVB that does not spit water when the air inlet opens, which is what makes it usable indoors and in finished spaces.
Installation: At least 12 inches above the highest downstream outlet, same as a PVB.
DC / DCVA
Double check valve assemblyASSE 1015
Two independent spring check valves in series. It handles both directions of backflow but is only approved for LOW hazard — a second check valve is redundancy, not containment.
Installation: In line, with shutoffs and test cocks, accessible for annual testing. May be installed below grade in an approved vault where permitted.
DCDA
Double check detector assemblyASSE 1048
A DC with a small metered bypass so the water utility can detect a leak or an unauthorised tap on a fire line that should never register flow.
Installation: On a fire service line, with a metered bypass around the assembly.
ASSE 1012
Backflow preventer with intermediate atmospheric ventASSE 1012
Two checks with a vent between them. Used on residential boiler make-up where the system carries no chemical treatment; add treatment and it becomes a high-hazard connection needing an RPZ.
Installation: In line on a small branch, typically 1/2 or 3/4 inch.
RPZ / RPBA
Reduced pressure principle backflow preventerASSE 1013
Two checks with a pressure-differential relief valve between them that dumps to atmosphere the moment the zone loses its differential. The only in-line assembly approved for high hazard under backpressure — and the only one that will flood a room when it fails, which is why the discharge has to be planned for.
Installation: Above grade, above the flood level of the space, with clearance for the relief port to discharge freely. Never in a pit or vault.
RPDA
Reduced pressure detector assemblyASSE 1047
An RPZ with the fire-line bypass meter. Required where a sprinkler system carries antifreeze, foam, or any additive rather than plain potable water.
Installation: On a fire service line, above grade, with a metered bypass.
Minimum air gaps
The whole table is one rule: 2× the effective opening, or 3× where the outlet sits close to a wall, with a floor of 1 inch and 1.5 inches respectively. The values below are computed from that rule rather than transcribed, so they cannot disagree with it.
| Effective opening | Away from a wall | Close to a wall |
|---|---|---|
| 1/2" in | 1.00 in | 1.50 in |
| 3/4" in | 1.50 in | 2.25 in |
| 1" in | 2.00 in | 3.00 in |
| 1-1/4" in | 2.50 in | 3.75 in |
| 1-1/2" in | 3.00 in | 4.50 in |
| 2" in | 4.00 in | 6.00 in |
Measured from the flood level rim
The gap runs from the lowest point of the outlet down to the rim water would spill over — not to the normal water line, and not to the bottom of the fixture. A faucet spout that swings below the rim has no air gap at all while it is there.
Effective opening, not pipe size
For an odd-shaped outlet, the effective opening is the diameter of a circle of the same area. That is what gets multiplied — so a wide flat outlet needs a bigger gap than its nominal connection size would suggest.
Nothing to test, nothing to fail
An air gap has no moving parts, needs no annual test, and cannot be defeated by a stuck check or a tired spring. Where a fixture can take one, it is the better answer — which is why the code reaches for a device only when an air gap is impractical.
It costs you the pressure
The trade-off is that an air gap breaks the pressurised line completely. Anything downstream has to run on gravity or be re-pumped, which is exactly why air gaps live at fixtures and tanks rather than mid-run on a distribution main.
What each common connection takes
These pairings follow directly from the direction-and-hazard matrix above. They are the usual answer, not a substitute for the purveyor's own cross-connection control programme — which is frequently stricter, and which is the body that actually approves the installation.
| Connection | Direction | Hazard | Usual assembly | Why |
|---|---|---|---|---|
| Hose bibb or sillcock | Backsiphonage | High | HVB (ASSE 1011) | A hose can reach anything — a bucket, a pool, a sprayer of weedkiller. Cheap device, worst-case hazard. |
| Lawn irrigation, no chemical injection | Backsiphonage | High | PVB or SVB | Buried heads sit in soil and standing water, so the code treats irrigation as high hazard even with nothing injected. An RPZ is required instead wherever any head can be below the assembly. |
| Irrigation with fertiliser or pesticide injection | Both | High | RPZ (ASSE 1013) | An injection pump creates backpressure, which rules out every vacuum breaker on its own. |
| Fire sprinkler, potable water only | Backpressure | Low | DC or DCDA | Stagnant sprinkler water is a taste-and-odour problem, not a health one — and elevation in the riser is a permanent backpressure source. |
| Fire sprinkler with antifreeze or foam | Backpressure | High | RPZ or RPDA | The additive turns a low-hazard system into a contaminant under standing backpressure. |
| Boiler feed, no chemical treatment | Backpressure | Low | ASSE 1012 or DC | System pressure sits above supply pressure whenever the boiler is hot. |
| Boiler feed with treatment chemicals | Backpressure | High | RPZ (ASSE 1013) | Glycol and oxygen scavengers are contaminants; the boiler supplies the backpressure. |
| Carbonated beverage dispenser | Backpressure | High | Dual check with vent, and no copper upstream | CO2 pushes back into the line and carbonic acid dissolves copper, so the material rule matters as much as the device. |
| Chemical or detergent dispenser | Both | High | Air gap or RPZ | An air gap is preferred where the fixture allows it, because nothing can fail. |
| Fixture with a submerged inlet, or a tank fill | Backsiphonage | High | Air gap, or AVB where an air gap is impossible | Any inlet below the flood level rim is a cross connection by definition. |
| Water heater on a metered service with a check valve | Backpressure | Low | Thermal expansion tank, not a backflow device | The check valve is already there — it is what makes the system closed. IPC 607.3 then requires somewhere for the expansion to go. |
A backwater valve is not a backflow preventer
The names are close enough that they get swapped constantly, but they solve unrelated problems. A backflow preventer protects the potable water supply from contamination flowing back into it. A backwater valve is a check valve in a drain, stopping sewage backing into a basement when the public sewer surcharges. Neither does the other's job, and fitting one where the other belongs leaves the actual hazard untouched.
Testing and approval
Testable assemblies are tested annually
Anything with test cocks — the PVB, SVB, DC, DCDA, RPZ and RPDA — is a testable assembly and is normally required to be tested when installed, when repaired or relocated, and at least once a year after that, by a certified tester. The AVB and hose connection vacuum breaker have no test cocks; they are devices, not assemblies, and are replaced rather than tested.
Approval is a listing, not a size
An assembly has to be on the approved list your purveyor recognises — commonly the USC Foundation for Cross-Connection Control and Hydraulic Research listing, alongside the ASSE standard for the type. A device that meets the standard on paper but is not on the list will still fail inspection, so confirm the listing before ordering rather than after.
Plan for the relief port
An RPZ discharges to atmosphere when it does its job, and a failed first check can dump a serious volume. It needs an air-gapped drain of adequate capacity, and it may not be installed below grade or anywhere it could be submerged. This is the detail most often left out of a quote and discovered on site.
Freezing kills assemblies
A PVB or RPZ on an irrigation line sits above grade with water in it. In a freezing climate it has to be drained down for winter or housed in a heated enclosure — a cracked bonnet in spring is the most common irrigation callback there is.
Sources & standards: IPC 2021 Section 608 (protection of potable water supply), including the air gap provisions and the application of backflow preventers, with device designations from the ASSE standards named in each entry and ASME A112.1.2 for air gaps. The air gap figures on this page are computed from the 2× and 3× rule rather than transcribed. This is a summary for planning, not a substitute for the code text or for your water purveyor's cross-connection control programme — purveyor requirements are frequently stricter than the model code, approved-assembly lists vary, and roughly fifteen states use the UPC or a derivative instead of the IPC. Testing and certification requirements are set locally. A licensed plumber, a certified tester and the AHJ have final say on anything installed.
Keep going
A pressure-reducing valve or backflow preventer closes the system — which is what makes the next calculation necessary.