Backflow preventer types, explained
In a reduced pressure principle assembly the central chamber is kept below supply pressure by a relief valve that discharges to the atmosphere.
Reduced pressure zone (RPZ) assemblies
EPA lists plating plants, car washes, funeral parlors and hospital autopsy rooms as typical high hazard RP installations.
Because check valves can leak from wear or obstruction, the RP adds a relief valve that opens to the atmosphere if a check leaks.
An RPZ has two check valves plus a discharge port as a third line of defense, and is approved for high hazard applications.
RPZs are approved for high hazard (health hazard) applications.
Manhattan requires RPZs to sit at least one foot off the ground or floor and not in locations subject to flooding such as a pit or underground valve box.
Boulder calls the RPZ the standard assembly and the one with the highest degree of protection among testable assemblies; it will dump water so needs a drain.
Boulder requires RPZs installed 12 inches above grade and protected from freezing and flooding.
Sources: How an RP assembly works: reduced pressure zone (EPA manual); RPZ discharge port (City of Manhattan); RPZ is the highest protection among testable assemblies (City of Boulder)
Double check valve assemblies
A double check valve is two single check valves in one body with test cocks and two gate valves.
EPA says double checks protect low to medium hazard installations and may be used under continuous pressure.
USC defines a double check valve assembly as two independently acting check valves with shutoff valves and test cocks.
USC states the double check valve assembly shall only be used for a non-health (pollutant) hazard.
A DCVA has two spring-loaded check valves between two shut-off valves, so if one fails there is a backup.
Portland says DCVAs protect against backflow hazards that do not pose a health risk, work under continuous pressure and handle backsiphonage and backpressure.
Portland says the DCVA and the pressure vacuum breaker assembly are most commonly installed on residential irrigation systems.
Boulder describes the DC as protecting low hazards; it normally does not dump water and may be below grade in some cases but must be protected from freezing and flooding.
Sources: Double check valve construction (EPA manual); DC assembly definition (USC Foundation); DCVA has two check valves, one backs up the other (City of Manhattan); DCVA for hazards that do not pose a health risk (Portland Water Bureau); Double check protects low hazards and normally does not dump water (City of Boulder)
Pressure vacuum breakers (PVB)
Boulder says PVBs are used specifically for outdoor irrigation and must be 12 inches above the highest sprinkler head with no continuous backpressure.
Boulder describes the PVB as used specifically for outdoor irrigation systems; PVBs can dump water.
CSU Extension: a PVB goes in a vertical position at least 12 inches above all downstream piping, and one PVB serves a whole sprinkler system.
CSU Extension says the PVB has test cocks and must be tested yearly by a backflow technician.
Atmospheric and pressure vacuum breakers are effective only against back-siphonage and are not for back-pressure situations.
EPA manual: PVBs may be used under constant pressure but do not protect against backpressure, so install them 6 to 12 inches higher than the outlet.
Manhattan: PVBs can only be used where there is no danger of backpressure, so not where a downstream pump or other pressure source exists.
A pump downstream creates pressure, which rules out a PVB.
Sources: PVB is for outdoor irrigation, 12 inches above the highest sprinkler head (City of Boulder); PVB 12 inches above all downstream piping (Colorado State Univ. Extension); PVB cannot protect against backpressure (EPA manual); PVB backsiphonage only, not with a downstream pump (City of Manhattan)
Spill-resistant vacuum breakers
EPA manual: the spill resistant pressure vacuum breaker is a modification of the standard PVB designed to minimise water spillage.
EPA: SVB installation and hydraulic requirements are similar to a standard PVB and SVBs are recommended for internal use.
USC defines the SVB as a check valve plus loaded air inlet valve with a bleed/vent port and a single test cock; backsiphonage only.
The USC tester course covers field testing PVBs, SVBs, double check assemblies and RP assemblies.
Ohio EPA: a differential pressure gauge tests RP assemblies and the newer spring-loaded PVBs and also double check assemblies.
Sources: SVB minimises water spillage (EPA manual); SVB definition (USC Foundation); SVB is one of four assemblies certified testers learn to test (USC Foundation); Spring-loaded newer PVB type needs a differential gauge (Ohio EPA)
Atmospheric vacuum breakers and hose bibbs
EPA: hose bibb vacuum breakers are small devices attached to sill cocks for hose-supplied outlets like garden hoses.
A spring loaded check valve seals against an atmospheric outlet when supply pressure is on; when water is off it vents to atmosphere.
EPA: hose bibb vacuum breakers protect against backsiphonage and should not be used as backpressure devices.
EPA: AVBs (about 1/2 to 3 inch) must be installed vertically, with no shutoff downstream, at least 6 inches above the final outlet; they cannot be tested in place.
EPA notes AVBs cannot be tested once installed in the plumbing system.
Manhattan: AVBs depend on gravity, are not rated for continuous pressure (12 hours or more), and can stick if left under pressure.
Manhattan notes newer sillcocks have integral vacuum breakers and older ones can be fitted with inexpensive add-on devices.
CSU Extension: use a hose connection vacuum breaker as a minimum when applying fertilizers or pesticides with a hose-end sprayer.
Sources: Hose bibb vacuum breaker: what it is (EPA manual); AVB not rated for continuous pressure (City of Manhattan); Hose connection vacuum breaker for hose-end sprayers (Colorado State Univ. Extension)