What is a backflow preventer?

What is the purpose of a backflow preventer?

Its purpose is to stop a cross-connection, any link between drinking water and something that could contaminate it, from fouling the supply. EPA names the household hazards behind the home version, and Washington's rule says each cross-connection must be eliminated or controlled by a backflow preventer: “Any actual or potential connection between the public water supply and a source of contamination or pollution.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “Protection of the main potable supply from household hazards such as home photograph chemicals, toxic insect and garden sprays, termite control pesticides used by exterminators, etc., reinforced, a true need for such a device.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. In Washington, “The purveyor shall ensure that cross-connections between the distribution system and a consumer's water system are eliminated or controlled by the installation of an approved backflow preventer commensurate with the degree of hazard.” Source: Washington State Legislature, Washington Administrative Code, read 2026-10-29.

How does EPA define a backflow preventer?

A backflow preventer is a valve assembly or a gap in the piping that lets drinking water flow only one way, into the house, so used or dirty water can never be pulled or pushed back into the clean supply. EPA's Cross-Connection Control Manual defines backflow and the device this way: “A device or means designed to prevent backflow or backsiphonage.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “The flow of water or other liquids, mixtures or substances, under positive or reduced pressure in the distribution pipes of a potable water supply from any source other than its intended source.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29.

How does a backflow preventer work?

Most models are two spring-loaded check valves in a row: water pushes them open going in, and the springs close them if flow tries to reverse. The reduced pressure type adds a relief valve between the checks that keeps that middle zone at least 2 psi below supply pressure and dumps water if a check leaks. EPA explains: “Each check is spring loaded closed and requires approximately a pound of pressure to open.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “These devices are essentially modified double check valves with an atmospheric vent capability placed between the two checks and designed such that this “zone” between the two checks is always kept at least two pounds less than the supply pressure.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “If some obstruction prevents check valve 2 from closing tightly, the leakage back into the central chamber would increase the pressure in this zone, the relief valve would open, and flow would be discharged to the atmosphere.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29.

What causes backflow in plumbing?

Two things cause it: backsiphonage, when supply pressure suddenly drops and pulls water backward, and backpressure, when a pump, boiler or other system pushes harder than the city supply. Either needs a cross-connection to do harm. EPA defines both: “The flow of water or other liquids, mixtures or substances into the distribution pipes of a potable water supply system from any source other than its intended source caused by the sudden reduction of pressure in the potable water supply system.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “A condition in which the owners system pressure is greater than the suppliers system pressure.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “A reversal in differential pressure may occur when pressure in the potable system drops, for some reason, to a pressure lower than that in the system to which the potable water is connected.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “In addition to the negative pressure or reversed force necessary to cause backsiphonage and backflow, there must also be the crossconnection or connecting link between the potable water supply and the source of pollution.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29.

What are the types of backflow preventers?

EPA's manual lists six basic types, from a simple air gap to the reduced pressure principle assembly, and says the choice depends on how hazardous the connection is. This page only names them; the options guide linked below compares one against another. EPA says: “There are six basic types of devices that can be used to correct cross-connections: air gaps, barometric loops, vacuum breakers—both atmospheric and pressure type, double check with intermediate atmospheric vent, double check valve assemblies, and reduced pressure principle devices.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “Generally, the selection of the proper device to use is based upon the degree of hazard posed by the cross-connection.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “In general, the air gap must be twice the supply pipe diameter but never less than one inch.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29.

How often do backflow preventers need to be tested?

It depends on the rule that covers you. Texas irrigation rules require a yearly test for assemblies on health-hazard irrigation systems, EPA's sample program for water utilities sets at least twice a year for reduced pressure devices, and Washington requires an assembly that fails a test to be repaired or replaced. The rules say: In Texas, “Those backflow prevention assemblies used in irrigation systems designated as health hazards shall be tested annually.” Source: Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute, read 2026-10-29. EPA's sample utility program says: “Reduced pressure principle backflow devices shall be tested and inspected at least semi-annually.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. In Washington, “The purveyor shall ensure that an approved backflow prevention assembly or AVB, whenever found to be improperly installed, defective, not commensurate with the degree of hazard, or failing a test (if applicable) is properly reinstalled, repaired, overhauled, or replaced.” Source: Washington State Legislature, Washington Administrative Code, read 2026-10-29.

What is a backflow preventer in a sprinkler system?

It is the valve on the line that feeds a lawn sprinkler system, there so water standing in the irrigation pipes cannot siphon back into the house supply. Washington lets a water utility accept a simple atmospheric vacuum breaker on a home irrigation system; EPA says a pressure vacuum breaker must sit 6 to 12 inches above the outlets it serves: In Washington, “For example, the purveyor may accept an approved AVB on a residential irrigation system, if the AVB is properly installed under the UPC.” Source: Washington State Legislature, Washington Administrative Code, read 2026-10-29. “Again, these devices may be used under constant pressure but do not protect against backpressure conditions.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “As a result, installation must be at least 6- to 12-inches higher than the existing outlet.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. In Texas, “All backflow prevention assemblies installed per this chapter shall be installed according to manufacturer's recommendations and provided with sufficient clearance to facilitate testing.” Source: Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute, read 2026-10-29.

Where is the backflow preventer located?

For a whole house it usually sits at the water meter or where the service line enters the property; EPA says the home dual check goes right after the meter. Point-of-use devices sit at the hazard instead, such as an outdoor hose faucet or the sprinkler line. EPA and the Washington and Texas rules say: “It is sized for ½-, ¾-, and 1-inch service lines and is installed immediately downstream of the water meter.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. In Washington, “The purveyor shall ensure that approved backflow assemblies for premises isolation are installed at a location adjacent to the meter or property line or an alternate location acceptable to the purveyor.” Source: Washington State Legislature, Washington Administrative Code, read 2026-10-29. In Texas, “At any residence or establishment where an actual or potential contamination hazard exists, additional protection shall be required at the meter in the form of an air gap or backflow prevention assembly.” Source: Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute, read 2026-10-29.

Does a backflow preventer need a drain?

The reduced pressure principle type does, because its relief valve is built to discharge water to the open air whenever the zone between its checks loses pressure or a check leaks, so the spot needs somewhere for that water to go. EPA explains the relief valve: “An assembly consisting of two (2) independently operating approved check valves with an automatically operating differential relief valve located between the two (2) check valves, tightly closing shut-off valves on each side of the check valves plus properly located test cocks for the testing of the check valves and the relief valve.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “The pressure in the central chamber is maintained lower than the incoming supply pressure through the operation of the relief valve 3, which discharges to the atmosphere whenever the central chamber pressure approaches within a few pounds of the inlet pressure.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “Malfunctioning of one or both of the check valves or relief valve should always be indicated by a discharge of water from the relief port.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29.

Who is required to have a backflow preventer?

Rules are set by each state and water utility. In Texas, any home or business where an actual or potential contamination hazard exists needs protection before it can connect, and one with no such hazard does not; Washington applies the same isolation rules to single-family homes when they fit. The rules say: In Texas, “No water connection from any public drinking water supply system shall be allowed to any residence or establishment where an actual or potential contamination hazard exists unless the public water facilities are protected from contamination.” Source: Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute, read 2026-10-29. In Texas, “At any residence or establishment where there is no actual or potential contamination hazard, a backflow prevention assembly is not required.” Source: Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute, read 2026-10-29. In Washington, “For single-family residential service connections, the purveyor shall comply with the premises isolation requirements of (b) of this subsection when applicable.” Source: Washington State Legislature, Washington Administrative Code, read 2026-10-29.

Do I need a backflow preventer on my garden hose?

A garden hose links your drinking water to whatever its end sits in, and EPA describes a small device made for it: the hose bibb vacuum breaker, which is attached to the outdoor faucet and vents to the air when the supply shuts off. Whether one is required is set by local code and your water utility. EPA says: “A device which is permanently attached to a hose bibb and which acts as an atmospheric vacuum breaker.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “They are generally attached to sill cocks and in turn are connected to hose supplied outlets such as garden hoses, slop sink hoses, spray outlets, etc.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29. “When the water supply is turned off, the device vents to atmosphere, thus protecting against backsiphonage conditions.” Source: U.S. Environmental Protection Agency, Office of Water, read 2026-10-29.

Sources

• Cross-Connection Control Manual (EPA 816-R-03-002) — U.S. Environmental Protection Agency, Office of Water; published 2003-02; read 2026-10-29.
• WAC 246-290-490 Cross-connection control (Washington) — Washington State Legislature, Washington Administrative Code; published 2022-01-01; read 2026-10-29.
• 30 Tex. Admin. Code § 290.44 Water Distribution, subsection (h) Backflow, siphonage (Texas) — Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute; published 2023-12-21; read 2026-10-29.
• 30 Tex. Admin. Code § 344.50 Backflow Prevention Methods (Texas landscape irrigation) — Texas Commission on Environmental Quality rule, via Cornell Legal Information Institute; published 2020-07-26; read 2026-10-29.