A backflow preventer keeps water moving one way only, from the public supply into your pipes, so dirty water can't be pulled or pushed back into the drinking water. It does this with spring-loaded check valves, and on some types an air inlet or relief valve that opens to the air when pressure drops.
What problem a backflow preventer solves
A backflow preventer stops two kinds of reverse flow. Plumbers call them backsiphonage and backpressure, and our page on what backflow is walks through both.
- Backsiphonage happens when supply pressure drops, say during a water main break or heavy hydrant use. The pipes act like a straw and pull water backward. A garden hose lying in a bucket of soapy water is the classic victim.
- Backpressure happens when something downstream pushes harder than the supply. The EPA's Cross-Connection Control Manual defines it as a condition in which the owner's system pressure is greater than the supplier's system pressure. A pump, a boiler or water stored up high can do it.
Either way, the danger is a cross-connection, a link between drinking water and something that isn't. Our explainer on what a cross-connection is lists the common ones in a house.
How the check valves do the work
Every testable backflow assembly starts with check valves: a disc pressed against a seat by a spring. Forward pressure lifts the disc and water flows. When pressure reverses, the spring and the backward push slam the disc shut.
The EPA manual describes the double check this way: "Each check is spring loaded closed and requires approximately a pound of pressure to open." That spring loading is the point. The manual says it lets a check "bite" through small debris and still seal, a protection it says is not common in unloaded swing check valves.
A double check valve assembly stacks two of those checks, with a shutoff valve on each end and small test cocks so a tester can prove each check holds. Two checks are better than one, but both can still foul. That's why the double check is rated for lower-hazard uses.
How the stronger types add a safety exit
The higher-hazard designs don't trust check valves alone. They add a path to open air.
Reduced pressure principle assembly (RP or RPZ). Two checks, with a relief valve in the chamber between them. The EPA manual says the zone between the checks "is always kept at least two pounds less than the supply pressure." If the second check leaks or the supply pressure falls, water gets dumped out of the relief port instead of moving upstream. A spitting RPZ is the device doing its job or telling you a check is worn. Our page on an RPZ spitting water covers which is which.
Pressure vacuum breaker (PVB). One or two spring-loaded checks plus a spring-loaded air inlet on the outlet side. When supply pressure drops, the air inlet pops open and breaks the siphon. The EPA manual is blunt about the limit: these devices "may be used under constant pressure but do not protect against backpressure conditions." That's why the code makes a PVB sit high. The 2024 International Residential Code (IRC) that Phoenix uses for houses sets its critical level at least 12 inches above the highest downstream piping (section P2902.3.4).
Atmospheric and hose bibb vacuum breakers. The simplest kind. A float or check seals the vent while water runs and drops open when it stops, letting air in. They guard against backsiphonage only. The little cap on your outdoor faucet is one; see our page on the hose bib vacuum breaker.
| Type | Stops backsiphonage | Stops backpressure | How it fails safe |
|---|---|---|---|
| Air gap | Yes | Yes | Open space, nothing to wear out |
| RP / RPZ assembly | Yes | Yes | Relief valve dumps water to the air |
| Pressure vacuum breaker | Yes | No | Air inlet opens on pressure drop |
| Double check assembly | Yes | Yes | Two spring-loaded checks, no vent |
| Hose bibb vacuum breaker | Yes | No | Vents to air when the tap is off |
Arizona's drinking water rule, A.A.C. R18-4-215, ranks the testable options in this order of protection: "an air-gap separation (AG), a reduced pressure principle backflow prevention (RP) assembly, a pressure vacuum breaker (PVB) assembly, and a double check valve (DC) assembly." For the RPZ and double check side by side, see RPZ vs double check.
Where you'll find one at a house
At a typical house, the testable assembly is usually on the sprinkler line, not at the meter. IRC P2902.5.3 calls for an atmospheric vacuum breaker, a PVB or an RP on the irrigation supply, and an RP if chemicals are fed into the system. The rest of the home's protection is built into fixtures you already own: the antisiphon fill valve in each toilet tank, the vacuum breaker on each hose bibb, and the air gap under every faucet spout. Our page on whether all homes have a backflow preventer walks through that list and the Arizona exemption for single-family homes. To spot the hardware itself, here's how to find your backflow preventer and tell the types apart.
Why a backflow preventer needs testing
Springs weaken and rubber seals wear or pick up scale. Phoenix water is hard, and mineral grit is exactly what keeps a check from seating. A backflow assembly can look fine and still leak backward, and you can't tell by looking.
So the IRC's section P2503.8.2 calls for RP, double check and PVB assemblies to be tested at installation, after repairs or relocation, and every year after that. A tester uses a differential gauge on the test cocks to confirm each check holds and the relief valve opens when it should. HQ Plumbing & Air tests, repairs and replaces home backflow assemblies across the Valley, and we give you the price up front before any repair starts. Simple hose bibb vacuum breakers have no test cocks. When one drips or won't vent, you replace it.
If you're not sure your setup is protected, start with the hose bibbs and the sprinkler riser. That's where most home backflow risk lives. For anything with a test cock, have it tested before you assume it works.