Is a backflow preventer supposed to drip?

A home can have one or more of these devices. You can usually tell them apart by where they sit.

• Hose bib vacuum breaker. A small brass or plastic cap screwed onto an outdoor faucet. It has little vent holes on the side or top.
• Pressure vacuum breaker (PVB). A brass assembly standing up above the ground near the sprinkler valves, with a domed cap on top. This is the common irrigation device.
• Anti-siphon valve. A sprinkler zone valve with a vacuum breaker built into its top, one per zone. It is a different device from a PVB, as our page on whether a backflow preventer is the same as an anti-siphon valve explains.
• Reduced pressure zone assembly (RPZ). A larger assembly with two shutoff valves, four small test cocks, and a relief valve hanging off the bottom middle.

Not sure which is yours? Where to find your backflow preventer shows how to spot each one.

A hose bib vacuum breaker works by sealing its vent while water is flowing and opening the vent when the water stops. The EPA's Cross-Connection Control Manual describes it this way: "When the water supply is turned off, the device vents to atmosphere, thus protecting against backsiphonage conditions."

So a short dribble from the vent holes right after you shut the faucet is expected.

A fault looks different. If water sprays or runs steadily from the vent while the faucet is on and the hose is flowing, the vent is not sealing. The internal disc or seal is usually worn. These caps are inexpensive to swap, and your outdoor faucet itself may be fine. If the water is coming from the faucet stem or the spout instead, that is a different repair, covered in why an outdoor hose bib leaks.

One more thing worth knowing: a hose bib vacuum breaker is only meant for brief hose use. The Watts product guide says its Series 8 hose connection breaker is "not to be used under continuous pressure," so do not leave a valve or nozzle shut on the end of the hose with the faucet open.

A PVB can spit a little at start-up. Watts says so in the installation manual for its Series 800M4: "Follow ASSE Standard 1020 that the atmospheric vent valve remains open until the valve body pressure exceeds 1 lb. Until this pressure is reached, some amount of spillage occurs at the atmospheric vent."

In everyday terms, the vent stays open until pressure builds, so a splash from under the cap when the sprinklers first charge is normal. Watts also lists "Install where discharge or spillage is not objectionable" among its installation requirements. That is why a PVB belongs out in the yard and not inside a closet or against a stucco wall you care about.

Watts gives a tip for cutting that start-up spill down: close the downstream shutoff valve and open the inlet shutoff valve quickly. That is for whoever is turning the system on at the assembly, such as after winterizing or a repair.

What is not normal for a PVB:

• Water that keeps coming from under the cap long after the sprinklers have shut off.
• Spraying or gushing from the top while the zones are running.
• A wet, soggy patch around the base that does not dry between watering days.

Those point to a fouled or worn seat, a damaged float or disc, or freeze damage. A PVB is serviceable, and rebuilding it with fresh internal parts is a normal repair. One thing to check first: the manual says to install it 12 inches above the highest point of downstream piping and warns against installing it where backpressure can occur. A PVB that is too low, or that sees backpressure from a raised sprinkler line or a pump, can leak even when its parts are good.

If you are weighing whether your yard system needs this device at all, see do I need a backflow preventer for sprinklers in Arizona.

An RPZ is the one device that is designed to discharge on purpose. Its relief valve opens and dumps water from the zone between the two check valves when something goes wrong, so questionable water never reaches the supply side.

A short discharge during start-up is fine. Zurn's Model 375 instructions say: "A brief discharge from the relief valve may occur while the device is pressurizing. The discharge should cease by the time the shut-off valve is fully open."

Quick bursts of "spitting" can also be normal. The same manual says that if "spitting" or intermittent discharges are noted, "it could be a result of pressure fluctuation and/or a water hammer condition in the system." In its troubleshooting section, Zurn adds that when the relief valve discharges intermittently, "it can be almost always assumed that the device is functioning correctly." The usual fix there is not inside the RPZ. It is calming the pressure swings or the hammer upstream or downstream of it.

Continuous discharge is the line you do not want to cross. Zurn states: "Continuous discharge of the relief valve signifies a failure of some part of the device." Its list of causes is a fouled number one check, a fouled relief valve seat, or a fouled number two check, and it says "the amount of discharge is proportional to degree of fouling." A light, intermittent drip can mean a slightly fouled first check. For the full diagnosis, read why an RPZ is spitting water.

Two RPZ cautions. First, never plug or cap the relief port. It has to be free to discharge or the assembly cannot protect you. Second, where an RPZ is installed inside a building, Zurn requires a drain arrangement with an air gap "at least two times the pipe diameter" between the relief valve and the drain piping, and cautions "Do not pipe the relief valve solidly to a floor drain, sewer or sump." That is covered in why an RPZ needs an air gap.

A short spill when the water turns on is normal for a PVB and for an RPZ while it pressurizes. A brief dribble from a hose bib breaker when you shut the faucet is normal too. Occasional spitting from an RPZ on a pressure swing is often normal. Steady dripping, spraying, or flow from any of the three is a fault.

When a device has a steady leak, do not just wait it out. A backflow assembly that is stuck open or fouled may not be protecting your drinking water, and a steady dump also wastes water around the clock.

Start by noting the pattern. Does it happen only at start-up, only after shutoff, or all the time? Does it change when the sprinklers are off? That one detail narrows the cause quickly.

For a hose bib cap, unscrew it and replace it. A PVB or RPZ is a testable assembly, and repairs to one are typically followed by a retest. See what happens after a backflow assembly fails its test for how that goes. Cities and water providers set their own rules on who can repair and test an assembly and how to report it, so check yours.

HQ Plumbing & Air replaces and installs backflow assemblies. Licensed and insured, with free estimates and upfront pricing before any work starts. If yours keeps dripping and you cannot tell which kind of drip it is, we will help you sort it out. A splash at the edges of use is normal. Steady water is not.