A little water from a PVB's top vent while the line pressurizes is normal. Steady dripping usually points to debris, a damaged poppet seal or a cracked bonnet. A heavy gush points to low downstream pressure or a broken poppet or bonnet. Anything beyond a brief start-up spit calls for repair and a retest.
When a little water is normal
Some spitting at start-up is built into the design. Watts' 800M4 manual explains that under ASSE Standard 1020, "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." Its start-up steps say to open the inlet shutoff "until water discharges from the bonnet then quickly open to pressurize the valve."
FEBCO's Model 765 manual says the same thing from the other side: install the device "where some spillage is not objectionable," because "instantaneous siphon conditions and pressure surges" will make it spit. So a short burst when the system fills, or when pressure in the street dips, is expected. Water that keeps coming once the line is fully pressurized isn't.
What a steady drip means
FEBCO's troubleshooting guide lists four causes for "Minor leakage through air vent":
- Damaged poppet seal
- Cracked or damaged poppet
- Cracked bonnet or damaged sealing edge
- Debris on sealing surface
The fixes are all repairs to the top half of the device: clean it, or replace the seal, poppet or bonnet assembly. Debris is the one you can often prevent. FEBCO warns that if the lines aren't flushed before the device goes in, the check members can "become fouled and require disassembly and cleaning."
What a gush means
Significant discharge through the air vent is a separate line in the same guide, with these causes:
- Poppet not properly guided
- Major poppet or seal failure
- Low downstream pressure
- Insufficient inlet volume to operate device
- Poppet and or bonnet broken
Pressure is the cause people miss. The FEBCO manual notes that "The downstream pressure must be maintained above 5psi to keep the spring loaded air inlet poppet closed." If a big irrigation zone pulls more water than the supply can deliver, pressure in the body sags and the poppet opens while water is flowing. Watts' list of requirements includes "Do not undersize supply or oversize the valve relative to demand."
Broken poppets and bonnets usually come from pressure the device wasn't built to hold. FEBCO's manual cautions that "Thermal water expansion and/or water hammer downstream of the backflow preventer can cause excessive pressure increases," and its guide lists a broken poppet "due to thermal expansion" among the causes of a gush.
Install mistakes that cause leaks
Some PVBs leak because of where they are. The 2024 IPC, which Phoenix adopted without changing this section, requires the critical level of a PVB to be "not less than 12 inches (305 mm) above all downstream piping and outlets," and says these assemblies "shall not be installed in locations where spillage could cause damage to the structure." Watts' manual says to install it "bonnet side up," and FEBCO's says the air inlet must operate vertically or the device will malfunction.
Arizona and Phoenix also limit where a PVB can be used at all. The state rule allows one on landscape irrigation only when, among other things, "The irrigation system is designed and constructed to be incapable of inducing backpressure" and chemigation is not used (A.A.C. R18-4-215). Phoenix's code allows a PVB for an irrigation "System not subject to backpressure (PVB)," and says "A pressure vacuum breaker assembly must be installed above ground" (City Code 37-144). If yours sits below the highest sprinkler head, serves a line with fertilizer injection, or could see backpressure, the leak may be the smaller problem: you may need a different assembly.
What to do about it
- 1Watch it through a full cycle. A spit at start-up that stops is normal. A drip or stream that keeps going is not.
- 2Don't cap, plug or cover the vent. The vent is the backflow protection. Blocking it defeats the device.
- 3Check downstream pressure if it only gushes with a big zone running. That points to supply, not parts.
- 4Have it repaired and retested. Phoenix's code says "The customer shall test all backflow prevention assemblies at least once a year" (City Code 37-143), and the state rule requires a test after installation, relocation or repair.
- 5Fix the cause as well as the part. Debris, thermal expansion and water hammer can break a new poppet the same way they broke the old one.
HQ tests and repairs backflow assemblies on domestic water and irrigation lines for Valley businesses and HOAs.
For the RPZ version of this problem, read why an RPZ backflow preventer spits water. If the leak turned into a failed test, what happens after a backflow assembly fails its test covers the next steps, and backflow testing for HOA common-area irrigation covers the yearly schedule for landscape assemblies.