An RPZ spits from its relief valve when pressure between its two check valves gets too close to the supply pressure. A short spit when street pressure dips can be normal. Steady or repeated discharge usually means a fouled first check, a leaking second check with backpressure, or a worn relief valve. It needs a test and repair.
How an RPZ decides to dump water
An RPZ has two check valves in a row with a small chamber between them, the "zone." The relief valve watches the difference between the incoming supply pressure and the pressure in that zone. As BAVCO's explainer puts it, the relief valve opens when the pressure past the first check becomes "greater than the inlet pressure minus the relief valve spring loading."
So anything that shrinks that gap makes it spit. Either the supply pressure falls, or water sneaks into the zone from a check that doesn't seal. Dumping water to the ground is the fail-safe. It's how the assembly keeps questionable water from ever reaching the public main. BAVCO says it plainly: "Just because a relief valve discharges water, does not always mean the RP is not working."
Occasional spit vs. steady discharge
This is the first thing to figure out, because it splits the likely causes in half.
| What you see | What it usually means |
|---|---|
| A short burst, then nothing, often when a big fixture opens or city crews work on the main | Supply pressure dropped for a moment. Watts lists "fluctuating inlet pressure" as a cause of relief discharge |
| Drips or discharge whenever no water is being used (nights, weekends) | Watts's first listed cause for discharge in a no-flow condition is a fouled first check |
| Discharge while water is flowing | Watts points to a first check "wedged open," a displaced O-ring in the relief valve, or a relief valve that doesn't close |
| Steady flow that won't stop | A leaking check valve, a relief valve diaphragm leak, or backpressure from downstream |
The first check is the usual suspect. Nick Azmo, writing in Working Pressure Magazine, puts the odds bluntly: of the three possible sources of an RP leak, the first check is the culprit about nine times in ten. A bit of debris on the check's seat is enough. Watts's manual for its 009 series says a brand-new assembly may discharge "due to inadequate initial flushing of pipe lines to eliminate dirt and pipe compounds," and it names water hammer as another trigger.
Backpressure is the other big one. Watts's troubleshooting table lists "outlet pressure higher than inlet pressure & leak in No. 2 check valve" and says to clean the second check and find the cause of the backpressure. On a boiler feed, a booster pump or a hot water loop, the building side can push harder than the street for a moment. If the second check doesn't seal, that water heads into the zone and the relief valve dumps it.
What a tester does to find the cause
Diagnosing an RPZ takes a differential pressure test kit, and it's work for a certified tester. Watts's manual lays out the logic. With a gauge across the first check, a reading of 2 to 3 psi points to a leak in one of the check valves. A reading that holds steady at 4 to 7 psi while the valve still dumps points to the relief valve itself, and a reading that swings between 2 and 7 psi points to fluctuating inlet pressure.
Without a gauge, the tester closes the No. 2 shutoff. If the discharge stops, the No. 2 check is leaking. If it keeps going, they open a test cock to draw a flow bigger than the discharge; if that stops it, the No. 1 check is the leak, and if not, the relief valve is the problem.
There's also a formal check on the relief valve itself during the annual test. Watts's test procedure says the relief valve "must operate to maintain the 'zone' between the two check valves at least 2 psi less than the supply pressure." The tester slowly lowers the pressure difference and records the reading at the moment the first drop comes out. A relief valve that opens too early can spit for no reason. One that doesn't open until the gap drops below 2 psi fails the test.
The fixes are usually parts, not a new assembly:
- Fouled check: take it apart and clean it, or rebuild it with the maker's kit.
- Relief valve leaking: service the relief valve, often a new diaphragm or seals.
- Pressure swings with no bad parts: control the supply pressure. For dead-end lines like boiler feeds and cooling tower makeup, Watts's 009 manual recommends a "spring-loaded rubber seated check valve ahead of the backflow assembly" to lock in the pressure.
In Phoenix, a repair has to be done by a certified repairer following ASSE Standard 5130 (City Code 37-143). Arizona's state backflow rule then requires a test "after installation, relocation, or repair." Mesa gives you 24 hours after a repair to get that retest done (City Code 8-1-11).
What not to do, and what you can do now
Don't plug, cap or pipe the relief port into a closed line. A plugged RPZ can't do its only job. Watts says a drain line with an air gap should be piped from the relief valve connection, ending about 12 inches above the ground or through an air gap to a floor drain. Mesa's code shows what a proper setup looks like for indoor units: an air-gapped drain line at least as large as the assembly, running to a floor sink, mop sink or other drain, with no flexible hose (City Code 8-1-9). That's also why Phoenix won't allow an RP in a basement, meter box, pit or vault (City Code 37-144). A flooded pit would put the relief port under water.
What you can do safely while you wait for a tester:
- 1Note when it spits. All the time, only at night, only when a big fixture runs, or only after city work. Tell the tester.
- 2Check your pressure. If other fixtures surge or hammer, mention it. Pressure swings are a cause the tester can't fix by rebuilding the valve.
- 3Keep the drain path clear. Make sure water can leave the area and isn't pooling around the assembly or a nearby electrical panel.
- 4Don't wait for the annual date. Mesa's code gives a customer 15 days to repair an assembly that fails between annual tests. A steady dump also wastes water around the clock.
For the difference between this valve and the simpler alternative, see RPZ vs double check backflow preventers. If the spitting turns into a failed test, what happens after a backflow assembly fails its test walks through the repair, retest and report. And if you're not sure which valve on the property is the RPZ, where to find your backflow preventer helps you spot it.
HQ Plumbing & Air tests and repairs RPZ assemblies on domestic water and irrigation lines, with upfront pricing before any parts go in.