Why Is the Relief Valve on My RP Assembly Discharging Water?
If water is coming out of the port on the underside of your reduced pressure assembly, the device is doing exactly what it was built to do. The question is whether it should be doing it right now.
A short spit as pressure stabilizes is normal. Steady flow, or discharge that recurs several times a day, means something upstream, downstream, or inside the device needs attention. Here is how to tell them apart.
What the relief valve is reacting to
An RP has two check valves in series with a zone between them. The relief valve watches the difference between the supply pressure ahead of the first check and the pressure inside that middle zone. It is designed to keep the zone meaningfully below supply pressure. When that margin collapses — supply pressure drops, zone pressure climbs, or a check stops sealing — the relief valve opens and dumps to atmosphere rather than let anything move backward.
So the relief valve is not really the problem most of the time. It is the alarm.
The common causes, roughly in order
Debris on a check valve seat
Grit or scale lodges on the first or second check and it no longer seals. This is the single most common cause we find, and it usually shows up right after nearby construction, a water main repair, or spring irrigation startup when lines that sat idle get flushed through the device. Fixed with a disassembly, clean and reseat, plus rubber if the parts are marginal.
Worn rubber, springs, or relief valve internals
Discs, o-rings and springs degrade from chlorine, heat and cycling. The symptom is persistent dripping, or a relief valve that opens earlier than it should. Fixed with a manufacturer rebuild kit and a re-test to confirm the differentials.
Thermal expansion in a closed system
This one gets missed constantly. Once a property has check valves or a pressure reducing valve, the plumbing is a closed system. Heat water in a closed system and it expands with nowhere to go, downstream pressure climbs, and it pushes back against the assembly.
The tell is timing: the relief valve opens after a long hot water draw, or overnight when nothing is running. If that matches what you are seeing, the fix is an expansion tank, or an existing one that has lost its charge. No amount of rebuilding the RP will solve it.
High or fluctuating supply pressure
Plenty of Southern California neighborhoods run static pressure well above 80 psi. Spikes, surges and water hammer can lift the relief valve on a device that is otherwise healthy. The fix is a working pressure reducing valve, and sometimes arrestors.
Downstream backpressure
Booster pumps, irrigation pumps, and significant elevation change downstream can all create conditions that open the relief. The tell is that discharge coincides with a pump starting.
Installation and drainage problems
An RP has to sit above grade with the relief port free to discharge to atmosphere through a proper air gap. If the port is restricted, submerged, or piped into a sealed drain, you get pooling and nuisance discharge, and the device cannot protect anything. Orientation matters too — install it the way the manufacturer and the approval list allow, which for most models means horizontal.
Freeze or sun damage
Rare here, but real in the high desert and inland valleys, and UV degradation of exposed plastic bonnets happens everywhere. Look for body leaks alongside the relief activity.
Wrong size or wrong model for the flow
An undersized RP chatters and discharges at peak demand. If the discharge tracks exactly with heavy irrigation or process flow, sizing is worth checking.
Never cap, plug, or pipe the relief port into a sealed line. The relief has to discharge freely to atmosphere. Blocking it removes the protection entirely and is a code violation, not a workaround.
What you can check before calling
Four things, none of which require opening anything.
- Confirm the source. Water from the relief port in the center is a different problem than water from a test cock or a body seam. See our guide on diagnosing a leaking backflow preventer if you are not sure which you have.
- Note the timing. Startup? After hot water use? When a pump runs? Random? This is the most useful thing you can tell a tester.
- Look for a PRV and an expansion tank. If either is missing or well past its service life, they are prime suspects.
- Check the air gap. Make sure the discharge is not restricted or sitting in water.
What a test actually tells you
A certified test puts a calibrated gauge on the device and produces numbers: the holding differential across each check, and the point at which the relief valve opens. That isolates whether the first check, the second check, or the relief is the failure — or whether all three are fine and the problem is pressure conditions on the property.
Most discharge problems are resolved with a rebuild kit and a pressure correction, not a new assembly. We recommend replacement only for cracked or obsolete bodies, or a device that keeps failing after a proper rebuild. Either way, the passing re-test report goes to your purveyor, which is the part that puts you back in compliance.
Common questions
Is some dripping okay?
A brief discharge during a pressure change is normal. Continuous or frequent discharge is not.
Can I route the relief line to a drain?
Only to a code-compliant, air-gapped receptor sized for full relief flow. Never into a sealed or undersized line.
Do I need a whole new device?
Usually not. A rebuild plus fixing the pressure condition resolves most of these.