Why Is My RP Backflow Preventer Dripping From the Relief Valve?

A reduced pressure (RP) backflow preventer drips or spits from its relief valve because pressure in the zone between its two check valves has climbed too close to the supply pressure. The usual causes are a fouled first check, a second check that is leaking under backpressure, a dirty or damaged relief valve seat, or pressure swings in the line. The drip is the safety feature working. It still means the assembly needs service, and in Upstate SC it means a retest and a new report after the repair.

Here is how to read the symptom, narrow down the cause, and handle the paperwork with Greenville Water, Spartanburg Water, or Easley Combined Utilities.

What is the relief valve actually doing?

An RP assembly has two independent check valves with a "zone" between them, and a hydraulically operated relief valve that vents that zone to the atmosphere. Zurn Wilkins' description of its 975XL is a good plain-language version: with no flow, both checks are closed and the supply side sits about 8 psi above the zone, which holds the relief valve shut. If the zone pressure rises to within about 2 psi of the inlet pressure, the relief valve starts to open and dumps water. (Zurn Wilkins backflow catalog)

So water coming out of the relief port is the assembly refusing to let water move backward. That is why South Carolina requires an RP or an air gap on high hazard connections under R.61-58.7, and why you never plug the port.

Is it a constant drip or an occasional spit?

That one observation splits the problem in half. Watts' troubleshooting guide for its reduced pressure assemblies sorts symptoms the same way. (Watts RP troubleshooting guide)

Spits now and then. Watts points to fluctuating supply pressure or fluctuating downstream pressure. When the supply drops for a moment, say a big draw elsewhere on the main or a pump cycling, the margin between supply and zone shrinks and the relief valve burps. Watts' fix is a spring-loaded, soft-seated check valve immediately upstream of the assembly for supply swings, or downstream near the shutoff for downstream swings. None of those causes call for replacing the assembly.

Drips all the time. Watts lists these causes:

• A fouled first check. Flush the valve, and if that does not clear it, disassemble and clean or replace the first check.
• A damaged or fouled relief valve seat.
• A relief valve piston O-ring that cannot move freely because of pipe scale, dirt, or mineral buildup.
• A second check distorted by excessive backpressure, freezing, or water hammer.
• Electrolysis on the relief valve seat or first check seats.
• An assembly that was put back together with the check springs in the wrong places.

Debris is also why Greenville Water's manual says a strainer may be needed between the service connection and the assembly, to keep particles from fouling the checks (section 5-4.11).

How does a tester figure out which part is leaking?

There is a quick shutoff sequence for this that works on any brand. The Reece relief valve discharge guide lays it out step by step.

1. Close the outlet shutoff (downstream). If the discharge stops or slows a lot, the assembly is under backpressure and the second check is leaking. Water from downstream was pushing back into the zone.
2. If it keeps dripping, get water flowing through the assembly by opening a test cock downstream of the first check. If the discharge stops or slows, the first check is the one leaking.
3. If it still discharges, the problem is in the relief valve itself, most often debris on the seat or the rubber.

Each branch ends the same way: shut the water off, open up the part that failed, then clean it or replace the rubber.

A certified tester then confirms with a differential pressure gauge. South Carolina requires a tester's gauge to be accurate within 2% of full scale or plus or minus 0.3 psid, so the numbers from that test hold up. (R.61-58.7)

Why do so many RPs start dripping in late fall?

Two local reasons. The first is freeze damage. Watts lists freezing as a cause of a distorted second check, and an irrigation RP that was not drained before a hard frost can come back in spring with a cracked body or a check that no longer seats. Greenville Water's manual requires assemblies to sit in an enclosure or building that protects them from freezing, traffic, and vandalism. (Greenville Water Cross Connection Control Manual, section 5-4.8) If you have an irrigation RP, our winterizing guide covers the shutdown.

The second is thermal expansion. Once a backflow preventer is on a service, heated water can no longer push back into the main. Greenville Water's manual notes that thermal expansion often makes water heater relief valves discharge, usually above 150 psi, and suggests an expansion tank, a special ball cock and relief valve in the toilet, or a remote thermal expansion relief valve. (Greenville Water manual, section 6) That backpressure shows up at an RP as a second check that leaks and a relief valve that won't stay quiet.

Where is that water supposed to go?

A dripping RP inside a mechanical room or a below-grade vault turns into a flooding problem fast. Greenville Water's manual has specific RP rules (section 5-5):

• Relief valve drain piping must meet air gap requirements. The gap is at least twice the relief valve diameter or one inch, whichever is greater, and the piping must be sized to exceed the relief valve's discharge rate.
• The relief valve shall never become submerged.
• Underground RP installations are discouraged and only allowed in unusual cases where an adequately sized gravity drain to the surface can be installed.

The state rule says the same thing about flooding: RPs shall not go in locations subject to possible flooding, including pits or vaults without a gravity drain that can outrun the relief valve. (R.61-58.7)

What happens with the report after the repair?

This is the part owners and property managers miss. A repair is not done when the drip stops.

South Carolina regulation R.61-58.7 says all backflow prevention assemblies shall be tested immediately after repairs of any kind, and the certified tester submits a written report to the water system.

Greenville Water spells out the timing in section 7-2 of its Cross Connection Control Manual. If an assembly fails its test and cannot be repaired right away, for example because parts are on order, the tester sends Greenville Water a copy of the failed report explaining the failure that same day. Once the assembly is repaired, it gets tested immediately and the completed report goes back the same day. Testers upload results through the utility's portal, described on Greenville Water's backflow prevention page.

Spartanburg Water and Easley Combined Utilities run their own programs, so check their current instructions before you assume the Greenville clock applies.

For a tester, that means one dripping RP can turn into two reports in a day: the initial failed test and the passing test after the rebuild. Psid fills the published test report PDF for Greenville Water, Spartanburg Water, and Easley Combined Utilities from the readings you enter, so the second report starts from your entered readings instead of a fresh blank form. You still review, sign, and submit it yourself. The how it works page walks through it, and pricing is $49 a month after a 14-day free trial.

What should an owner do right now?

• Don't cap, plug, or bury the relief port.
• Note whether it drips constantly or spits now and then, and when. That detail saves your tester time.
• Check that the discharge has somewhere to go and the enclosure isn't filling with water.
• Call a certified tester or a plumber who services backflow assemblies, and ask them to test and report the assembly after the repair.
• If water heater relief valves are also popping, mention it. Thermal expansion may be part of the story.