Rinnai TPR Valve Leaking? Stop It Safely

The Short Answer: A Rinnai T&P (temperature-and-pressure relief) valve that drips is doing its job: pressure or temperature is over the setpoint, usually 150 psi / 210°F. Do not cap it. Check incoming water pressure, the expansion tank, and scale-driven overheat. If it dumps a stream, shut the unit down and call a licensed plumber.

Search Intent & The Real Problem

You walked into the utility closet or looked under an outdoor Rinnai and found a wet discharge pipe, a stained wall, or a puddle under the T&P. Hot water still works, so it feels “minor.” It is not. That valve is the last mechanical fuse between a blocked heat exchanger and a scalding discharge — or, on a closed plumbing system, between thermal expansion and a burst fitting.

Homeowners mix this up with condensate. Condensate is cool, slightly acidic, and comes from the drain fitting or trap. T&P discharge is hot (or at least house-temperature), comes from the brass relief valve on the hot outlet, and often pulses when the burner fires.

What other guides get wrong

Generic troubleshooting pages often jump to a part or a reset before identifying the unit, fuel, and failure sequence. Start with the model label and the safest outside checks. If the fault involves gas, combustion, pressure, or 240 volts, stop at that boundary and call a licensed professional.

Step-by-Step Troubleshooting

Step 1: Confirm it is the T&P — not condensate or a fitting

• What to look for: Brass valve with a test lever, usually on the hot-water outlet of the Rinnai. A pipe should run from that valve to a drain or to daylight. Wetness at the PVC condensate elbow is a different repair.
• Required tools: Flashlight, dry paper towel, phone camera.

Wipe the valve body, the threaded joint, and the discharge pipe separately. A weep at the threads is often a missing or crushed gasket on a new valve. A weep from the discharge opening means the valve opened.

Step 2: Measure incoming water pressure

• What to look for: Static pressure at a hose bib over 80 psi. Rinnai units are typically rated to 150 psi maximum; many T&P valves open at 150 psi. City pressure plus a failed or missing pressure-reducing valve (PRV) will make a healthy valve drip every heat cycle.
• Required tools: $12 hose-bib pressure gauge.

Screw the gauge onto an outside faucet, open it fully, and read static pressure. Over 80 psi: the house PRV is the repair, not the Rinnai. Under 80 psi and the T&P still drips during a hot-water call: go to expansion and overheat.

Step 3: Test the expansion tank (closed systems)

• What to look for: A tank-style expansion tank on the cold line that is waterlogged (the Schrader valve spits water) or that has 0 psi precharge. Closed systems (check valve or PRV on the city line) have nowhere for heated water to expand. The T&P becomes the expansion tank. That is a code failure, not a “bad Rinnai.”
• Required tools: Tire gauge, bicycle pump or compressor, isolation valve.

Tankless heaters hold less volume than a 50-gallon tank, so drips can be smaller — but they still happen, especially if a recirc line or buffer tank is in the system. Precharge should match static house pressure, typically 50–60 psi, with the tank isolated and drained.

Step 4: Check for overheat from scale or a failed sensor

• What to look for: Error history for overheat (Rinnai often logs 16 / 16), longer-than-normal heat-up, a roaring heat exchanger, or skip-flush maintenance past 12 months in hard water. Scale insulates the exchanger; outlet temperature overshoots; the T&P lifts.
• Required tools: Isolation valves with hose bibs, Rinnai-approved descaler or food-grade vinegar per the manual, two washing-machine hoses.

If you have service valves, a flush is legitimate DIY. If you do not, that is an install defect — stop and hire the job. Do not run acid through a unit that is actively dumping from the T&P; kill power/gas first.

Step 5: Only then replace the valve

• What to look for: Debris on the seat, a valve that dribbles after you lifted the test lever, or a valve older than the unit that never reseated. Replacement without fixing pressure/expansion is a callback waiting 48 hours.
• Required tools: Two pipe wrenches, pipe dope rated for potable water or a new gasket, matching 150 psi / 210°F T&P, bucket.

Match the pressure and temperature ratings stamped on the old valve. Discharge piping must be the full outlet size, no threads on the outlet, slope to drain, and terminate where you can see it — that is UPC/IPC language, not a suggestion.

When DIY Ends: Calling a Licensed Pro

Stop if any of these are true: the valve dumps a hard stream, you smell gas, the unit shows a lockout you cannot clear, incoming pressure is over 80 psi and you do not have a PRV, or the heat exchanger has never been flushed in a hard-water house. Capping the T&P, reducing the discharge with a 3/8” tube, or “just snugging it” are how water heaters become incident reports. A licensed plumber will gauge the house, set or replace the PRV, charge or replace the expansion tank, flush the Rinnai, and only then swap the valve — and will pull the permit if your city requires it.

Frequently Asked Questions

Q: Why is my Rinnai pressure relief valve leaking only when it runs? A: Heat raises pressure. On a closed system, that spike lifts a 150 psi T&P even if static pressure looked “fine” at the hose bib. Fix expansion and the PRV; do not keep replacing the valve.

Q: Can I cap a dripping Rinnai T&P valve overnight? A: No. A capped relief valve is a code violation and a scald/rupture risk. Close the cold isolation valve and shut the heater down instead.

Q: Is condensate dripping the same as a T&P leak? A: No. Condensate is cool and comes from the drain trap or neutralizer. T&P discharge is from the brass relief valve on the hot side and is a pressure or overheat event.