Rinnai Pressure Relief Valve Leaking: Causes
On this page (5 sections)
The Short Answer: The relief valve on a Rinnai tankless opens when water pressure in the heater gets too high. Rinnai requires a valve rated up to 150 psi, and the maximum supply pressure is also 150 psi, so frequent dripping usually means pressure spikes from high street pressure or thermal expansion in a closed system. Overheating from scale and a worn valve seat are other causes. Never cap or plug the valve.
It is a pressure relief valve, not a tank T&P
Tank water heaters use a combined temperature-and-pressure (T&P) valve. Rinnai’s installation manual (opens in a new tab) calls for a pressure relief valve on the tankless heater, rated up to 150 psi and at least to the heater’s Btu/hr input, with its discharge piped to the ground or a drain. If you see a valve with a different rating, the installer chose it; check its tag.
What causes it to open
| Pattern | Likely cause | What to do |
|---|---|---|
| Drips steadily, more at night when nobody uses water | High static pressure from the street | Check pressure; pressure-reducing valve service |
| Spurts after hot water use stops | Thermal expansion in a closed system (check valve, backflow preventer, or PRV on the main) | Plumber evaluates an expansion tank |
| Discharges during or right after heating, with overheat codes | Scale causing overheating | Flush; see code 14 |
| Keeps dribbling after someone tested it | Debris on the valve seat | Valve replacement |
What you can check
- Confirm the source. Wipe the valve, its threads, and the discharge pipe dry. A wet valve body or threaded joint is a fitting leak; water coming out of the discharge outlet means the valve opened. Condensate from a separate drain line or the vent is a different issue; see Rinnai dripping at the exhaust.
- Measure static pressure. A hose-thread gauge on an outdoor faucet takes a minute. Household pressure that sits well above 80 psi, or climbs overnight, is high enough to warrant a pressure-reducing valve.
- Note the timing. Write down when it drips. That tells the plumber whether it is supply pressure, expansion, or heat.
- If it is discharging a steady stream, turn the heater off and close the cold-water valve feeding it, then call a plumber.
Rinnai’s manual says the relief valve should be operated manually once a year to make sure it works. If it drips afterward, debris may be holding it open; a replacement is a minor repair.
What the plumber does
The plumber checks static and dynamic pressure, looks for a check valve or backflow device that makes the system closed, and adds or services an expansion tank if needed. They inspect the heater for scale or overheating history, replace the relief valve with one of the correct rating, and confirm the discharge pipe is full size, unobstructed, and ends where water can be seen.
Related: why a water heater relief valve keeps discharging, relief valve leaking after testing, and thermal expansion and expansion tanks.
Frequently Asked Questions
Q: A tankless holds so little water. Can thermal expansion really matter? A: Yes, though less than with a tank. In a closed system there is no room for heated water to expand, and the pressure can spike enough to lift a relief valve, especially with long hot-water runs or a recirculation loop.
Q: Can I replace the relief valve myself? A: It is a plumbing repair on a pressurized system and must match the required rating. Most owners have a plumber do it, and the plumber should also fix the cause so the new valve does not start dripping too.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Rinnai: RSC SENSEI installation and operation manual (100000773), section 4.8 and maintenance (opens in a new tab)Relief valve requirements and water pressure limits for Rinnai RSC SENSEI models; other models list their own figures.
- Rinnai: official document library (opens in a new tab)Select your exact model and owner manual before interpreting a fault or reset procedure.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
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