Rinnai Water Heater Leaking — What’s Happening
When your Rinnai water heater is leaking, the unit’s leak detector has sensed water inside the cabinet and triggered error code 79. Rinnai’s diagnostic system checks for water at the leak detector probe and then directs you to inspect the heat exchanger, internal piping, and the bottom pan of the cabinet for actual leaks.
If water is visible inside the unit, you have a real leak that needs immediate attention. If the cabinet is dry but the code persists, the issue may be in the leak detector wiring harness itself. Rinnai’s guidance is to identify the source of the water before resetting the unit, since the repair depends on which component failed.
Most Likely Causes
- Internal heat exchanger leak The heat exchanger has developed a crack or pinhole, allowing water to drip into the cabinet pan and trigger the leak detector.
- Cracked or loose internal water piping Water lines inside the cabinet have failed or connections have loosened, allowing water to pool in the bottom pan.
- Water pooling in bottom pan from condensate or drain issues Condensate or water from another source has entered the cabinet and collected in the pan, activating the leak sensor.
- Damaged or loose leak detector wiring harness The wiring to the leak detector is broken, corroded, or disconnected, causing a false leak code even when the cabinet is dry.
- External drip entering cabinet Water from a nearby plumbing component or overhead pipe is dripping into the unit rather than a failure of the heater itself.
How to Diagnose and Fix
- Shut down the unit and disconnect power before inspecting any internal components.
- Open the cabinet and inspect the bottom pan for standing water or evidence of previous leakage.
- Check the heat exchanger and all visible internal water piping for active leaks or corrosion.
- Examine the leak detector wiring harness for loose, broken, or corroded connections at the detector and control board.
- If water is present, trace the leak to its source by drying the area and watching for new drips while the unit is briefly powered.
- Replace the heat exchanger if it is the source of the leak, along with any components damaged by water intrusion.
- If no leak is found and the wiring is intact, test the leak detector circuit or replace the detector probe and harness.
- After repairs, reset the unit and monitor the cabinet for 24 hours to confirm the leak is resolved.
Parts You Might Need
| Part | Notes |
|---|---|
| Rinnai heat exchanger | Amazon | Required if the heat exchanger is the confirmed source of the leak. |
| Leak detector wiring harness | Amazon | Replace if wires are damaged, corroded, or causing intermittent false codes. |
| Internal water piping or tubing | Amazon | For cracked or loose water lines inside the cabinet. |
When to Call a Pro
Heat exchanger replacement and internal cabinet work on a tankless water heater require soldering or compression fittings, pressure testing, and knowledge of Rinnai’s wiring and plumbing layout. If you find water inside the unit or the leak code persists after checking the wiring, call a qualified plumber or Rinnai technician to identify the failed component and complete the repair safely. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.