Rinnai code 79 means "Water Leakage Detected": the leak sensor at the bottom of the cabinet is wet, or its wiring is faulty. Turn off power, gas and water, then look for drips, pooled water and damp fittings. Condensation can set it off without a real leak, but only a technician should open the cabinet.
Figures checked September 2026. Code meanings from Rinnai's service documents; costs from dated sources listed below.
| Cause, in Rinnai's order | What it looks like | Who handles it |
|---|---|---|
| Loose, broken or damaged wiring at the leak detector | Code with a dry cabinet | Technician |
| Leak from the heat exchanger or internal piping | Water inside, drips under the unit | Technician |
| Water pooled in the bottom pan with no leak found | Damp cabinet, often in humid or wet weather | Technician dries it; see TB-101 below |
| Failed sensor | Unit fires with the sensor unplugged during a test | Technician replaces sensor and screw |
| Optional drain-pan float switch lifted (RE and REP indoor models) | Water in the pan under the unit | Owner checks the pan, technician finds the source |
Sources: steps 1-4 from Rinnai, Troubleshooting Tankless Water Heater Diagnostic Codes, p. 62, May 2024; float switch from Rinnai, Non-Condensing Leak Detection Kit instructions, February 2022.
Code 79 is Rinnai's water leakage code. A sensor mounted low in the cabinet reports moisture, and the controller displays 79. The sensor cannot tell a failed heat exchanger from a film of condensation, so the code says water is present, not where it came from.
Rinnai titles the code "Water Leakage Detected" on page 62 of its diagnostic code guide, document 100000652. Its leak kit instructions quote the owner's manual wording as "Water Leak Detected."
Two pages ranking for this query get it wrong. One installer's code list labels 79 a "Flame Sensing Device Fault," a name Rinnai does not use for this code. A Q&A answer suggests unplugging the sensor so the heater runs again. In Rinnai's guide, unplugging the sensor is a test for a technician, covered below.
Rinnai's guide applies code 79 to its Non-SENSEI series only. That group covers the Value V models, the Luxury RL models, the RE and REP line, and older K-series units. The RU-series SENSEI manual's code table does not list 79 at all, so a 79 on a SENSEI unit calls for Rinnai's help line.
| Group in Rinnai's guide | Example models | Code 79 listed? |
|---|---|---|
| Value (VC) | V53De, V65i, V75e, V94i, V94Xi | Yes, Non-SENSEI |
| Luxury (VB and VC) | RL75i, RL75e, RL94i, RL94e, RLX94i | Yes, Non-SENSEI |
| RE and RE with Pump | RE140i, RE199e, REP160i, REP199i | Yes, Non-SENSEI |
| KA, KB, KBD, KBP, KCM | RC98i, RU98e, RUC98i, RUR98i, RUCS75i | Yes, Non-SENSEI |
| SENSEI RU and RUR | RU199i, RU160e, RUR199i | Not in the RU manual's code table |
Sources: Rinnai diagnostic guide, model list p. 2 and code 79 p. 62, May 2024; Rinnai RU-series manual 100000467(06), code table pp. 73-75, June 2020.
The model and serial label tells you which group you own. Rinnai's warranty documents give 1-800-621-9419 as its phone line.
There is no separate reset button for code 79. Rinnai's documents clear it by cycling power once the cause is dealt with. Resetting over water that is still there brings the code straight back, and it can hide a leak that is getting worse. Do the owner checks first.
The leak kit instructions, dated February 2022, test the float switch by lifting it until "79" appears, then clear it "by resetting power to the tankless water heater" (Rinnai document 100000789, p. 4). The diagnostic guide's third step, for a wet cabinet with no leak, is to dry the sensor and restart the heater.
Condensation is Rinnai's own explanation for a code 79 with no leak. Where the air is very damp, moisture can collect at the bottom of the cabinet and wet the sensor. Rinnai calls these "nuisance code 79s" and published a shorter replacement sensing element to stop them.
That fix is Rinnai Technical Bulletin 101, "Code 79 Sensing Element", dated 2013. The sensing element is a screw held in a bracket at the bottom of the unit. The bulletin's procedure, for a qualified technician:
Q&A threads ranking for this code describe the same pattern: a 79 that shows up in rainy or damp conditions. That matches the bulletin. It does not rule out a slow leak, which is why Rinnai's guide checks the wiring, heat exchanger and internal pipes before blaming moisture.
No. In Rinnai's guide, unplugging the sensor is step four of a technician's test, done only after the cabinet has been checked and found dry. If the heater fires with the sensor unplugged, the guide says to replace the sensor and screw. It never treats running without the sensor as a repair.
The same diagnostic guide (May 2024) restricts its steps to trained and qualified professionals, warns of live tests near energized parts, and says not to use a heater if any part has been under water. On RE and REP units with the optional kit, the float switch also closes an internal water valve to stop further leakage, per Rinnai's leak kit instructions (February 2022). Bypassing the sensing circuit removes that protection.
Safety: water, gas and electricity together. Rinnai's guide says to turn off electrical power, gas and incoming water before any diagnosis, to keep clear of energized parts during live tests, and not to run the heater with the front panel off. Do not use the heater if any part has been under water. If you smell gas, leave the lights and switches alone and call the gas supplier from a neighbor's phone, or the fire department. Call a licensed technician for any water inside the cabinet, a code that returns after a power cycle, or a unit that was flooded. Source: Rinnai diagnostic guide, p. 3.
It ranges from a service call to a heat exchanger. A condensation code may need only a visit and a sensor screw. A real internal leak can mean a heat exchanger, which HomeGuide lists at $600 to $1,300 in its August 2025 repair cost guide. Leak sensor part prices are not published in a form we could confirm.
| Repair | Cost | Source |
|---|---|---|
| Minimum service call-out fee | $50 to $200 | HomeGuide, August 2025 |
| Leak repair, any water heater | $100 to $1,000 | HomeGuide, August 2025 |
| Minor tankless repair | $50 to $300 | HomeGuide, July 2025 |
| Major tankless repair (heat exchanger, circuit board) | $300 to $1,300 | HomeGuide, July 2025 |
| Heat exchanger replacement, part plus labor | $1,200 to $1,300 | HomeGuide, August 2025 |
Sources: HomeGuide, water heater repair cost, August 2025; HomeGuide, tankless water heater maintenance cost, July 2025.
HomeGuide's leak range covers tank heaters too, where a leak at a fitting or drain valve is often a small job. On a tankless unit the source decides the bill, so ask the technician to name the leaking part before quoting. The water heater repair prices guide lists other parts, and the leak repair guide covers which leaks are fixable.
A heat exchanger that leaks from a defect is covered for a long term on Rinnai's Non-SENSEI models, but labor coverage is short. The warranty also excludes damage from missing drain pans, freezing, condensate and poor water quality, so the cause the technician records matters.
| Series | Heat exchanger (residential) | Other parts | Labor |
|---|---|---|---|
| Value V53De, V65, V75, V94 | 10 years | 5 years | 1 year |
| Luxury RL75, RL94, RLX94 | 12 years | 5 years | 1 year, or 5 years if registered within 30 days |
Sources: Rinnai HE Series (Value) warranty 100000656, May 2019; Rinnai HE+ Series (Luxury) warranty 100000657, May 2019; Rinnai Technical Bulletin 114, July 2015, for the 12-year Luxury term.
Both documents list "Absence of a drain pan under the appliance" and "Condensate damage" as improper installation, and "freeze damage" as improper maintenance. The Value document also excludes "Any damage caused by poor water quality." Coverage terms shrink in some cases: the Luxury heat exchanger drops to 5 years above a 160°F setting, and both series drop to 3 years in certain circulating-loop setups, per the Luxury and Value warranties (May 2019). Rinnai requires genuine parts and a qualified professional. For claims and servicers, see the Rinnai warranty and support guide.
Repair when the cause is the sensor, wiring or condensation. Those are small jobs. The decision gets harder when the heat exchanger is leaking on a unit past its heat exchanger term, since that is the most expensive repair on a tankless heater.
| Finding | Suggested move |
|---|---|
| Damp cabinet, no leak | Dry it, ask about the TB-101 sensing element |
| Damaged sensor wiring or failed sensor | Repair |
| Leaking internal pipe or fitting | Repair |
| Leaking heat exchanger inside its warranty term | Warranty claim |
| Leaking heat exchanger outside its term | Compare a repair quote with a replacement quote |
The tankless replacement cost guide covers what a new unit costs to install.
At the bottom of the unit, inside the cabinet. Rinnai's Technical Bulletin 101 shows the sensing element as a screw held in a bracket, reached by removing the front panel. On indoor units the controller bracket also comes off. That is technician work.
Rinnai's RU-series manual (section 6.3, p. 80) specifies undiluted, food-grade white vinegar circulated by a pump, then a rinse. Other series have their own flushing steps, so use the manual for your model. Flushing treats scale; it does not fix a code 79.
Airflow, ignition and flame faults come up most in owner searches. See the guides to Rinnai code 10, code 11 and code 12, or the Rinnai tankless error code guide.
Expect it to. Owners report losing hot water when 79 appears, and on RE and REP units with Rinnai's leak kit, the float switch closes an internal water valve by design. Treat the code as a reason to shut the unit down and check it.
Independent Research
Published ranges for tank, tankless and heat pump replacement, the failures that justify a repair, and the ones that do not. Sourced, dated, and free to read.
