Code 79 comes from a leak-sensing element inside the cabinet. Rinnai has also noted in a technical bulletin that condensation in very humid locations can trigger nuisance code 79s.
Safety first
Turn off the water supply to the heater. Do not open the cabinet: it contains gas and electrical components. If you smell gas, leave and call SDG&E or 911 from outside.
What can cause code 79
- A leak inside the unit, at the heat exchanger or internal piping.
- Water pooling in the cabinet.
- Condensation in very humid locations (a known nuisance cause, per Rinnai's technical bulletin).
What you can do
Turn off the water supply to the heater and look for water around and under it. Rinnai says the code can be cleared by resetting power, but if the leak is real it will come back.
When to call a plumber
Any real leak inside a tankless heater needs a technician to open the cabinet and find it. If the heater is indoors, deal with it quickly so the water does not damage the floor or wall.
General information, not professional or legal advice; follow your manufacturer's manual, and your installer and local permit office have the final word. Sources:Rinnai, Non-Condensing Leak Detection Kit Installation Instructions; Rinnai Technical Bulletin TB-101, Code 79 Sensing Element; Rinnai, Troubleshooting Tankless Water Heater Diagnostic Codes. SD Water Heaters is not affiliated with any manufacturer; brand names are used only to identify products, and codes can differ by model, so confirm in your own manual.
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