Your Rinnai tankless water heater stopped making hot water and the controller is showing a number. That number is a diagnostic code: the heater telling you what it noticed before it shut down.
One thing first: codes vary by model. Rinnai's own FAQ says to contact Rinnai or your local installer to find the root cause. The list below comes from Rinnai's published code sheets; check your model's manual for the exact meaning.
If you smell gas
Don't reset anything. Southwest Gas says to leave the area immediately, avoid anything that could make a spark (switches, phones, lighters), and call 911 and Southwest Gas at 877-860-6020 from a safe distance.
Common Rinnai codes
- 10: air supply or exhaust blockage
- 11: no ignition
- 12: flame failure
- 14: thermal fuse
- 16: over temperature
- LC: scale build-up in the heat exchanger
Code 12: flame failure
Code 12 means the heater lit, but the flame went out or the heater couldn't keep sensing it, so it shut down for safety. Rinnai's list of things to check for code 12 is long, and most of it is technician work: gas pressure and gas line sizing, venting, the flame rod, wiring and the burners.
What you can safely check:
- Is the gas on? Rinnai's first check is that gas is on at the water heater and at the meter. If your stove or other gas appliances aren't working either, the problem may be the gas supply, not the heater.
- Is the vent clear? On an outdoor or wall-vented unit, look at the vent outside for anything blocking it: plants, a nest, debris. Don't take anything apart.
- Did it happen once, or keep happening? A single code 12 after a gas service interruption is different from one that comes back every day.
If the code comes back, call a qualified technician. Rinnai's troubleshooting guide says only trained and qualified professionals should follow its repair steps, because they involve gas and combustion.
Code 11: no ignition
The heater tried to light and couldn't. Rinnai's first checks are the same: gas turned on at the heater and meter, and the right gas type and pressure. After that it's igniter, gas valve and burner work for a technician.
Code 10: air supply or exhaust blockage
Something is restricting the air coming in or the exhaust going out. Check the outside vent for obvious blockages. Vent length, vent materials and the fan are technician checks.
LC: scale build-up
LC means scale is building up in the heat exchanger and it's time to flush. Rinnai notes LC is the only code that lets the heater keep running, alternating with the temperature on the display. In the East Valley's hard water, LC is common. See flushing a tankless water heater.
14 and 16: thermal fuse and over temperature
These point to overheating, restricted air flow or scale. They're not codes to keep resetting. Have it checked.
When it's time to call
- The same code keeps coming back after a reset
- Any code along with a gas smell, soot, or a hot or scorched look around the unit (leave and call first, per the steps above)
- Codes on a heater that hasn't been flushed in years
Rinnai's customer care line is 1-800-621-9419. For service in the East Valley, OX Plumbing installs and services tankless water heaters, including Rinnai, and we answer 24/7. Our $99 service call covers the visit, the diagnosis and an upfront price before any work begins. Wondering whether yours is worth repairing? See how long tankless water heaters last.
Sources
- Rinnai: Residential FAQ (lifespan, flushing, error codes, ignition, recirculation)
- Rinnai: Troubleshooting tankless water heater diagnostic codes (for qualified professionals)
- Rinnai TB-75: Water heater error codes (10, 11, 12, 14, 16, LC)
- Southwest Gas: How to recognize a natural gas leak (what to do)