A tankless water heater error code tells you why the unit shut itself down, usually no ignition, flame failure, a blocked vent, overheating or mineral scale in the heat exchanger. A few codes have a safe first check you can do yourself, such as confirming the gas is on, but a code that comes back after a reset needs a technician.
Why Tankless Units Show Codes
A tankless water heater fires its burner only when hot water is flowing, and it watches itself constantly while it does. When a sensor reads something outside its limits, such as no flame, too much heat or restricted exhaust, the unit stops and displays a code instead of running unsafely.
Codes are brand-specific. The numbers below come from Rinnai's own service documentation, because Rinnai units are widely installed, but Navien, Noritz, Rheem and others use their own numbering. Always check the code against your unit's manual or the label inside the cover.
Ignition and Flame Codes (Rinnai 11 and 12)
Rinnai lists code 11 as "No Ignition" and code 12 as "Flame Failure." One means the burner never lit, the other means it lit and then lost the flame.
The first step in Rinnai's own list for both is the one you can do yourself: "Check that the gas is turned on at the water heater, gas meter". If another gas appliance in the house is also out, the problem is likely the supply, not the water heater.
Past that, the checks are inside the unit: the igniter, gas valves, flame rod, gas pressure and whether the gas line and meter are sized properly. An undersized gas line is a common cause in homes that switched from a tank, because a tankless unit burns far more gas at once. Our post on converting a tank to tankless explains why.
Venting and Overheating Codes (Rinnai 10, 14 and 16)
Rinnai uses code 10 for "Air Supply or Exhaust Blockage," 14 for "Thermal Fuse" and 16 for "Over Temperature Warning."
- Code 10 often has an outside cause. Look at the vent terminal on the outside wall and clear anything blocking it, such as leaves, snow or a bird's nest, without taking any vent pipe apart. If the code returns, the vent run itself needs checking.
- Codes 14 and 16 mean the unit got too hot. Rinnai's checks include restricted air flow, low water flow, a clogged heat exchanger and materials in the combustion chamber. A tripped thermal fuse is a part replacement, not a reset.
In Delaware winters, snow drifts and ice at a low side-wall vent are a real cause of blockage codes, so check outside first after a storm.
Scale Build-Up Codes
Rinnai's LC code means "Scale Build-up in Heat Exchanger." Minerals from the water coat the inside of the heat exchanger, the unit overheats in spots and loses efficiency, and it warns you before it gets worse.
An LC code is not fixed by resetting. The fix is a flush: descaling solution is pumped through the heat exchanger through the service valves, then rinsed out, and the inlet water filter is cleaned. Wells across Chester County, PA and Cecil County, MD carry more minerals, so units there scale faster. Our guide on how often to flush a tankless water heater covers the schedule, and yearly maintenance keeps the code from coming back.
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What You Can Safely Check Yourself
Before calling, these are safe to do:
1. Write down the code. Give it to the technician on the call. It saves diagnostic time. 2. Check the gas. Is the valve at the heater open? Do other gas appliances work? 3. Check power. Tankless units need electricity to run, even gas models. 4. Look at the outside vent terminal for snow, ice or debris. 5. Reset once. Turn the unit off and on. If the code comes straight back, stop resetting.
Do not open the cover. Rinnai's documentation is direct about it: "Only trained and qualified service technicians should attempt to repair this product." There is gas, high voltage and a burner inside.
If you smell gas, leave the house and call your gas utility from outside before calling anyone else.
When a Code Means Calling a Technician
Call when the code returns after one reset, when gas and power are fine but the unit will not fire, when there is any overheating code, or when you see a scale code and the unit has not been flushed in a year or more. Our post on a tankless water heater not producing hot water covers the problems that do not throw a code.
Most tankless faults are cleared by descaling or replacing a sensor or igniter, not a new unit. On a service call we read the code history, test the part it points to, and give you a flat price for the repair before any work starts.
Book a Tankless Repair in Wilmington
Water Heater Now repairs and services tankless water heaters in Wilmington and across New Castle County, Chester County, PA and Cecil County, MD, under Delaware plumbing licence PL-0002475. Call (302) 240-2818 with your error code. We answer 24 hours a day.
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Source, verified October 20, 2026: Rinnai, RL75i / RL94i technical sheet: error codes and troubleshooting (PDF). Error code numbers vary by brand and model; check your unit's manual.
