Tankless Water Heater Repair

Read the error code first

Every major brand shows a code when something's wrong. Look it up in your owner's manual before you do anything else. Our guides cover the common codes by brand, starting with Rinnai error codes. Navien, Noritz and Rheem pages are linked at the bottom of this page.

A few Rinnai examples show how much a code tells you. On the RL series, code 11 means no ignition, code 14 is the thermal fuse, and LC flags scale buildup in the heat exchanger. Rinnai's technical guide ties code 10 to restricted airflow, including a clogged or frozen condensate line on condensing models.

Many units beep along with the code. Beeping plus no hot water just means the unit has shut itself down and wants attention. The code is the useful part.

Common problems, most likely first

1. 1

   Cause 1: Scale buildup

   In hard-water areas this is the number one tankless problem. Minerals coat the heat exchanger, the unit overheats or throttles back, and eventually it throws a code. A.O. Smith recommends descaling once a year, especially in hard water. Rinnai's diagnostics also call for treating hard water to prevent scale.

   Descaling is the one big maintenance job owners can do themselves. It uses the service valves on the unit's water lines: you close them, hook a small pump and hoses to the flush ports, and circulate a descaling solution through the heat exchanger. Rinnai's guide mentions vinegar for flushing. If your unit has no service valves, a plumber can add a service valve kit, and it pays for itself the first time you flush. Our tankless water heater flush guide has the steps.

2. 2

   Cause 2: Ignition failure

   The unit tries to fire and gives up. A.O. Smith's list of causes is gas supply, venting and the ignition pack. Rinnai links its code 11 to a loose flame rod among other causes. You can check that the gas is on, that other gas appliances work and that the vent and intake are clear. Flame rods, igniters and gas valves are technician work.

3. 3

   Cause 3: Venting and airflow

   Tankless units watch their exhaust closely. A blocked vent termination, a bird's nest, snow drift or a clogged air filter will shut them down. Rinnai's guidance is to keep the area around the vent terminal clear and to maintain the required clearances. Rinnai also calls for cleaning the inlet air filter.

4. 4

   Cause 4: Condensate drain problems

   Condensing units make acidic water that has to drain away. Rinnai's checks for its condensate code 25 are to confirm the drain line isn't clogged or frozen, that the condensate pump has power, and that the drain line has an air gap. A frozen condensate line in winter can shut the unit down suddenly.

5. 5

   Cause 5: Not enough flow, or too much demand

   A tankless heater only fires when water flows fast enough, and it can only heat so much at once. A.O. Smith calls undersizing a common tankless issue, and the fix is to cut back on simultaneous use or move to a bigger unit. If the water goes lukewarm whenever two showers run, the unit may simply be too small. See what size tankless water heater you need.

6. 6

   Cause 6: The cold water sandwich

   Hot, then a burst of cold, then hot again. This happens when you turn the tap off and on quickly and the unit has to restart. A.O. Smith suggests reducing quick on-off use or choosing a model with a built-in buffer tank. It's a quirk of the design, not usually a fault.

Resetting a tankless unit

Rinnai's reset for many codes is to turn off all hot water taps, then press the ON/OFF button twice. If the code keeps coming back after three resets, Rinnai's guide calls for a circuit board replacement, and that's a technician job. Some codes, like Rinnai's code 14 for overheating, Rinnai says should be taken seriously. Don't keep resetting those.

Electric tankless units

Electric tankless units, sold by brands like Titan and Eemax, have no burner, gas valve or vent. When they stop heating, check the breakers first, since larger units often run on more than one. Then check the manual's troubleshooting table. Scale still matters, because the heating elements sit right in the water stream.

What it costs

HomeGuide's 2026 figures for tankless water heaters:

Add a service call fee of $50 to $200 in many areas. HomeGuide suggests weighing replacement if the unit is more than 10 years old or the repair costs more than half the price of a new one. Compare on tankless water heater cost.