Tankless Water Heater Problems

Find the most common tankless water heater problems, how to avoid the cause, how to recognize the symptoms and find solution for the flame failure, fluctuation, not enough and no hot water...

Modern tankless water heating systems today are smart; they are able to check its own operation, performance and efficiency continuously. Manufacturers are installing electronic circuit boards known as PC boards, a computer chip that will help in diagnosing and solving problems on tankless water heaters.

The issue or part failure that exist inside the tankless water heater will be shown on the display located at the front of the unit, as the error code or flashing light, or on the display of the remote controller.

For example Noritz error codes are: 11, 90, 99...

Bosch has these codes: A2, C1, E1, FA, 8E...

Rinnai error codes are shown as: 02, 10, 11, 52, LC...

AO Smith has these error codes: 02, 10, 11, 12...

This is what helps in diagnosing the tankless water heater problems and successful troubleshooting. When this error appears, our recommendation is to call a qualified technician to service the unit or replace the faulty component part.

Noritz, Rinnai, Bosch and other tankless manufacturers will approve the warranty if the problem occurred within the warranty period and only if the service has been performed by the licensed technician.

Common Problems

No ignition

Flame failure

Overheating and thermal fuse failure

Temperature fluctuates

Water is not hot enough

Water is too hot

Other problems

Preventing problems

... or simply ask.

To review tankless water heater problems more efficiently, we will consider or assumed that the water, electrical and gas lines and parts' installation are done properly, and that unit was working good before the problem occurred. I have seen many users complaining: "I don't have any hot water for my shower when I open the tap." And they call technician, pay a lot because they didn't follow simple rules; to check is there a gas, water or electricity at all.

Troubleshooting no ignition problem

Burner does not ignite when the hot water tap is open

  • Proper gas pressure coming to your tankless heater has to be provided.

  • To avoid problems with the heater, ensure that the unit is properly grounded.

  • Verify the fuses in the control unit are OK.

  • Ensure wiring harness and igniter are operating properly and that the electrode can produce a spark.

  • Check the electrical short on components.

  • Check gas valve, does it open.

  • Check inside the combustion chamber that burners are properly installed and seated.

  • Check that there is no condensation or any kind of debris on the burner that can lead to the gas burning problem.

  • Check is the flow sensor works correctly.

  • The flow switch doesn't open as the minimum pressure in the incoming supply is not met. If the flow is too low, the burner will not ignite. Low pressure is one of the most frequent tankless water heater problems easy to fix.

  • An inlet filter is dirty and clogged.

  • Check for the correct hot and cold water installation (make sure that lines are not installed in reverse order).

  • You might experience sediment build up in the faucet aerators or shower head which causes the problem where the tankless unit doesn't activate.

  • Crossover in plumbing due to failed washer at a single lever faucet, incorrect plumbing or faulty mixing valve.

Troubleshooting flame failure problem

  • Check is the proper venting installed.

  • Is the heater condensation collar correctly installed?

  • Vent length should be within limits specified by the manufacturer.

  • Check for the loose electrical connections on the electrode.

  • Ensure that the proper voltage is within limits.

  • Check components for electrical short.

  • Gas valve works properly?

  • Is flame rod wire connected?

  • Deposits on the flame rod? Carbon build-up can be cleaned with the sand paper.

  • Presence of condensation and debris on the burner?


emergency plumber

Overheating, thermal fuse failure or heat exchanger sensor fault

Tankless water heaters are using temperature sensors to protect the heater from overheating and exceeding maximum allowable temperature on the heat exchanger. Follow the list below to check, diagnose and troubleshoot tankless water heater problems with the symptoms mentioned above.

  • Is the wrong gas type used?

  • Air flow around the heater and its vent terminal is blocked.

  • There is a low flow inside the heat exchanger.

  • The presence of the foreign materials inside the combustion chamber and venting.

  • Cracks on the heat exchanger.

  • Scale build up inside the heat exchanger creates an uneven temperature distribution and hot spots.

  • Clogged heat exchanger.

  • Dirty sensor.

Hot water temperature fluctuates

  • Ensure that there are no restrictions in the flow (dirty or clogged shower heads, faucet aerators).

  • Clean the filter if it is dirty.

  • Check for plumbing crossover.

  • Check for the water pressure that it doesn't fluctuate.

Water is not hot enough

  • Temperature selection is too low. Increase the temperature either on the unit or on the remote control.

  • If the water runs cold, increase the flow so the minimum required pressure is met. Keep in mind that you will not get the hot water immediately, it takes time for hot water to reach the tap. The longer the distance between tankless water heater and your fixture the longer it gets for hot water.

  • Tankless water heater doesn't produce enough heating power. An adequate gas pressure and gas line has to be provided for different models and manufacturers.

  • Ensure that temperatures sensors are making contact and are firmly mounted on the pipes.

  • Crossover in plumbing - cold water is mixing with the hot water and therefore decreasing the outlet water temperature.

Water is too hot

  • Selected temperature on your tankless water heater is too high. Select the lower temperature.

  • Inlet filter is dirty... clean it.

  • Make sure that there are no water path obstructions, including faucet aerators, shower heads...

  • Is the heater using the specified gas type?

  • Temperature sensors are loose... ensure that sensors are firmly mounted and are making contacts.

Other problems

If your heater freezes during the cold winter days, the heat exchanger can burst. This is why electricity and gas has to be supplied so the freeze protection can activate. Another solution can be to install the auto drain down solenoid valves.

If you experience the low water flow or pressure you might have too many hot water applications running at the same time. Greater draw will result in the increased pressure drop. Another cause for low flow might be dirty cold water inlet screen.

For problems with and related to the pilot light check out the article why my pilot light goes out.

Preventing problems

Regular annual inspection and periodic maintenance are imperative to keep your tankless water heater running and performing well. Water heating system and unit parts should be checked at least once a year to reduce chances for tankless water heater problems.

  • Clean the burner, control compartments and circulating air passageways.

  • Inspect the venting system.

  • Do the visual inspection of the flame to ensure that it burns with a clean, blue and stable flame and evenly on the entire surface of the burner.

  • Keep fan and motor free of dust and dirt.

  • Flush the heat exchanger with a descaling solution if mineral build up is evident.

  • Keep the area around the vent exhaust free of snow and ice.

Note: There are some prolonged activities done by the specific heater elements, like when the fan continues to run after you turn the hot water tap off. This is a normal occurrence and it last for a short period of time to ensure that any residual products of combustion are vented out.

Another frequent complaint or "problem" which is normal in tankless water heater operation is the white smoke coming out of the exhaust. This is normal during the cold weather, when the exhaust fume temperature is much hotter than the outside air, as the fume contains lots of the water vapor and condenses.

It is also good to know that sometimes safety features will be triggered, and these are not tankless water heater problems, and the unit will stop its operation in the next occasions:

  • Overheating - the unit will automatically shut down when the appliance exceeds a predetermined temperature.

  • Flame failure - the water heater will shut down if the flame is gone.

  • Power failure - if the appliance loses the power the safety feature will cut the gas off.

  • The fuse will blow when overcurrent occurs.

  • Fusible link will break if the extreme temperature occurs.

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