Tankless Water Heater Common Problems and Troubleshooting Guide

Explore the most common tankless water heater problems, causes, and symptoms. Learn how to fix the flame failure, fluctuation, not enough and no hot water, covering Rinnai, Takagi, Noritz, Bosch, and other tankless models.

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Most modern and advanced tankless water heating systems are "smart" so they can continuously control, monitor, and adjust operation, performance, and efficiency. Models are equipped with the electronic circuit board that utilizes a computer chip to help in diagnosing problems and displaying one of many error codes.

The great thing about this advanced feature is that any issue or part failure that exists inside the tankless water heater will be shown as the error code or flashing light on a display located at the front of the unit or screen of the remote controller (if applicable).

For example Noritz error codes will display: 11, 90, 99...

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

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

Takagi: 121, 311, 321...

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 these errors appear, call a qualified technician to service the unit or replace the faulty part.

Noritz, Rinnai, Takagi, Rheem, Bosch, and other tankless manufacturers will recognize the warranty if the problem occurred within the warranty period and only if the licensed technician performs a service.

Common problems

To review tankless water heater problems more efficiently, we will consider or assume that the water, electrical, and gas lines and installation are correctly done and that unit was in a good working condition 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 a technician, pay a lot because they didn't follow simple rules; to check if there is gas, water or electricity, sufficient water flow and others.

Troubleshooting no ignition problem

A 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 that the fuses in the control unit are OK.
  • Ensure that the wiring harness and igniter are operating properly and that the electrode can produce a spark.
  • Check the electrical short on components.
  • Check the gas valve and does it open.
  • Check inside the combustion chamber that burners are correctly installed and seated.
  • Check that there is no condensation or debris on the burner that can lead to gas-burning problems.
  • Check if the flow sensor is working 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 and is easy to fix.
  • An inlet filter is dirty and clogged.
  • Check for the correct hot and cold water installation (ensure that lines are not installed in reverse order).
  • You might experience sediment buildup in the faucet aerators or showerhead, which causes the problem where the tankless unit doesn't activate.
  • Crossover in plumbing due to the failed washer at a single lever faucet, incorrect plumbing, or faulty mixing valve.

Troubleshooting the flame failure problem

  • Check if the proper venting is installed.
  • Is the heater condensation collar correctly installed?
  • Vent length should be within the 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.
  • Does the gas valve work properly?
  • Is the flame rod wire connected?
  • Deposits on the flame rod? Carbon buildup can be cleaned with the sandpaper.
  • Presence of condensation and debris on the burner?

Overheating, thermal fuse failure or heat exchanger sensor fault

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

  • Is the wrong gas type used?
  • The airflow around the heater and its vent terminal is blocked.
  • There is a low flow inside the heat exchanger.
  • The presence of foreign materials inside the combustion chamber and venting.
  • Cracks in the heat exchanger.
  • Scale buildup inside the heat exchanger creates an uneven temperature distribution and hot spots.
  • Clogged heat exchanger.
  • Dirty sensor.
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Hot water temperature fluctuates

  • Ensure that there are no flow restrictions (dirty or clogged showerheads, faucet aerators).
  • Clean the filter if it is dirty.
  • Check for plumbing crossover.
  • Check for the water pressure so it doesn't fluctuate.

Water is not hot enough

  • The temperature selection is too low. Increase the temperature either on the unit or a remote control.
  • If the water runs cold, increase the flow so the minimum required pressure is met so the unit can fire up. Keep in mind that you will not get hot water immediately; it takes time for hot water to reach the tap. The longer the distance between the tankless water heater and a fixture, the longer it gets for hot water to reach the fixture.
  • A tankless water heater doesn't produce enough heating power. Adequate gas pressure and gas line have to be provided for different models and manufacturers.
  • Ensure that temperature sensors are making contact and are firmly mounted on the pipes.
  • Crossover in plumbing - cold water mixes with the hot water, decreasing the outlet water temperature.

Water is too hot

  • The selected temperature on the tankless water heater is too high. Select the lower temperature.
  • If the inlet filter is dirty, clean it.
  • Make sure that there are no water path obstructions, including faucet aerators, showerheads...
  • 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 the heater freezes during the cold winter days, the heat exchanger can burst. This is why electricity and gas have to be supplied so that freeze protection can be activated. Another solution can be to install the auto drain down solenoid valves.

If you experience low water flow or pressure, you might have too many hot water applications running at the same time. The greater draw will result in an 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 its parts should be checked at least once a year to reduce the chances of 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 the fan and motor free of dust and dirt.
  • Flush the heat exchanger with a descaling solution if the mineral buildup is evident.¬†More about tankless flushing here. You can also buy a flushing kit to make a job easier.
  • Keep the area around the vent exhaust free of snow and ice.

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

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

It is also good to know that sometimes safety features will be triggered, which will not be considered an issue. The unit will stop its operation on the next occasions:

  • Overheating - the unit will automatically shut down when the appliance exceeds the predetermined temperature.
  • Flame failure - the water heater will shut down if the flame is gone.
  • Power failure - if the appliance loses power, the safety feature will cut the gas off.
  • The fuse will blow when an over-current occurs.
  • The fusible link will break if an extreme temperature occurs.

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