Tankless Water Heater Problems and Troubleshooting Guide
(updated 2020)

Explore the most common tankless water heater problems, reasons 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.

If you need any help for your water heater, contact a plumber here!

Modern and advanced tankless water heating systems are "smart"; they are able to control, monitor and adjust the operation, also performance and efficiency continuously. All the models are equipped with the electronic circuit board known as the electric boards that utilizes a computer chip to help in diagnosing a problem and displaying an error code.

The great thing about this advanced feature is that any issue or part failure that exist 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, Rhee, Bosch and other tankless manufacturers will approve 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 parts' installation are correctly done, 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 a technician, pay a lot because they didn't follow simple rules; to check is there a gas, water or electricity at all, sufficient water flow and other.

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 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 the gas valve and does it open.
• Check inside the combustion chamber that burners are properly installed and seated.
• Check that there is no condensation or any debris on the burner that can lead to the gas burning problem.
• 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 (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 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 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 build-up 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 the temperature sensors to protect the heater from overheating and exceeding the 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.

• The wrong type of gas is used?
• The airflow 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 in 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 flow restrictions (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

• The 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 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. Longer the distance between the tankless water heater and a fixture, longer it gets for hot water to reach the fixture.
• 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 is mixing with the hot water and therefore 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, 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 the heater freezes during the cold winter days, the heat exchanger can burst. This is why electricity and gas have to be supplied so the 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 unit parts should be checked at least once a year to reduce the 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 the fan and motor free of dust and dirt.
• Flush the heat exchanger with a descaling solution if the mineral buildup 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 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 "problem" which is normal among tankless is the white smoke coming out of 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, and these are not something that will be considered as an issue. The unit will stop its operation in 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 the power the safety feature will cut the gas off.
• The fuse will blow when over-current occurs.
• Fusible link will break if the extreme temperature occurs.

Related articles

Troubleshooting tankless water heaters
Comparing tankless
How to winterize tankless
Installing tankless
Benefits