Common Tankless Water Heater Problems and Troubleshooting Guide

Dealing with common tankless water heater problems can be frustrating, but understanding their causes and solutions can make a big difference.

Tankless heaters offer efficient, on-demand hot water, but like any appliance, they can experience issues from time to time. This guide will help you identify common problems, understand their causes, and provide troubleshooting steps to get your system back up and running smoothly.

Whether you’re dealing with error codes, temperature fluctuations, or flow rate concerns, we’ve got you covered with practical solutions.

Common Tankless Water Heater Problems

To efficiently review tankless water heater problems, we’ll assume that the water, electrical, and gas lines are properly installed, and the unit was functioning correctly before the issue arose.

Many users have reported issues such as, “I don’t have any hot water when I turn on the shower,” only to later find out that they missed simple checks. Before calling a technician and incurring unnecessary costs, it’s important to verify basic factors such as gas, water, and electricity supply, along with sufficient water flow.

No Ignition

A burner does not ignite when the hot water tap is open.

• Gas pressure: Ensure the proper gas pressure is being supplied to the tankless heater.
• Proper grounding: Make sure the unit is properly grounded to prevent electrical issues.
• Fuse check: Verify that the fuses in the control unit are functioning correctly.
• Wiring and igniter: Check that the wiring harness and igniter are operating properly, and confirm that the electrode can produce a spark.
• Electrical shorts: Inspect components for any signs of electrical shorts that could affect performance.
• Gas valve functionality: Test the gas valve to ensure it opens properly and delivers gas as needed.
• Burner installation: Check the combustion chamber to ensure that the burners are properly installed and seated.
• Condensation or debris: Ensure there is no condensation or debris on the burner, as this could interfere with proper gas combustion.
• Flow sensor: Verify that the flow sensor is working correctly to regulate water flow.
• Flow switch: Ensure the flow switch is opening correctly. If the incoming water pressure is too low, the burner may not ignite. Low pressure is a common and easily fixable issue with tankless water heaters.
• Clogged inlet filter: Check for any clogs in the inlet filter that could restrict water flow.
• Hot and cold water lines: Confirm that the hot and cold water lines are correctly installed and not reversed.
• Sediment buildup: Sediment buildup in faucet aerators or showerheads can prevent the tankless unit from activating properly.
• Plumbing crossover: Look for plumbing crossover issues caused by a failed washer in a single-lever faucet, incorrect plumbing, or a faulty mixing valve.

Flame Problems

• Proper venting installation: Verify that the venting is installed according to the manufacturer’s specifications.
• Condensation collar: Ensure the heater’s condensation collar is correctly installed to prevent moisture issues.
• Venting length: Check that the vent length stays within the limits specified by the manufacturer.
• Loose electrical connections: Inspect the electrode for any loose electrical connections that could affect performance.
• Voltage check: Confirm that the voltage is within the acceptable range for optimal operation.
• Electrical shorts: Inspect all components for any signs of electrical short circuits.
• Gas valve functionality: Test the gas valve to ensure it is operating properly and delivering gas as needed.
• Flame rod wire: Ensure the flame rod wire is properly connected to the unit.
• Flame rod condition: Look for deposits on the flame rod. Carbon buildup can be cleaned with sandpaper for better performance.
• Burner condition: Check for the presence of condensation or debris on the burner, which could affect combustion.

Hot Water Temperature Fluctuates (Cold Water Sandwich)

A “cold water sandwich” occurs when you turn on the shower and experience warm water initially, followed by a burst of cold water, and then a return to warm water. This happens because some hot water, left over from a previous use, was trapped in the pipes—that’s the initial warmth you felt.

Once the trapped hot water is used up, the delay in your water heater producing new hot water causes the sudden blast of cold. It takes a moment for the heater to catch up and deliver warm water again.

• Check for Flow Restrictions: Inspect showerheads and faucet aerators for dirt or clogs. Clean them if necessary to restore proper flow.
• Clean the Filter: If the water filter is dirty, clean it to ensure unobstructed water flow.
• Inspect for Plumbing Crossover: Verify that there’s no plumbing crossover, where cold water mixes with hot water inappropriately.
• Monitor Water Pressure: Ensure water pressure is stable and not fluctuating, as this can impact temperature consistency.
• Check Gas Line Size and Capacity: Confirm that the gas line is properly sized and capable of delivering the maximum BTU needed for your water heater.
• Assess Venting Pipe Length: Make sure the venting pipe length falls within the manufacturer’s recommended limits.
• Inspect the Flow Sensor: If the flow sensor is damaged, it might need repair or replacement.

Tip: If your hot water temperature fluctuates during a shower, consider pairing your tankless water heater with a buffer mini tank. The mini tank ensures a ready supply of hot water, eliminating the delay while the tankless unit heats up.

No Hot Water

• Electricity, Water, and Gas Supply: Verify the unit has power, water is flowing, and the gas line is open; confirm the shut-off valve is fully open.
• Gas Burner and Ignition: Check if the flame rod generates sparks when the unit is turned on; if not, inspect for ignition failure and refer to troubleshooting guides.
• Error Codes: Review the control panel for error codes, as these indicate specific issues; resolve the problem and reset the system before attempting to restart.
• Water Flow Rate: Ensure the faucet is open wide enough to meet the minimum flow rate required for the heater to function and check for blockages in the pipes.
• Freezing Temperatures: Inspect if the heater’s elements or pipes are exposed to freezing conditions; insulate all exposed pipes and cover the heater with an insulation jacket to prevent freezing.

Water Is Not Hot Enough

• Temperature Setting: Check if the temperature selection is too low; increase it using the unit’s control panel or remote control to achieve the desired level.
• Cold Water Flow: If the water runs cold, increase the flow to meet the unit’s minimum required pressure; note that hot water takes time to travel to the tap, especially if the fixture is far from the heater.
• Heating Power: Ensure the tankless water heater has adequate gas pressure and that the gas line is compatible with the model’s specifications; check the manufacturer’s requirements for proper operation.
• Temperature Sensors: Confirm that the temperature sensors are securely mounted on the pipes and making proper contact for accurate readings and functionality.
• Crossover in Plumbing: Inspect for plumbing crossovers where cold water might mix with hot water, reducing the outlet temperature; address this issue to restore consistent hot water.
• Sediment and Limescale Buildup: Check for sediment or limescale inside the heat exchanger; descale and flush the system to remove deposits, as scale acts as an insulator and hinders heat transfer.
• Venting System: Inspect the venting system for debris and ensure it provides enough fresh air for proper combustion and efficient operation.

Water Is Too Hot

• Temperature Setting: Check if the selected temperature is too high; lower it using the unit’s control panel or remote control to prevent overheating.
• Inlet Filter: Inspect the inlet filter for dirt or debris; clean it to ensure proper water flow into the heater.
• Water Path Obstructions: Verify that there are no obstructions in the water path, such as clogged faucet aerators or showerheads; clean or replace them if needed.
• Gas Type: Confirm that the heater is using the specified gas type as recommended by the manufacturer; incorrect gas types can cause performance issues.
• Temperature Sensors: Ensure the temperature sensors are securely mounted and making proper contact with the pipes for accurate readings and functionality.

Overheating

• Incorrect gas type: Ensure the correct gas type is being used for the unit.
• Blocked airflow: Check for obstructions around the heater and its vent terminal, which can restrict airflow.
• Low flow inside the heat exchanger: Inspect for any blockages or issues that may reduce water flow through the heat exchanger.
• Foreign materials in the combustion chamber or venting: Look for debris or other contaminants that may have entered the combustion chamber or venting system.
• Cracked heat exchanger: Check the heat exchanger for any visible cracks that could cause malfunction.
• Scale buildup: Mineral deposits inside the heat exchanger can lead to uneven temperature distribution and hot spots, which can cause overheating.
• Clogged heat exchanger: Inspect for any debris or buildup that could clog the heat exchanger and restrict heat transfer.
• Dirty sensor: Clean the temperature sensor, as dirt or debris may prevent it from accurately reading the water temperature, leading to overheating.

How Error Codes Help Troubleshoot Tankless Water Heaters

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).

When problems arise, these error codes provide valuable insights, allowing technicians to quickly pinpoint the issue. For example:

• Noritz error codes: 11, 90, 99…
• Bosch error codes: A2, C1, E1, FA, 8E…
• Rinnai error codes: 02, 10, 11, 52, LC…
• Takagi error codes: 121, 311, 321…
• AO Smith error codes: 02, 10, 11, 12…

This diagnostic feature greatly aids in troubleshooting and ensures efficient repairs. If an error code appears, it’s best to contact a qualified technician to service the unit or replace the faulty part.

Note that tankless water heater manufacturers, including Noritz, Rinnai, Takagi, and others, will honor warranties for issues occurring within the warranty period, provided the service is performed by a licensed technician.

Helpfull Tips

If you’ve just switched to a new tankless water heater, note that some behaviors may seem unusual but are entirely normal:

• Fan Motor Operation: The fan motor may continue to spin briefly after the water heating operation stops. This helps expel exhaust gases through the vent system.
• White Smoke: You may see white smoke coming from the exhaust vent in cold weather. This occurs when the exhaust gases are much warmer than the outside air.
• Cold Water During a Shower: If the water turns cold during a shower, it could be due to the flow rate falling below the minimum requirement. Maintaining the correct flow rate can help prevent or resolve this issue and others.

Proper care of your heater is essential for its long life and efficient operation:

• Freezing Prevention: If the heater freezes during cold winter days, the heat exchanger can burst. Ensure electricity and gas are supplied to activate freeze protection. Installing auto-drain solenoid valves is another solution.
• Low Water Flow or Pressure: If you experience low water flow or pressure, you might have too many hot water applications running at the same time. The higher demand can result in a pressure drop. A dirty cold water inlet screen can also cause low flow.
• Regular Maintenance: Regular annual inspection and periodic maintenance are crucial to keep your tankless water heater running efficiently. Be sure to check the water heating system and its components at least once a year to reduce the chances of problems.

It’s also good to know that sometimes safety features will be triggered, which is not considered a malfunction. The unit will stop operating in the following situations:

• 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 goes out.
• Power Failure: If the appliance loses power, the safety feature will cut off the gas supply.
• Fuse Blown: The fuse will blow in case of an overcurrent.
• Fusible Link Breakage: The fusible link will break if exposed to extreme temperatures.