If your tankless water heater is not igniting, the system will fail to produce hot water. This issue is usually related to ignition components, gas supply problems, or airflow restrictions.
Fortunately, many ignition problems can be diagnosed with basic troubleshooting.
This guide explains why a tankless water heater may fail to ignite and how to fix the most common causes.
Tankless Water Heater Ignition Troubleshooting Flowchart
Below is a simple diagnostic flow that homeowners can use to identify the most common ignition problems.
Tankless Heater Not Igniting
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Check Power Supply
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Check Gas Valve
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Inspect Air Intake & Vent Pipes
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Look for Error Codes
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Flush Heater if Mineral Buildup
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Call Technician if Problem Persists
Why Is My Tankless Water Heater Not Igniting?
A tankless water heater may fail to ignite due to gas supply problems, blocked vents, electrical issues, or worn ignition components.
Why Tankless Water Heaters Fail to Ignite
Tankless water heaters rely on an electronic ignition system that activates the burner when hot water is needed. If the system cannot detect a proper flame, the heater will shut down to prevent unsafe operation.
Ignition problems are usually caused by one of the following:
• gas supply interruption
• blocked intake or exhaust vents
• worn ignition components
• electrical power issues
• airflow restrictions
Understanding these possible causes helps determine which troubleshooting steps to try first.
Gas Supply Problems
Gas-powered tankless heaters require a steady gas supply to ignite.
Ignition problems may occur if:
• the gas valve is closed
• gas pressure is too low
• air is trapped in the gas line
Check that the gas shutoff valve near the heater is fully open.
If the heater was recently installed or serviced, air trapped in the gas line may temporarily prevent ignition.
Air in the Gas Line
In some cases, a tankless water heater may fail to ignite because air has entered the gas line. This can happen after:
• a new installation
• recent plumbing work
• gas line maintenance
When air is trapped in the gas line, the burner may not receive enough gas to ignite properly.
Running hot water for several minutes may allow the system to purge the air and restore normal operation.
If the heater continues failing to ignite, the gas supply may need to be inspected by a technician.
Blocked Vent Pipes
Tankless gas heaters require proper airflow for safe combustion.
Blocked intake or exhaust vents can prevent the system from igniting.
Inspect vent pipes for:
• leaves or debris
• snow or ice buildup
• damaged vent components
Clearing obstructions may restore normal operation.
Blocked Air Intake
Gas tankless water heaters require a proper air supply to ignite the burner. If the air intake becomes blocked, the system may prevent ignition for safety reasons.
Common causes include:
• debris blocking the intake pipe
• insect nests or dirt inside the vent
• snow or leaves covering outdoor vents
• damaged vent screens
Inspect the intake and exhaust pipes outside the home to ensure airflow is not restricted.
Proper airflow is essential for safe combustion and reliable ignition.
Ignition Component Problems
Over time, ignition components may wear out or become dirty.
Possible issues include:
• worn ignition electrodes
• faulty flame sensors
• damaged wiring connections
If these components fail, the heater may attempt to ignite but shut down immediately.
Routine tankless water heater maintenance helps keep these components functioning properly.
Electrical Power Problems
Tankless water heaters require electrical power to operate ignition systems, sensors, and control boards.
If the heater does not receive adequate power, ignition may fail.
Check for the following issues:
• tripped circuit breakers
• loose electrical connections
• damaged power cords
• blown fuses
Resetting the breaker or restoring power may resolve the problem if the system shut down due to an electrical interruption.
Mineral Buildup Affecting Performance
Mineral buildup inside the heat exchanger can reduce efficiency and interfere with system sensors.
In some cases, heavy scale buildup may affect ignition performance.
Flushing the heater helps remove these deposits and maintain proper operation.
👉 See our guide on how to flush a tankless water heater.
Low Gas Pressure
In some situations, a tankless water heater may fail to ignite because the gas pressure is too low.
Low gas pressure can occur when:
• multiple gas appliances operate at the same time
• gas supply lines are undersized
• the gas regulator is malfunctioning
If gas pressure is insufficient, the burner may not ignite properly.
A licensed technician can measure gas pressure and determine whether adjustments or upgrades are necessary.
Resetting the Tankless Heater
Sometimes resetting the unit can clear temporary ignition errors.
Typical reset steps include:
- Turn off the heater using the power button.
- Wait about 30 seconds.
- Restore power to the unit.
- Run hot water from a faucet.
If the system continues to fail to ignite, additional troubleshooting may be required.
Dirty Flame Sensors
Some tankless heaters use flame sensors to confirm that the burner has ignited properly.
If the sensor becomes dirty or coated with residue, the heater may incorrectly detect that ignition has failed.
Symptoms may include:
• heater attempting to ignite repeatedly
• ignition starting briefly and shutting down
• intermittent ignition errors
Routine tankless water heater maintenance helps keep sensors clean and functioning properly
Error Codes Related to Ignition Problems
Many tankless water heaters display diagnostic codes when ignition problems occur.
Common examples include:
• Error Code 11 – ignition failure
• Error Code 12 – flame failure during operation
• Error Code 29 – venting or combustion issues
If your heater displays one of these codes, consult the manufacturer manual or follow troubleshooting steps for that specific error.
When to Call a Professional
If the heater repeatedly fails to ignite, a technician may need to inspect the system.
Professional diagnosis may involve:
• testing gas pressure
• inspecting ignition electrodes
• checking internal sensors
• verifying combustion airflow
A licensed technician can ensure safe operation of the heater.
Related Tankless Water Heater Troubleshooting
If your system is experiencing additional issues, see our complete tankless water heater troubleshooting guide.
Frequently Asked Questions
Why does my tankless water heater click but not ignite?
Clicking usually means the ignition system is trying to start the burner. If the burner does not ignite, the problem may be related to gas supply, airflow restrictions, or worn ignition components.
Can low water flow prevent ignition?
Yes. Tankless water heaters require a minimum flow rate to activate the heating system. If the water flow is too low, the heater may not begin the ignition process.
Does flushing a tankless heater help ignition problems?
Flushing removes mineral buildup that can affect sensors and internal components. In some cases, regular flushing helps prevent ignition issues.