How to Troubleshoot and Fix Tankless Water Heater Ignition Failure

Tankless water heaters, also known as on-demand or instantaneous water heaters, heat water directly without the use of a storage tank. Because they only heat water as needed, they can be more energy efficient. However, one common problem homeowners face is ignition failure, where the burner fails to light. This results in no hot water being available.

When your tankless water heater isn‘t igniting properly, it requires some troubleshooting and repairs to get it functioning again. In this comprehensive guide, I‘ll provide key steps for diagnosing ignition problems and detail fixes to get your hot water flowing again.

What Causes Ignition Failure in Tankless Water Heaters?

Before diving into troubleshooting, it‘s helpful to understand what can cause ignition failure in the first place. Some of the most common culprits include:

Pilot Light Outage

Tankless water heaters use a pilot light—a small, constant flame—to ignite the main burner when hot water is needed. If this pilot light goes out, the burner cannot ignite. Pilot outages account for about 25-30% of ignition failures, according to water heater manufacturer Rheem.

Causes of pilot light outages include:

• Strong drafts or winds blowing out the flame
• Built up sediment or debris blocking the pilot orifice
• A loose or damaged thermocouple (more details below)

Thermocouple Issues

The thermocouple is a heat-sensitive device that sits in the pilot light flame. When the pilot is lit, it produces a very small electrical signal that tells the gas control valve to open and allow gas flow to the burner.

If the thermocouple malfunctions or fails, this signal is disrupted and the valve will not open. Faulty thermocouples are responsible for approximately 40% of ignition failures according to Rheem.

Problems with thermocouples stem from:

• Loose wiring connections causing electrical open circuits
• Cracking or damage to the thermocouple sensor probe
• Excessive soot buildup on sensor probe tip
• Normal wear and tear over time. Thermocouples degrade after 2-3 years.

Clogged Components

Sediment from hard water as well as other debris can build up in the pilot assembly and burner over time. This includes the orifices, tubes, and ports where both the pilot and main burner flames pass through.

Excessive blockages disrupt proper gas flow and prevent ignition. It‘s critical to keep these components clear and clean.

Gas Control Valve Defects

The gas valve regulates the flow of gas and acts as a switch to supply gas to the pilot and burner when heat is called for. If this valve is stuck, jammed, or defective, it will fail to open and allow gas to pass. Gas valves on tankless water heaters do degrade and eventually fail over time.

Igniter or Flame Sensor Issues

The igniter and flame sensor are electrical components that create a spark to light the pilot and detect when the pilot is lit respectively. If either is cracked, damaged, or worn out, the system will not function properly. As with other parts, they have a typical lifespan of 2-3 years before possible failure.

Inadequate Gas Supply

Finally, you may have ignition failure if there are leaks or cuts in the gas supply lines leading to the heater, improper line sizing leading to insufficient fuel pressure, or if the main gas supply to the building is interrupted. There must be an uninterrupted gas supply to the heater for proper ignition.

Troubleshooting Ignition Problems on Tankless Water Heaters

If your tankless unit fails to ignite and provide hot water, don’t despair. In most cases, ignition failure is repairable with some troubleshooting. Here is a systematic process you can follow to diagnose the problem:

Inspect the Pilot Flame

The first step is verifying if the pilot light is lit. Locate the view port where you can observe the pilot flame. It is typically found on the right side of the heater.

Peer inside and look for the small blue flame that should be present at all times. No flame means the pilot is out.

Location of pilot light view port on a typical tankless heater.

If the pilot is out, you will need to relight it by depressing the igniter button and following the manufacturer’s instructions.

However, a pilot that repeatedly goes out indicates an underlying problem like a failing thermocouple, inadequate gas supply, or excessive drafts (more on this shortly).

Check Gas Supply and Pressure

Make sure the shut-off valves on the gas supply line leading to the heater are fully open. Tankless heaters require a gas pressure between 3.5” to 10.5” WC for natural gas models, or 8” to 13” WC for propane models.

Use a manometer to check the gas pressure at the pressure port to ensure adequate fuel supply. If pressure is lower than recommended, there is a downstream blockage or the main gas line is too narrow.

Also check for leaks using leak detector fluid on all fitting connections upstream of the heater. Any leaks must be repaired.

Checking gas inlet pressure using a manometer.

Inspect Fuel Lines and Orifices

Examine the pilot assembly and main burner area for any visible obstructions. Use a flashlight to peer inside (when cool) and look for any dirt, lint, or debris that could block the small orifices the gas flows through.

Clogs are more common with propane units since propane is heavier than air and can pick up sediment. Use compressed air or a pipe cleaner to gently clear out any impacted areas.

Check Igniter and Wiring

The igniter sits next to the pilot light and creates a high voltage spark to light the pilot when the red igniter button is pushed.

Remove the igniter carefully and inspect it for any cracks in the ceramic insulator around the metal electrode tip. This area creates the spark. Any cracks mean it should be replaced.

Also check that the wire connections are secure and free of corrosion. Loose connections can impact sparking performance. Verify the electrode gap spacing with a feeler gauge, adjusting as needed per manufacturer specs.

Test the igniter by connecting it back to the heater and pushing the igniter button. You should see a bright blue spark. If not, replace the igniter.

Inspecting an electronic igniter module.

Test the Thermocouple

The thermocouple has a temperature sensor probe at the end that sits directly in the pilot flame. Use a multimeter to test that the thermocouple circuit has continuity and is providing the required millivolts when heated (typically 15-30 mV).

Set your multimeter to the millivolt scale, disconnect the thermocouple from the gas valve, place the positive lead on the copper end of the thermocouple, and the negative lead on the body of the gas valve. If you don’t get a millivolt reading, the thermocouple is likely bad and needs replacement.

Testing a thermocouple with a multimeter.

Evaluate Gas Valve Performance

The gas valve or control system manages gas flow to both the pilot and main burner. Listen for an audible click when you turn the water heater on to initiate heating. No click indicates a stuck valve.

Use a multimeter to check that the electrical gas valve coil is providing the proper Ohms resistance and is activating when power is applied. If not, the gas valve must be replaced.

Check Flame Sensor

If equipped, check that the flame sensor rod (next to the pilot) is secure, wires are intact, and ceramic insulator is free from cracks. Clean any soot deposits off the sensor tip for best performance. Replace if any damage is found.

Review Service History

Check service records to understand if ignition problems are a recent development or recurring issue, and which components have been repaired/replaced previously. Review for any clues about the problem origin.

By methodically working through these troubleshooting steps, you can determine what is causing the ignition failure in your tankless water heater.

Fixing and Replacing Faulty Components

Once you’ve diagnosed the problem through troubleshooting, specific repairs can be made to components that have failed or are malfunctioning. Here are some of the common fixes:

• Replace thermocouple – If cracked or not providing proper voltage signal, replace this part with a matching OEM thermocouple. Ensure tight, secure connections.
• Clear obstructions – Use compressed air or a pipe cleaner to gently clean out any debris or sediment clogging the pilot assembly, burner, or gas lines.
• Replace igniter – If cracked or no longer sparking, replace the igniter and check electrode gap spacing against factory specs during reinstallation.
• Clean flame sensor – Remove any soot buildup on the sensor probe. Be careful not to damage the ceramic insulator tip.
• Install new gas valve – If the gas valve is defective or sticking, install a replacement gas control assembly making sure to use a part number matched for your specific heater model.
• Adjust gas pressure – If gas pressure is off, adjust regulator on propane tanks or main home gas line as needed.
• Repair leaks – Seal any gas leaks upstream of the heater using approved pipe thread sealant. Pressure test to confirm leaks are eliminated.
• Clear vent blockages – Remove any debris blocking exterior exhaust/intake vents. This can reduce air flow needed for proper combustion.

Assortment of common replacement parts for tankless water heater repair.

More extensive repairs like gas valve or heat exchanger replacements are best left to a qualified technician. But many components like thermocouples, pilot orifices, and igniters can be replaced DIY with basic mechanical skills. Always exercise caution when working with gas systems.

Preventing Ignition Problems Through Regular Maintenance

While parts won’t last forever, you can maximize the service life of your tankless water heater and avoid untimely ignition failures by performing regular maintenance. This includes:

Flush mineral deposits – Annually flush the heat exchanger to clear out scale-forming mineral deposits from hard water that can accumulate over time. Use a descaling solution specifically designed for tankless heaters.

Check/replace air filters – Clean or replace air intake filters 2-4 times per year to maintain proper air flow to the burner.

Inspect burner – Check for any sign of soot buildup or debris clogging the burner assembly. Use compressed air and a soft brush to gently clean.

Monitor gas pressure – Periodically verify proper gas pressure at the heater inlet using a manometer.

Test electrical components – Check igniters, thermocouples, flame sensors and wiring for damage annually. Replace components proactively if aged over 2 years.

Clear condensate lines – Confirm condensate drain lines are clear and condensate flows freely. Blockages can lead to shut downs.

Check pipe connections – Inspect gas and water connections for any sign of leaks annually. Tighten or re-apply approved sealant as needed.

Clean exterior – Keep exterior cabinet clean and remove any accumulated dust/debris which can restrict air flow.

Flushing the heat exchanger annually prevents scale buildup.

Following the manufacturer’s maintenance guidelines and being vigilant with preventative care goes a long way towards avoiding ignition failure.

Safety Tips for Troubleshooting and Repairs

It’s critical to exercise caution and follow proper safety protocols when troubleshooting and servicing a tankless water heater ignition:

• Turn off power and gas supply before any repairs. Tag switches/valves appropriately.
• Allow sufficient time for components to cool before handling to prevent burns.
• Have a fire extinguisher on hand in case of accidental ignition.
• Use care when testing electrical parts to avoid shocks.
• Check for gas leaks with soapy water or detector fluid after any gas line repairs.
• Verify proper combustion after repairs by analyzing exhaust gasses either with test equipment or by observing the burner flame. A proper blue flame is desired.
• Follow all manufacturer precautions and warnings exactly.
• Consider having extensive repairs done by a licensed professional technician if DIY ability is limited.

Tankless water heaters offer the convenience of instant, continuous hot water, but only if the ignition system functions reliably. Stay proactive with preventative maintenance and follow a systematic troubleshooting process to get your heater firing properly again after any ignition failures. With some vigilance and proper safety practices, you can enjoy consistent hot water from your tankless heater for years to come.