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Ignition System Repair

Tankless Water Heater Ignition & Flame Sensor Repair

Gas tankless water heaters depend on precision spark igniters and flame ionization rods to light the gas burner safely and maintain continuous combustion.

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Tankless Water Heater Ignition & Flame Sensor Repair diagnostic overview
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Quick answer

How do igniters and flame sensors work in a tankless water heater?

When hot water flow is detected, the control board sends high voltage to the spark igniter to light the gas burner. The flame sensor rod then sits directly in the burner flame, measuring a micro-amp electrical current (flame rectification) to confirm ignition safety.

Updated ยท Reviewed by the TanklessFixer technical team

Technical Overview

Ignition & Flame Sensor Diagnostics

Gas tankless units use sophisticated electronic ignition systems rather than standing pilot lights. When a fixture opens, a combustion fan purges the chamber, gas valves open, and spark igniters generate an electrical arc to ignite the gas air mixture.

If the flame sensor rod becomes coated with carbon deposits or mineral oxide, it cannot detect the flame. The control board assumes combustion failed and shuts off gas flow to prevent unburned gas accumulation.

Symptoms & Warnings

Common Signs Your System Needs Attention

  • Unit attempts to spark multiple times but fails to ignite the burner
  • Burner lights for 2 to 5 seconds and then immediately shuts off
  • Error Code 11 or Code 12 flashing on wall control displays
  • Clicking spark sounds heard from cabinet without gas burner ignition
  • Frequent system lockouts occurring during morning peak usage
  • Soot accumulation visible on igniter ceramic insulators or flame rods
Core Factors

Common Ignition Failures

Oxidized Flame Sensor Rods

Carbon and mineral buildup on the metal rod insulates it from sensing current.

Worn Spark Igniter Electrodes

Corroded or misaligned spark gaps fail to create a strong electric arc.

Gas Supply Pressure Drops

Insufficient inlet gas pressure prevents proper air-to-fuel ignition mixtures.

Igniter Transformer Faults

Failed high-voltage ignition modules fail to supply current to igniter electrodes.

Diagnostic Categories

Key Focus Areas

01

Dirty Flame Rod

Cleaning carbon buildup from the flame sensor rod with fine steel wool restores electrical signal conduction.

02

Misaligned Spark Gap

Adjusting electrode gap distance ensures a hot spark directly in the gas path.

03

Gas Valve Activation Fault

Verifying gas solenoid valve coil resistance ensures gas opens during ignition sequence.

04

Combustion Air Balance

Ensuring proper intake fan speed delivers correct air-fuel ratios for smooth ignition.

Service Process

Ignition System Service Steps

  1. Inspect Flame Assembly

    Remove burner cover plate to access ceramic igniter mounts and flame sensor rods.

  2. Clean Sensor Electrodes

    Gently polish metal sensing rods to remove non-conductive oxidation crust.

  3. Check Spark Gap & Wiring

    Inspect wire leads for cracks and verify gap distance meets manufacturer specs.

  4. Test Firing Sequence

    Reassemble chamber, restore power, and verify stable flame signal micro-amp readings.

Brands

Brand Compatibility & Service

Tankless systems from major manufacturers can differ in components, diagnostics, maintenance procedures, and error-code definitions. Service resources cover major brands found in service areas across the country.

  • Condensing & non-condensing gas systems
  • Single-phase & 3-phase electric tankless
  • Whole-home & point-of-use equipment
  • RinnaiCondensing & non-condensing gas
  • NavienCondensing units & combi boilers
  • NoritzIndoor & outdoor gas models
  • RheemGas & electric tankless
  • TakagiResidential gas tankless
  • A. O. SmithCondensing gas tankless
  • BoschGas & electric tankless
  • Bradford WhiteInfiniti gas series
  • Stiebel EltronElectric tankless
  • EcoSmartElectric tankless
  • RichmondGas tankless
  • StateGas tankless
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FAQ

Frequently Asked Questions

Why does my gas tankless water heater light briefly then shut off?

This is a classic symptom of a dirty or faulty flame sensor. The unit ignites, but because the sensor fails to detect the flame, the gas valve closes for safety.

Can a flame sensor on a tankless water heater be cleaned?

Yes, technicians clean flame sensor rods by gently rubbing them with fine steel wool or emery cloth to clear carbon buildup.

What causes a tankless igniter to stop sparking?

Igniter failure can be caused by cracked ceramic insulation, worn metal tips, loose wiring leads, or a failed high-voltage ignition transformer.

What error code indicates ignition failure?

On most major gas tankless brands, Code 11 indicates ignition failure and Code 12 indicates flame loss during operation.

How often should ignition components be inspected?

Igniter electrodes and flame sensors should be inspected and cleaned during annual preventative maintenance.

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