Tankless Water Heater Ignition Failure: Causes and Remedies
Ignition failure is one of the most common service conditions encountered with tankless water heaters, occurring when the unit's burner assembly fails to light during a call for hot water. The fault can originate in fuel supply, venting, electronic controls, or component degradation — each with a distinct diagnostic pathway. This page covers the mechanical and electronic scope of ignition failure, the conditions that produce it, and the classification boundaries that separate DIY-accessible maintenance from licensed trade work or permit-required repair.
Definition and scope
Ignition failure in a tankless water heater is the condition in which the appliance receives a hot water demand signal — registered by a flow sensor — but fails to establish a stable burner flame within the control board's ignition trial window. The unit typically responds by issuing an error code (commonly displayed as error codes 11, 12, or 16 depending on manufacturer) and shutting down the heating cycle. The flow of cold water continues, but no thermal output is produced.
The scope of this failure mode applies primarily to gas-fired tankless units — both natural gas and liquid propane — because electric tankless heaters (electric units operate on resistance heating elements, not combustion) have no ignition system. Gas units divide into two categories relevant to ignition behavior:
- Non-condensing gas units — Operate at flue temperatures typically above 300°F and use standard venting materials. Ignition components are exposed to higher thermal stress cycles.
- Condensing gas units — Operate at lower flue temperatures (below 140°F) and extract latent heat from combustion gases. These units use plastic or CPVC vent materials and are more sensitive to vent blockage causing ignition lockout.
The International Fuel Gas Code (IFGC, published by the International Code Council) governs gas appliance installation standards adopted by most U.S. jurisdictions. Local amendments apply in states including California, which enforces its own California Plumbing Code (Title 24, Part 5) through the California Department of Housing and Community Development.
How it works
Gas-fired tankless water heaters execute a specific ignition sequence each time a flow event is detected. Understanding the discrete phases of this sequence identifies where failure can occur:
- Flow detection — A flow sensor (typically a turbine or paddle type) detects water movement above the minimum activation threshold, commonly 0.5 to 0.75 gallons per minute (GPM). Below this threshold, the ignition sequence does not begin.
- Pre-purge — The combustion fan runs for a timed interval (typically 5 to 15 seconds) to clear residual gases from the heat exchanger and verify vent path integrity through a pressure switch.
- Gas valve opening — The control board energizes the gas valve to allow fuel flow to the burner assembly.
- Ignition spark activation — The igniter — either a piezoelectric spark igniter or a hot surface igniter (HSI) — fires repeatedly during the ignition trial window (typically 5 to 10 seconds).
- Flame sensing — A flame rod (ionization sensor) detects the presence of a flame by measuring current conducted through the ionized combustion gases. If the flame rod does not detect ignition within the trial window, the control board closes the gas valve and logs a fault code.
- Lockout — After one or more failed ignition trials, the unit enters safety lockout, requiring manual reset or automatic retry after a timed delay depending on manufacturer programming.
The pressure switch in step 2 is a critical gating component: if vent static pressure does not reach the required threshold — because of a blocked exhaust, a disconnected vent pipe, or a failed inducer motor — the ignition sequence will not advance to gas valve opening. This means ignition failure codes can originate upstream of the igniter itself.
Common scenarios
Ignition failures in the field group into four primary cause categories, each with distinguishable diagnostic indicators.
Gas supply interruption — Insufficient gas pressure is among the most frequent root causes. Minimum inlet pressure for most residential natural gas tankless units is 4 to 7 inches water column (WC) at the unit; propane systems require 8 to 11 inches WC. A partially closed service valve, undersized gas line, or competing appliance load can drop pressure below the operating threshold. Reviewing unit specifications through the tankless resource provider network helps identify model-specific pressure requirements before field testing.
Igniter and flame rod degradation — Spark igniters wear over ignition cycles. Ceramic insulator cracking, electrode gap widening beyond manufacturer specification (typically 3–4 mm), and carbon deposits on the flame rod ionization surface are all documented failure patterns. Flame rod contamination from silicone-based sealants used in nearby pipe joints is a recognized cause of false flame-sensing failures documented in manufacturer service bulletins.
Venting blockage or configuration error — Blocked intake or exhaust terminals — from debris, nesting material, or ice formation in cold climates — prevent the inducer from establishing the pressure differential needed to advance ignition. Incorrectly pitched vent runs (condensate pooling in non-condensing units) and vent runs exceeding manufacturer-specified equivalent feet produce the same pressure switch failure signature.
Control board and wiring faults — Moisture intrusion, rodent damage to wiring harnesses, and capacitor degradation on control boards produce erratic or absent ignition signals. These are distinguished from component-level failures by the absence of any spark sound or gas valve click during an ignition trial.
Decision boundaries
Not all ignition failure remedies fall within the same trade category or permitting tier. The following boundaries define how work is classified:
Homeowner-accessible maintenance includes cleaning combustion air inlet screens, verifying service valve positions, inspecting vent terminals for visible blockage, and resetting the unit per the manufacturer's documented reset procedure. These actions require no licensure and no permit.
Licensed contractor scope includes gas pressure measurement and adjustment, gas valve replacement, igniter or flame rod replacement, vent system modification or extension, and control board replacement. In all U.S. jurisdictions, work on gas piping and gas appliance connections requires a licensed plumber or gas fitter. The National Fuel Gas Code (NFPA 54) and IFGC both require that gas system work be performed by qualified personnel meeting local licensing requirements.
Permit-required work includes any alteration to the gas supply line, vent system relocation, or appliance replacement. The International Residential Code (IRC), Section G2427 covers venting requirements for gas appliances in one- and two-family dwellings. Jurisdictions adopting the IRC require permits and inspections for vent system changes even when the appliance itself is not replaced.
The distinction between non-condensing and condensing unit vent repairs carries safety implications recognized by Underwriters Laboratories (UL) under UL 1738, which establishes provider standards for venting systems. Installing non-verified vent materials on a condensing unit — or using plastic vent materials rated for condensing service on a non-condensing unit — violates both the appliance provider conditions and the adopted mechanical code.
Ignition failure that recurs after verified component replacement — particularly in units with more than 10 years of service — enters the territory of heat exchanger scaling assessment and whole-unit evaluation. Scaled or fouled heat exchangers reduce combustion airflow and alter gas-to-air ratios enough to prevent stable flame establishment even when ignition components are functional. This assessment falls within the scope of a qualified service technician. The tankless providers provider network supports locating licensed contractors qualified for this diagnostic work.