Tankless Water Heater Not Igniting / No Hot Water
When a tankless water heater stops producing hot water, it usually flashes an error code and the cause is gas, air/venting, water flow, or scale. Many are homeowner-fixable. Always note the error code on the display first — it points right at the problem.
Read the Error Code
Tankless units (Rinnai, Navien, Noritz, Takagi) show a numeric code when they fault. Codes like Rinnai 11 (no ignition), 12 (flame failure), or 10/code families (air/exhaust) tell you exactly where to look. Write it down and check it against your model.
Check the Gas Supply
Confirm the gas is on and other gas appliances work. A tankless needs a lot of gas flow to fire; an undersized gas line, a partly closed valve, or low propane can cause no-ignition (code 11). For propane, check the tank level.
Check Venting and Air
Blocked intake or exhaust venting (debris, ice, a nest, or snow) stops ignition for safety. Inspect the outdoor vent terminations and clear any blockage. Combustion needs clean air in and exhaust out.
Clean the Inlet Water Filter
Tankless units have a small inlet water filter screen that clogs with sediment and reduces flow below the minimum needed to fire. Shut off the water, remove the inlet filter, and rinse it clean.
Descale It (Annual)
Hard-water scale inside the heat exchanger is the slow killer — it causes error codes, reduced output, and eventual failure. Flush the unit with a descaling/vinegar solution using the service valves once a year (every 6 months in hard water).
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FAQ
Common causes are insufficient gas supply, blocked intake/exhaust venting, a clogged inlet water filter, or scale buildup. Read the error code first — it points to the cause.
Once a year, or every six months in hard-water areas. Scale buildup is the leading cause of tankless faults and lost efficiency.
Always unplug an appliance and shut off its water supply before servicing. This guide is informational and not a substitute for a qualified technician.