Rinnai · Tankless Water Heater Serious
12 — Flame Failure
The unit lit successfully but lost flame signal during operation. Indicates intermittent gas, combustion, or sensing problem.
- Keep using it?
- Limited use only
- DIY difficulty
- Moderate
- Verified against
- rinnai.us ↗
How to fix it
- Remove and clean the flame rod with fine emery cloth — do not over-polish.
- Verify chassis ground and that the unit is bonded properly.
- Have inlet gas pressure measured at high fire (dynamic) — must stay in spec.
- Confirm gas line sizing and length are correct per install manual.
- Inspect vent terminations for partial blockage.
- If flame rod and gas pressure are good, replace the gas valve assembly.
Why it happens
Typical signs: Hot water starts then goes cold · Burner cycles on/off rapidly · Code 12 mid-shower
- Dirty flame rod Carbon or scale on flame sensor preventing flame detection.
- Low gas pressure under load Inlet pressure drops below spec when burner ramps up.
- Undersized gas line Line too small for BTU load over the run length.
- Restricted air supply Partially blocked intake/exhaust causing unstable combustion.
- Faulty gas valve modulation Solenoid not metering correctly at high fire.
- Poor grounding Flame rectification requires good chassis ground.
Parts you may need
If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.
More Rinnai tankless water heater codes
Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: rinnai.us.