HeatingCodesRinnai tankless water heater code 12: flame failure
What code 12 means
Rinnai's guide titles code 12 Flame Failure and applies it to SENSEI, SENSEI RX/CX and non-SENSEI models. The first thing the guide says is: "You must be qualified to service gas systems before proceeding." That is the right frame for this page: it explains the likely causes so you can talk to a technician, not so you can open the unit.
Causes in the Rinnai guide
| Area | What Rinnai says to check |
|---|---|
| Gas supply | Gas turned on at the heater, meter or propane tank; propane tank not empty; inlet pressure maintained on high fire. Pressure drop should be no more than 0.3 inch w.c. on natural gas (0.5 inch for propane) with all heaters firing, otherwise the gas piping may be undersized. |
| Venting | Loose joints, improper fittings, or vent terminal clearances that allow recirculation of combustion gases, which can cause incomplete combustion and a code 12. Equivalent vent length and approved vent materials. |
| Ignition and flame sensing | Ceramic sparker electrode and flame rod tight in their bracket, electrode gap 0.138 in (3.5 mm), flame rod wiring, and the flame rod silicone sleeve (a cracked sleeve can let the spark go to ground outside the chamber). |
| Electrical | Ground connections intact and tight, correct polarity, and no moisture in the wiring harnesses. Rinnai says to contact a licensed electrician if unsure. |
| Air path | Debris or insects in the fan blower wheel or burner assembly. |
| Regulators | Two-stage regulators at least six feet from the heater; inlet pressure must never exceed 10 in. w.c. on natural gas or 13.5 in. on propane. |
For SENSEI and SENSEI RX/CX units, Rinnai also notes that a code 12 that appears immediately after water starts flowing, with no flame visible, points to an electrical short in a component on the DC circuit, such as a wiring harness or a flow or bypass valve.
Frequently asked questions
Why does my Rinnai code 12 only show up in cold or windy weather?
Rinnai's guide links vent terminal clearance problems to recirculation of combustion gases and incomplete combustion, and also lists inlet gas pressure. Wind and snow at the vent, and cold propane tanks, are plausible reasons for changes, but the guide does not state a weather cause.
Can I fix code 12 by resetting the heater?
A reset can clear the display, but the causes in the manual remain. If code 12 recurs, schedule a technician.
Sources
- Troubleshooting Tankless Water Heater Diagnostic Codes (Rinnai America) Covers: Rinnai tankless water heaters in the SENSEI, SENSEI RX/CX and non-SENSEI series. Not every code applies to every model.
Safety note: this page is general information drawn from the sources above. Gas, combustion and line-voltage repairs belong to a licensed professional. Your own equipment's label and manual take priority over anything here. Read the full disclaimer.