Rinnai Water Heater Error 12 – Causes & Fix
The Rinnai lit but then lost its flame or stopped sensing it – usually gas supply, venting or the flame rod.
Safety first · Consult a professional
Leave the gas valve, burner and flame rod to a qualified technician. At the first hint of gas odor, get out without switching anything on or off and phone 911 or your gas company from outside; if a CO alarm sounds, go outdoors, call 911 and keep the heater off until it's inspected.
- Gas appliance: no DIY repairs. If you smell gas, leave the building, do not use switches and call your gas utility’s emergency line from outside.
- Carbon monoxide is odorless. Keep a CO alarm near fuel-burning appliances; if it sounds, get everyone outside and call 911.
At a glance
Severity: Moderate
- Meaning
- Flame Failure – Flame Lost or Not Sensed
- Typical repair cost
- $0–$1,500
- DIY difficulty
- Hard
Code 12 on a Rinnai tankless heater is a flame failure: the burner lit, but the flame went out or the flame rod stopped reporting it, so the unit shut the gas valve. Rinnai's own troubleshooting guide starts with the gas supply – valve on at the heater, the meter or the propane tank, and propane actually in the tank – then moves to inlet pressure measured while every gas appliance in the house fires at high, vent terminal clearances that can cause exhaust recirculation, and a loose or coated flame rod. On SENSEI models there's a special case: a code 12 the instant water starts flowing, with no flame visible, points to an electrical short in a component rather than a combustion problem. Owners can check valves and propane level; everything beyond that is a licensed technician's job.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| Non-SENSEI models (V-series, RL-series, older RU/RUR) | 12 | Shown on the unit-mounted or remote temperature controller; usually flashes until cleared. |
| SENSEI series (RU/RUR/RX SENSEI) | 12 | Same meaning; if it appears instantly on water flow with no flame, Rinnai points to an electrical short in a component. |
| Control-R app / Wi-Fi module | 12 | The phone notification simply mirrors the controller; check gas and reset at the heater. |
Symptoms
- Water goes hot, then cold mid-shower, and the controller shows 12
- Faults mostly when the furnace, dryer or a second shower runs at the same time
- Propane homes: shows up as the tank gets low or during very cold weather
- Code appears after windy weather or snow near the vent termination
- SENSEI units: code 12 immediately on opening a tap, without the burner ever lighting
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Gas supply sagging at high fire – undersized piping, meter or regulator, or several appliances firing together
- Exhaust recirculating into the intake because terminal clearances aren't met or vent joints aren't fully engaged
- Flame rod with carbon or white deposits, or loose in its bracket (a 'floating' flame rod)
- Propane tank low, or a two-stage regulator mounted too close to the heater / letting pressure creep
- Poor grounding or wrong polarity on the heater's power supply
- Moisture in a wiring harness or a shorted DC component (SENSEI immediate code 12)
- Gas valve not opening reliably
Fix it step by step
Check the air in the room first
A gas smell or CO alarm overrides everything else – follow the safety box. If all is normal, go on.
Confirm gas is on everywhere
Check the manual valve at the water heater, the meter valve, and on propane systems the tank valve. Rinnai's guide lists these before any other check.
Tools: Flashlight
Propane users: check the tank level
Read the gauge on the tank. A low tank loses pressure, especially in cold weather, and a big tankless burner feels that first. Order a refill and mention the flame-failure code to the supplier.
Note what else was running
Did the code appear while the furnace, a gas dryer or another hot tap was in use? Rinnai sizes gas piping on the assumption that every appliance fires at once, so that detail tells the technician where to look.
Look at the vent termination
From outside, check that the termination is clear of snow and debris and isn't tucked under an overhang or near another vent where exhaust could be drawn back in. Report placement concerns; don't move or open vent parts.
Tools: Flashlight
One reset, then call a Rinnai-trained pro
Turn the heater off and on at the controller and run hot water at full flow. If 12 returns, a licensed plumber or gas fitter measures the inlet pressure (Rinnai allows only about 0.3 in. WC drop on natural gas at full fire), inspects the flame rod and checks grounding.
How to reset 12
- Close all hot taps and make sure there's no gas smell.
- Press the power button on the temperature controller to turn the heater off, then press it again to turn it back on.
- Open a hot tap at full flow and watch the controller for a couple of minutes.
- If 12 comes back, leave it off and arrange service – don't keep cycling the power.
Where the parts are
- 1Flue
- 2Fan
- 3Heat exchanger
- 4Ignition electrode
- 5Gas valve
- 6Circulation pump
- 7Expansion tank
- 8Pressure gauge
- 9Temperature sensor
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Propane refill or regulator check by supplier | No | $0–$200 |
| Service call: gas pressure test and flame-rod cleaning | No | $150–$300 |
| Replace flame rod / igniter assembly and gasket | No | $180–$380 |
| Correct vent joints or termination clearance | No | $250–$900 |
| Upsize gas piping or relocate regulator | No | $400–$1,500 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Call a licensed plumber or gas fitter if code 12 returns after one reset with gas confirmed on, if it happens mainly under combined gas load, or immediately on water flow on a SENSEI model. Pressure testing, regulator work, vent corrections and flame-rod or gas-valve service all require a qualified technician.
Frequently asked questions
What's the difference between Rinnai code 11 and code 12?
Code 11 means the burner never lit at all. Code 12 means it did light, then the flame went out or wasn't sensed any more – so gas pressure under load, venting and the flame rod move to the top of the list.
Why do I only get code 12 when the furnace is running?
When the furnace and a 199,000 Btu/h tankless heater fire together, an undersized gas line or regulator can't hold pressure. The water heater's flame becomes unstable and it reports flame failure. A pressure test at full load confirms it.
Can wind cause a code 12?
Indirectly, yes. If the termination is too close to a wall corner, overhang or another vent, wind can push exhaust back into the air intake. The burner then runs on its own exhaust and the flame fails.
Is code 12 dangerous?
The code itself is the unit protecting you by shutting off gas. It becomes an emergency only if there's also a gas smell or a CO alarm, or if repeated resets are used to keep it running.
Sources
- Official Rinnai user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy