Quick answer
Rinnai error code 12 means flame failure: the burner lit and the flame was then lost. The burner shuts down by itself, so what you notice is hot water that cuts out. Make sure gas is on at the heater and the meter, and note whether the code appears only while other gas appliances are running. If it repeats, call a licensed gas technician.
A shower that goes cold halfway through and a 12 on the controller belong together: Rinnai error code 12 is flame failure, meaning the burner lit normally and the flame was then lost. That is the difference from code 11, where the burner never lights at all. Rinnai's diagnostic guide opens its code 12 page with a warning that you must be qualified to service gas systems before going further. The label inside the panel of your own heater governs if its wording differs from this page.
Is a Rinnai with code 12 safe to use?
The burner shuts down by itself the moment the control loses the flame signal, so gas does not keep flowing into a dead burner. Expect intermittent cold water: hot water that cuts out and comes back after a reset. A reset to finish the day is reasonable.
Repeated flame loss is not something to live with, because one of its causes is combustion gas recirculating at the vent terminal. Stop resetting and switch the heater off if the code returns several times a day, and leave the house and call Enbridge Gas if you ever smell gas.
What you can check without tools
- Gas is on at the heater: the shut-off handle under the unit runs parallel to the gas pipe.
- Gas is on at the meter, or there is propane in the tank.
- Timing: write down what else was running when the code appeared. A code that shows up only while the furnace, range, dryer or fireplace is firing points at the gas supply.
- Weather: a code that appears only on windy days points at the vent terminal.
Those notes are worth more to the technician than any number of resets. They decide whether the diagnosis starts at the gas line or at the vent.
What makes a Rinnai lose its flame
- Inlet gas pressure that cannot hold at high fire. Rinnai's guide says a properly sized natural gas system drops no more than 0.3 inches of water column with every gas appliance in the building on high fire; more than that suggests an undersized gas system. A tankless heater pulls much more gas at once than a tank, so piping that suited the old tank can starve it.
- A loose or dirty flame rod or sparker electrode. The flame rod is how the control proves flame; if it moves in its bracket, or is coated with carbon or a white deposit, the signal drops out and the burner is shut off with a flame still present.
- Vent-terminal clearances that let combustion gases recirculate. Exhaust drawn back into the intake gives incomplete combustion and an unstable flame.
- A vent run outside the allowed equivalent length, or vent joints that are not fully engaged.
- Less common: a cracked silicone sleeve on the flame rod, a poor electrical ground, moisture in a wiring connector, or a gas valve fault.
How a technician diagnoses flame failure
Three measurements settle it, and all three need instruments. A manometer on the heater's test port reads inlet pressure under load, with the other gas appliances firing. A meter in series with the flame rod reads the flame signal in micro-amps. The vent run and terminal are then checked against the venting instructions for length, joint depth and clearances.
If pressure is the problem, the fix is in the piping or the regulator, done under the CSA B149.1 gas code by a TSSA-licensed technician. If the flame rod is the problem, it is cleaned or replaced, and Rinnai requires a new gasket whenever that seal is broken. Whether a tankless needs a bigger gas line explains the sizing side.
Cost of a code 12 repair
- $90–$150 for the diagnostic visit, applied to the repair.
- $150–$800 for most tankless repairs, flame rod and electrode work included.
- +$400–$1,200 when the gas run to the heater has to be upsized.
Houses in the 1970s–90s subdivisions of Burlington, Oakville and Mississauga were piped for a tank water heater and a furnace, and a tankless heater added to that original line without upsizing is a common source of code 12. If your Rinnai drops its flame when the furnace starts, say so when you call (647) 801-1252. The visit is described on the tankless water heater repair page, the remaining codes are in the Rinnai error code guide, and you can book online.