MyRinnai R75LS tankless water heater has started to vibrate loudly and then shut down with a code 12 when hot water is opened up a little over half way to full in the sink or shower. It is 10yrs old and never a problem. There are two in my home and the other is fine. The vibration does not start immediately but comes on quickly in about 30 seconds. It is a loud strong vibration that can be heard and felt throughout the house. I greatly appreciate any help or direction.

Have you ever cleaned the heat exchanger? It sounds like steam is being generated by too slow water flow for the gas input.

Embarrassingly no. I have never done any maintenance on it whatsoever. Cleaning the heat exchanger, would that be flushing the system? And thank you for responding.

flushing AND deliming the coil. Supposed to be an annual servicing. One of the joys of a tankless system.



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Well I spent the afternoon gathering the necessary supplies, hoses,white vineger, bucket, etc, and flushed my Rinnai. Sadly, the exact same issue continues to exist. With a faucet anywhere on the system open a little over half way the unit begins to vibrate or shudder violently, the flame goes out and it codes. (12) Any more ideas other than call an expert anyone?

The Rinnai error code 12 in relation to your continuous flow hot water system indicates that there is either a flame failure or a low gas flow.
I got that info off the internet.
if that is the case then someone needs to figure out if the burner is functioning as it should and if not address that issue.

Thank you, have investigated gas flow and pressure, system runs fine until water above half volume. Have second unit in other part of the house so gas is not the issue.Problem starts with the heavy vibration about 30 seconds after startup unless hot water outflow volume is half capacity or less. At about the 30 second mark, vibrations start, then flame flickers and dies, then shutdown.

check that the unit is getting sufficient make up air to the burner.
does that unit have a fan on the air inlet ?
make sure that is running and not partially blocked.
again, code 12 is a flame problem.
so if the burner is getting sufficient gas and the orfice is clean and the burner is clean then that leaves make up air as the culprit.

"so if the burner is getting sufficient gas"

how do you go about confirming or measuring that? And is this something that requires specialized training?

not my field of expertise but I assume you use an natural gas flue analyzer like this one.

there usually is a removable tab that allows for the insertion of the tool's probe.

It sounds like a blower problem. Even if you tear into it, and confirm the blower is bad, you may not be able to get the parts without a license. If you make a mistake assembling the unit, you can conceivably wind up with nothing left of the house but the foundation. It's the only part of the system you really should leave to a factory trained pro. There's plenty of projects that can be done by a homeowner, that's not one of them.

I appreciate all the responses and suggestions. I wonder though, if I have a flame or air issue, why the unit functions without issue when hot water outflow at the tap is at halfway or less. Would it not be a constant non function issue if either air or fuel was somehow restricted?

from what I can find, your heater modulates. the more water you run the bigger the flame gets.
so, apparently when you increase the demand the flame is supposed to get bigger BUT it is struggling to make a bigger flame.
impossible to tell from here whether there is a restriction to the incoming air, the fan bringing air in/out is failing, the burner is dirty or who knows what else it could be.
might be best to get someone who services rinnai's to have a look.