What does the LC code on a Rinnai tankless mean?

Rinnai's service manual lists it as Code LC, LC0 through LC9, and describes it as scale buildup in the heat exchanger. When you look at the maintenance code history on the controller, the manual says "00" is shown in place of LC. The fix Rinnai lists is to flush the heat exchanger using the procedure in your owner's manual.

Which behavior you get depends on the age of your heater. Rinnai's manual says that on older products you can clear an LC code by turning the power off and back on, and the heater carries on. That makes it easy to ignore. The warning does not go away because the display did, since the scale is still in the heat exchanger.

Two details set LC apart from the rest of Rinnai's codes:

• The heater may still run. Rinnai says the LC codes are the only codes that allow a water heater to keep operating. On these units the display alternates between the LC code and the temperature setting, and the controller keeps beeping.
• Newer units add a lockout. Rinnai says newer products have a shutdown feature built in to prevent damage from scaling. The LC codes then lock the water heater out until it is reset.

That second point is why the number after LC matters.

On the newer units, the digit counts how many times the heater has been given a temporary reset without a flush. Rinnai's manual describes it this way: a technician or owner can push the On/Off button on the temperature controller five times to run the heater temporarily until the heat exchanger can be flushed. The heater will then operate for 70 hours before locking out again with another LC code. Rinnai's example: LC2 means the water heater has been reset for three 70 hour periods without a flush and a hard reset.

Put simply, the number is a tally of borrowed time. A low number means the warning is new. A high number means the heater has been running on temporary resets for a long while, and the scale problem has had that whole time to get worse.

After a temporary reset, Rinnai says the controller cycles between the set temperature and the LC number. A permanent reset, which returns the count to zero, comes only after the heat exchanger is flushed. Rinnai's manual describes that hard reset as a step done on the circuit board, and notes that it is not in the owner's manual or tech sheet. That makes it a technician task, not a homeowner one.

Rinnai's manual has three codes that all point at scale, and owners often mix them up:

• SS (Service Soon). This is a time based reminder set at installation. Rinnai says it means it is time for service, and that the heat exchanger should be flushed. It is a calendar prompt, not a sign that scale has already been detected. The manual says you clear it by pushing the On/Off button on the temperature controller five times in five seconds. The reminder interval is a parameter that can be set to not active, half a year, one year or two years.
• LC, LC0 to LC9. The heater has detected scale buildup. This is a warning about a condition, not a reminder about a date.
• Code 58. On non-SENSEI models, this code appears when one of the two bi-metal switches on the secondary heat exchanger opens. Rinnai says it indicates scale buildup inside the secondary heat exchanger that needs to be flushed. To reset it, the underlying problem has to be corrected first, and then the power is turned off and back on.

Rinnai's code 54 entry belongs in the same family. When the manual lists causes for a code 54, it says hot spots on the heat exchanger surface indicate blockage from scale, and the fix is flushing the heat exchanger following the owner's manual instructions. If the exchanger shows damage from poor maintenance, the manual says to replace it.

### Why is this a bigger deal in Phoenix?

Rinnai says it plainly in the manual: hard water must be treated to prevent scale buildup or damage to the heat exchanger. Phoenix tap water is hard, which means it carries dissolved minerals that leave a crust inside the water passages as the water is heated. Our page on how hard water damages plumbing explains the process.

In a tankless heater that scale forms on the heat exchanger, the part that moves heat into the water. Rinnai's manual ties scale to hot spots on the exchanger and to overheating codes such as 54, which is why the LC warning exists and why Rinnai built a lockout into newer models.

Other makers build in the same warning. Noritz's installation manual describes a service reminder that displays code 05 when flushing is due, and Navien's operation manual includes its own procedure for flushing the heat exchanger. The pattern across brands is the same. The heater counts down to a flush, and it expects one.

### Can I flush it myself?

Rinnai's manual sends you to the flushing procedure in your owner's manual, which involves isolation valves, a circulation pump, hoses and a cleaning solution. Navien's manual, for comparison, warns that flushing is "a somewhat complicated procedure" and that improper maintenance can void the warranty. If your heater has isolation valves already installed and you are comfortable with the steps, the owner's manual is the authority. If it does not, or if the LC number is climbing, this is a good job to hand to a technician. Our page on how often to descale a tankless water heater in Phoenix covers timing.

Before you call, note these things:

• The LC number. LC0 and LC5 are different situations.
• The temperature controller model, and whether the heater is a SENSEI series or an older model, since the reset steps differ.
• The date of the last flush, even if you are only guessing.
• Any second code such as 14 or 54, which would point to overheating as well.

### What happens if I ignore it?

On the newer units, the heater eventually locks out, and you have no hot water until it is reset. On any unit, Rinnai's manual links scale to hot spots on the heat exchanger and to overheating codes, and it lists flushing as a fix for code 54 and code 14. It also says that a heat exchanger showing damage from poor maintenance should be replaced. The warning is easiest to act on early, while a flush can still prevent that damage.

Call when you see any LC code and you have not flushed the heater, when the number keeps rising, when the heater has locked out, or when LC shows up along with an overheat code such as Rinnai code 14. Rinnai's own manual treats a heat exchanger damaged by poor maintenance as a replacement item, so a flush done early is the better route.

HQ Plumbing and Air is licensed and insured, and answers emergency calls around the clock. Estimates are free and pricing is given upfront. If a scaled heater is well past its best years, our page on whether to repair or replace a water heater can help you weigh it up.