Rinnai LC code: scale buildup, what it means and how to clear it

LC stands for lime scale. When minerals from hard water coat the inside of the heat exchanger, the Rinnai detects it and shows LC, sometimes with a number from 0 to 9. In the unit's code history, LC can appear as 00.

Rinnai's diagnostic guide describes LC this way, and how it behaves depends on the age of the heater.

Older models: a warning

On older products, LC is the only code that lets the heater keep running. The display alternates between LC and the set temperature, and the controller beeps. Turning the power off and back on resets the code, but it returns until the scale is removed.

Newer models: a lockout

Newer products have a shutdown feature to prevent scale damage. LC locks the heater out and needs a reset.

• Temporary use: pressing ON/OFF on the temperature controller five times lets the heater run for about 70 hours. Then it locks out again with another LC code.
• What the number means: it counts those temporary resets. Rinnai's example: LC2 means the heater has been reset for three 70-hour periods without a flush.
• Full reset after flushing: a technician presses the two switches (A and B) on the PC board together for five seconds, which returns the heater to normal and sets the LC count back to 0. Rinnai notes this step is not in the owner's manual.

How to clear it properly

1. Flush the heat exchanger using the procedure in your Rinnai manual, or follow our guide to descale a tankless water heater.
2. Clean the inlet water filter while the unit is isolated.
3. Reset the code: power cycle on older models, or have the board reset done on newer models.

Why LC keeps coming back

If LC returns within months, your water is hard enough that yearly flushing is not enough. Rinnai says hard water must be treated to prevent scale damage. A water softener or a scale-prevention filter on the cold supply slows scale buildup, and a lower temperature setting helps too.

When to call a technician

Call a technician if you cannot descale the unit yourself, if LC returns within months of a proper descaling, or if overheating codes such as 14 or 16 start to appear. A quick return means scale is still building up faster than maintenance removes it.

What happens if you ignore it

Scale insulates the heat exchanger, so the metal runs hotter. Left alone, it can lead to overheating codes such as 14 or 16 and shorten the life of the heat exchanger, the most expensive part of the heater. Using the 70-hour override again and again only delays that.

See every code on our Rinnai error codes page.