Rinnai Tankless Error Code LC: What Lime Scale Buildup Means for East Cooper Homes

If your Rinnai tankless keeps beeping and the display flips between LC and your temperature setting in Mount Pleasant, Daniel Island, or somewhere else in East Cooper — here's the short version.

Code LC means lime/scale buildup in the heat exchanger. The unit is warning you that mineral scale is starting to form inside. Unlike most Rinnai codes, this one usually lets the heater keep running — the controller beeps and the screen alternates between LC and the setpoint. That's not a free pass to ignore it. Rinnai is clear: flush the heat exchanger. Scale damage is not covered by warranty.

I'm Christian with Avery & Sons Plumbing + Tankless. We work these units all over Charleston, and Code LC is one of the most common homeowner-facing warnings we see around here — especially where the water is hard and folks run hotter setpoints.

What the unit is doing

Your tankless heats water as it flows through a heat exchanger. Hard water leaves mineral deposits (lime scale) on those surfaces. When buildup gets far enough to affect performance, Rinnai shows LC (on some models you may also see LC0–LC9 as the alert repeats). In the maintenance history, Rinnai often stores this as 00 instead of LC — same issue, different label in the log.

The important local detail: LC is the rare code that does not shut you down cold. Hot water may still come out. The beep and the flashing LC are the warning. Clearing the display without flushing only resets the reminder — it does not remove the scale.

If you're comparing codes: we already covered Code 10, Code 11, Code 12, Code 14, Code 16, and Code 03 for East Cooper homes. Code LC is about scale inside the heat exchanger, not combustion, venting, or Bath Fill.

Common causes we see around here

Hard water + hot setpoint. Scale forms faster when minerals in the supply water meet high heat. Plenty of homes from Charleston through Mount Pleasant and Daniel Island run tankless at temperatures that cook minerals onto the exchanger over time.

Skipped or overdue flushes. Rinnai expects periodic flushing in hard-water areas. Put it off, and LC shows up as the early warning before flow and heat transfer get worse.

No water treatment. Softening or a scale-control setup on the cold supply slows the problem. Without it, coastal hard water does what hard water does.

LC returns soon after a power reset. Turning power off and back on clears the LC display — Rinnai says so. If the exchanger was never flushed, the code comes back. That's the unit telling you the minerals are still there.

Safe checks you can do yourself

Keep it simple. If you smell gas, leave the area and call your gas utility first — don't troubleshoot.

1. Read the display. Confirm it's LC (or LC0–LC9), not 14, 16, or another overheat-related code. Write down exactly what you see.

2. Note whether hot water still works. LC often keeps the unit running while it beeps. That does not mean you're fine — it means the warning is active.

3. Do not open the cabinet or try a DIY vinegar flush unless you know the Rinnai procedure cold. Flushing needs the right service valves, pump setup, and steps from the manual. Wrong chemistry or a half-flush can make things worse.

4. Don't keep power-cycling to silence the beep. A reset without a flush only hides the reminder. Schedule the flush.

Gas appliances aren't where you get curious. Scale work is maintenance — not a "hit the breaker and hope" fix.

When to call a pro

Call if you see Code LC (or 00 in the history), if the controller won't stop beeping, if hot water is weaker or takes longer than it used to, if you're under warranty and want the flush documented, or if LC returns after someone only power-cycled the unit.

A proper service flush follows Rinnai's heat-exchanger flushing procedure, then resets the code by cutting power and restoring it after the flush. Around Charleston, Daniel Island, Mount Pleasant, and the rest of East Cooper, untreated hard water is the usual story behind LC. Catching it at the warning stage is a lot cheaper than waiting until scale has cooked the exchanger.

Soft next step

If your Rinnai is throwing Code LC and you'd rather not guess — Avery & Sons Plumbing + Tankless can flush it the right way and tell you straight whether treatment on the cold side makes sense for your house. Call (843) 305-5555 or visit averysplumbing.com. We're local, we work these units every week, and we'll tell you what it needs.

Stay on top of scale — Code LC is the unit asking for a flush before the heat exchanger pays for the delay.