Rinnai · Tankless Water Heater Minor

LC — Scale Buildup / Descale Reminder

LC (Lime/Calcium) indicates scale buildup in the heat exchanger. Unit may still operate but performance is degrading.

Keep using it?
Yes — fix when convenient
DIY difficulty
Easy
Typical cost
$60-100
Verified against
rinnai.us ↗

The fix

Tankless water heater descaling flush kit (pump, hoses, bucket)

Lime/calcium scale buildup in the heat exchanger

$60-100 moderate difficulty 60-90 min Tools: flush kit pump, hoses, bucket

Diagnose first: This is an external maintenance flush through the service valves — no gas or combustion parts are touched. Circulate a descaling solution (or white vinegar) through the heat exchanger per Rinnai's manual. If you're on hard water, an inline scale-reduction/water softener cartridge ahead of the unit reduces how often this recurs. If LC keeps returning shortly after a flush, that points to a plumbed-in water softener need or a professional descale, not a bigger DIY job.

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How to fix it

  1. Schedule a descale with vinegar or manufacturer-approved tankless descaler.
  2. Use isolation/service valves with a submersible pump — circulate 60-90 minutes.
  3. Clean the inlet water filter screen.
  4. After descale, reset the LC counter per service manual.
  5. Install a softener or scale-prevention device to extend service intervals.

Why it happens

Typical signs: LC or LC0-LC9 on display · Lower hot water output · Temperature swings

  • Hard water deposits in heat exchanger Calcium scale insulating tubes.
  • No prior descale maintenance Recommended yearly with hard water.
  • No softener / scale inhibitor Untreated supply water.

Other parts sometimes needed

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

More Rinnai tankless water heater codes

02 02 — Power Interruption / Memory Loss 03 03 — Loss of Communication / EZConnect Issue 10 10 — Air Supply or Exhaust Blockage 11 11 — No Ignition 12 12 — Flame Failure 14 14 — Thermal Fuse / Overheat Cutoff

All Rinnai codes → All tankless water heater codes →

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: rinnai.us.