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Rinnai · Tankless water heater

LC Scale build-up in heat exchanger (LC, LC0–LC9, 00)

Medium severity Non-SENSEIRH180 hybridSENSEI RX/CX

What the control is telling you

LC or LC0–LC9 means scale has built up in the heat exchanger and it must be flushed; in the maintenance code history it appears as 00. Newer units lock out until reset.

Applies to: SENSEI RX/CX Series, Non-SENSEI Series

Likely causes

  • Scale build-up in the heat exchanger
  • Untreated hard water
  • Heat exchanger not flushed at the recommended interval
  • Repeated temporary resets without flushing (LC# count rising)

Test and repair steps

  1. Flush the heat exchanger following the owner's manual procedure (section 6.3 on SENSEI RX/CX).
  2. Treat hard water to prevent further scale and heat exchanger damage.
  3. Non-SENSEI LC codes are the only codes that let the heater keep running; the display alternates LC and set temperature and the controller keeps beeping.
  4. Non-SENSEI older products reset LC by switching power off and back on.
  5. Newer Non-SENSEI and SENSEI RX/CX a built-in shutdown locks the heater out to prevent scale damage, so a reset is required.
  6. Temporary run until flushing press On/Off on the controller five times (within 5 s on RX/CX). The unit runs ~70 hrs then locks out again; display cycles set temp and LC#. E.g., LC2 = three 70-hr resets without flush.
  7. After flushing, SENSEI RX/CX LC clears automatically if the flush completes properly; if not, press PC board buttons A and B together for five seconds.
  8. After flushing, newer Non-SENSEI press PC board buttons A and B together for five seconds.
  9. The heater returns to normal and the LC# count resets to 0. This hard reset is not listed in the owner's/installation manual or tech sheet.
  10. SENSEI RX/CX Flushing history can be viewed in the Rinnai Central app.

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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.67–68. Summarized in our own words; check the original manual for your model.

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For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.