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

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.

Settings for these units

RX supply connection sizes · SENSEI RX/CX

Vent connection: 2 in. nominal PVC/CPVC/PP or 3 in./5 in. concentric.

Full specs (input, flow, weight) are only on each model's spec sheet, not in this manual.

RX water quality limits · SENSEI RX/CX

Water damage beyond these limits is not covered by warranty.

• RX water treatment and flushing by hardness · SENSEI RX/CX
• RX clearances to combustibles · SENSEI RX/CX
• RX temperature settings available · SENSEI RX/CX
• RUCS/RUS unit clearances · Non-SENSEI
• RUCS/RUS water quality limits · Non-SENSEI

Models that show this code

69 models in Non-SENSEI, RH180 hybrid, SENSEI RX/CX

Other codes on the same units

• 72 Flame rod fault
• 73 Burner thermocouple fault
• 79 Water leakage detected
• FE Flammable vapors detected
• FF Maintenance performed (history marker)
• SE Cascade diagnostic display (error on secondary unit)
• SEt Initial settings not yet confirmed
• SS Service Soon alert (also listed as 55)