Rinnai LC: Scale Build-Up Warning
Unlike most codes on this list, LC isn't an emergency — the unit keeps making hot water while the display alternates between "LC" and the temperature setting, nagging you that the heat exchanger is due for a flush.
Applies to: Rinnai tankless and condensing water heaters with a heat exchanger scale-monitoring feature.
What it means
Scale build-up detected in the heat exchanger. Unlike most Rinnai codes, LC is a maintenance warning and the unit keeps running — the display alternates between "LC" and the temperature setting and the controller beeps. It's a preventive alert, not a fault: Rinnai's control board is tracking runtime and water conditions and estimating when scale has likely accumulated enough to warrant maintenance.
Most common causes
Two things drive this alert:
- Hard water without a softener or filtration
- Heat exchanger has gone past its recommended flushing interval (Rinnai recommends annual flushing in most water conditions)
What you can safely check yourself
This is one of the few codes on this site with a real, documented homeowner fix:
- Flush the heat exchanger following the descaling procedure in your owner's manual (typically circulating a vinegar or manufacturer-approved descaling solution through isolation valves)
- After flushing, cycle power off and on — the LC code resets on power cycle
When to call a professional
You aren't comfortable performing the flush yourself, don't have isolation valves installed, or the code returns quickly after flushing (a sign your water is very hard and a softener may be worth considering).
This is a maintenance task rather than a safety repair, but if you're not confident working with the isolation valves or a descaling pump, a plumber can perform the flush for you.
What the fix costs
A DIY flush costs close to nothing — a jug of white vinegar (or a manufacturer-approved descaling solution) and, if you don't already own one, a small utility pump to circulate it, often under $50 total. Hiring it out typically runs $80–$200 for a standard tankless descaling service (Angi, HomeGuide). Skipping the flush repeatedly is the expensive path: scale build-up is also a listed cause behind the pricier 14 and 16 faults, so this is one of the cheapest problems on this site to stay ahead of.
Related codes
- Rinnai 14 — Thermal Fuse Tripped
- Rinnai 32 — Water Temperature Sensor Fault
- Rinnai 16 — Over Temperature Warning
- Symptom: water not hot enough or runs lukewarm
All Rinnai codes
Every Rinnai water heater fault code — the full list on one page.
Sources
- Rinnai heat exchanger maintenance and descaling procedure, owner's manual (manufacturer documentation)