Rinnai LC Code: Scale Buildup & Service Reminder — Flush Guide

Scale build-up in the heat exchanger — the heat exchanger must be flushed; LC0–LC9 count how many 70-hour temporary resets have been used without a flush

LC is the code Rinnai owners in hard-water areas know best, and it is a maintenance alarm rather than a breakdown. It means scale — mineral deposit from hard water — has built up inside the heat exchanger far enough that the heater is protecting itself. Rinnai's wording is consistent across every document: 'LC indicates that there is scale build up in the heat exchanger and that the heat exchanger needs to be flushed to prevent damage. Hard water must be treated to prevent scale build up or damage to the heat exchanger.' Scale damage is pointedly not a warranty defect — the C199 manual's water-quality section states that replacement of a heat exchanger damaged by water quality is not covered. The LC family has a counting system worth understanding. Plain LC (shown in the fault history as 00 — the controller substitutes '00' for 'LC' when you scroll past codes) is the first warning. On newer products a shutdown feature escalates it: the heater will lock out unless reset. The temporary reset — press the ON/OFF button on the temperature controller five times within five seconds — buys 70 hours of operation, after which the heater locks out again with the count advanced: LC1, LC2, and so on. Rinnai's own example decodes it plainly: LC2 means the heater has been reset for three 70-hour periods without a flush and a hard reset. The number on the display is therefore a procrastination counter, and the controller cycles between the set temperature and the LC code, with a beep on Non-SENSEI models, so it cannot be missed. The documented remedy is the flush, and Rinnai publishes the procedure in the owner's manual (section 'Flushing the Heat Exchanger' — 6.3 on SENSEI manuals, 6.2 in the tech sheet references) precisely because it is the one substantial maintenance job the documents hand to a competent owner with the right kit, while many owners still book it as a service. The SENSEI RX/CX flushing procedure's scale-control detail is representative: power and gas off, controller off, hot and cold shutoff valves closed, the pump outlet hose connected to the cold-side service valve, the drain hose on the hot-side service valve, four gallons of undiluted food-grade white vinegar in a bucket, pump and drain hoses into the solution, service valves opened, and the solution circulated through the heat exchanger. Isolation (service) valves are what make this possible without cutting pipe — Rinnai 'strongly recommends' them at installation for exactly this job. After a flush that completes properly, Rinnai states the LC codes reset automatically on RX/CX models. If the code does not clear, the hard reset is a PC-board procedure — the two switches A and B pressed together for five seconds — which returns the LC count to 0. Finally, the prevention half: LC is a water-quality verdict. A heater flushed and returned to the same untreated hard water will count LCs again on the same schedule. Rinnai's repeated sentence — hard water must be treated — means a softener or the model's documented scale-control option, plus the annual service rhythm. Flushing is the repair; treatment is the cure.

What causes the LC code on a Rinnai water heater?

Hard water mineral scale in the heat exchanger

The cause, by definition: calcium and lime from hard water deposit on the hottest surfaces inside the heat exchanger, insulating them. Rinnai detects the condition (on RX/CX a dedicated lime-detecting temperature function feeds it) and raises LC before the deposit damages the exchanger. Homes on well water or in known hard-water regions meet LC earliest and most often. The process accelerates itself: a light coating insulates the metal, the metal runs hotter to deliver the same water temperature, and hotter metal precipitates minerals faster, which is why LC intervals shorten when flushes are deferred.

Skipped annual maintenance

The flush LC demands is the same flush the manuals schedule as routine maintenance. A heater that has never been flushed is not unlucky; it is overdue. The LC count then records how long the warning has been deferred in 70-hour increments — which is why a technician reads LC4 very differently from a first LC. The annual rhythm is not arbitrary, either: the manuals bundle the flush with the annual professional inspection, the inlet filter clean and the vent check, so one visit discharges the whole maintenance debt the LC is counting.

Untreated water after previous flushes

Flushing restores the exchanger; it changes nothing about the water. Without treatment — a softener or the scale-control equipment Rinnai documents for the installation — scale re-forms at the same rate and LC returns on the same interval. Repeat LC on a short cycle is a water-treatment conversation, not a flushing-technique argument. Rinnai publishes water-quality limits in the installation manual, hardness and chemistry ceilings the supply must meet, so a treatment recommendation can be checked against printed numbers rather than taken on trust.

Running on temporary resets

The five-press temporary reset exists so a household is not without hot water while a flush is arranged. Used as a lifestyle — reset, 70 hours, reset — it converts a maintenance job into heat-exchanger damage, because the heater is being run deliberately in the condition LC exists to prevent. The escalating LC number is the record of that choice. There is also a warranty edge to the behaviour: the manuals exclude heat-exchanger damage caused by water quality, so a heater run for months on advancing LC counts is being run outside the protection its owner paid for.

A scaled inlet filter and companion restrictions

Scale rarely travels alone: the same water that coats the exchanger loads the inlet water filter with sediment and debris, and the manuals pair the two jobs — the filter is cleaned in the same maintenance rhythm as the flush, and a clogged filter is on Rinnai's list for flow and temperature complaints that accompany an LC history. A heater whose LC arrives together with falling flow or temperature swings is showing the whole water-side picture, and the service that answers it is flush plus filter plus a water-quality conversation, not the flush alone.

Checks to run before you replace anything

1. Read the exact display: LC, or LC with a digit (LC0–LC9)? The digit is the count of temporary-reset periods already used — note it before doing anything
2. Scroll the fault history (hold ON/OFF 2 seconds, then UP): LC events appear there as 00 — a history full of 00s is a long scale story, not a new fault
3. Distinguish LC from SS: SS (or 55) is the time-based 'Service Soon' reminder; LC is the scale-detection alarm. Both end in a flush, but LC is the one that locks the heater out
4. Check your water: known hard-water supply, well water, no softener or scale control fitted? That context confirms LC rather than questioning it
5. Look at the installation: are isolation/service valves fitted below the heater? Their presence decides whether the documented pump-and-vinegar flush is straightforward
6. Note performance symptoms that corroborate scale: rumbling/kettling noises, fluctuating temperatures, longer waits — and check for sibling codes such as 54 (high exhaust temperature from scale hot spots)

Rinnai error code 55 how to fix

1. Do the flush — it IS the fix: isolate the heater with its service valves and circulate the manual's flushing solution (SENSEI procedure: 4 gallons of undiluted food-grade white vinegar) with a small pump through the heat exchanger, following your model's flushing section step by step
2. If you do not have isolation valves, a pump kit, or confidence in the procedure, book the flush as a service visit — it is the single most routine tankless service call there is
3. Temporary running only while the flush is being arranged: press ON/OFF five times in five seconds for up to 70 hours of operation; the controller will keep cycling the LC reminder, and the count will advance if you repeat it
4. After a properly completed flush, RX/CX models clear LC automatically; if a code remains, the A/B-switch hard reset (both switches, five seconds, count back to 0) is the technician's documented completion step
5. Clean the inlet water filter as part of the same visit — the manuals pair filter cleaning with flushing in routine maintenance
6. Prevent the sequel: fit or service water treatment for hard water, and put the flush on the annual calendar — Rinnai's position is that scale damage from untreated water is not a warranty matter

The part that usually fails: The heat exchanger itself — protected, not replaced; plus the inlet water filter and isolation valve kit that make flushing possible

LC is unusual among fault codes in that the 'part' is the component being protected rather than one that failed: the heat exchanger. Rinnai's whole LC system — detection, counting lockouts, published flush procedure — exists to keep scale from destroying it, because a scaled-through exchanger is a replacement, and water-quality damage is excluded from warranty. The supporting hardware matters as much: the inlet water filter (cleaned at the same service) and the isolation/service valve kit, which Rinnai strongly recommends at installation precisely so the flush is a hose-and-pump job instead of a plumbing job. Where prevention is fitted, it is water-treatment/scale-control equipment sized to the home's hardness.

Rinnai error code 55 reset button

LC has three distinct resets and they are not interchangeable. The TEMPORARY reset — ON/OFF pressed five times within five seconds — restarts a locked-out heater for 70 hours per use and advances the LC count; it treats nothing. The AUTOMATIC reset happens on RX/CX models when a flush completes properly: the code clears itself and the flushing history is visible in the Rinnai Central app. The HARD reset — PC-board switches A and B held together for five seconds after flushing — returns the count to 0 when the automatic clear does not happen; Rinnai notes this procedure is not published in the owner's manual, so it is performed by the service technician. Older Non-SENSEI units could clear LC with a plain power off/on. Resetting without flushing, by any route, leaves the scale in place.

The standard owner reset is a controller cycle: close the hot taps, press ON/OFF to turn the controller off, then on again, and run one test draw (code 03's documented form is ON/OFF twice). Special codes have special resets that are not interchangeable: LC and SS use five ON/OFF presses within five seconds (LC's version is a 70-hour temporary reprieve, not a cure); codes 17 and 52 clear only through the PC-board A/B-button sequence ending in 'ECL'; codes 57 and 58 need a hard reset — power off, then on — after the cause is corrected; and the LC hard reset after flushing is the A/B switches held together for five seconds. One reset, then observe: a code that returns is a live fault reporting itself, not a reset that 'didn't take'. Document the outcome of every reset you perform — cleared and held, returned during the draw, or never cleared — because that record is what turns a home reset into diagnostic evidence.

Why does the LC code keep coming back?

Two displays get confused with LC constantly, and this page handles both. SS — also shown as 55 — is 'Service Soon': a purely time-based reminder set at installation (Parameter 03: off, 0.5, 1 or 2 years) meaning service and a flush are due; the heater keeps running, and the reset is ON/OFF five times in five seconds on SENSEI, or holding the PC-board 'A' button until SS disappears on RX/CX. LC, by contrast, is detection-based: the heater has measured a scale condition, counts temporary resets as LC0–LC9, appears in the history as 00, and will lock the heater out. An SS ignored long enough on hard water becomes an LC; an LC is never 'just the reminder'.

How to stop LC coming back

• Hard water mineral scale in the heat exchanger: The cause, by definition: calcium and lime from hard water deposit on the hottest surfaces inside the heat exchanger, insulating them. Rinnai detects the condition (on RX/CX a dedicated lime-detecting temperature function feeds it) and raises LC before the deposit damages the exchanger. Homes on well water or in known hard-water regions meet LC earliest and most often. The process accelerates itself: a light coating insulates the metal, the metal runs hotter to deliver the same water temperature, and hotter metal precipitates minerals faster, which is why LC intervals shorten when flushes are deferred.
• Skipped annual maintenance: The flush LC demands is the same flush the manuals schedule as routine maintenance. A heater that has never been flushed is not unlucky; it is overdue. The LC count then records how long the warning has been deferred in 70-hour increments — which is why a technician reads LC4 very differently from a first LC. The annual rhythm is not arbitrary, either: the manuals bundle the flush with the annual professional inspection, the inlet filter clean and the vent check, so one visit discharges the whole maintenance debt the LC is counting.
• Untreated water after previous flushes: Flushing restores the exchanger; it changes nothing about the water. Without treatment — a softener or the scale-control equipment Rinnai documents for the installation — scale re-forms at the same rate and LC returns on the same interval. Repeat LC on a short cycle is a water-treatment conversation, not a flushing-technique argument. Rinnai publishes water-quality limits in the installation manual, hardness and chemistry ceilings the supply must meet, so a treatment recommendation can be checked against printed numbers rather than taken on trust.

Retrieving stored fault codes on Rinnai water heaters

Rinnai tankless heaters report faults on the temperature controller — the integrated SENSEI controller or a remote such as the MC-91-2 or MC-601 — as flashing two-digit codes in place of the set temperature. Alongside the numbers sit four letter displays with their own jobs: LC (scale in the heat exchanger, counted as LC0–LC9 when temporary resets are used, and stored as 00 in the fault history), SS (the time-based Service Soon reminder, also shown as 55), FF (a history placeholder recording that maintenance was performed) and, on cascaded commercial systems only, SE. The controller keeps a fault history: turn the heater OFF, hold ON/OFF for 2 seconds and press UP at the same time — the last 9 maintenance codes flash one after the other. A separate performance-data mode (hold DOWN, then ON/OFF) shows live values such as water flow rate (item 01) and outgoing temperature (item 02) — data items, not fault codes.

Other Rinnai water heater error codes

See the full list on the Rinnai Water Heater error codes hub.

Codes LC gets confused with

10 — Air supply or exhaust blockage — on SENSEI condensing models the same code also reports a full condensate trap Code 10 is Rinnai's airflow fault: the heater cannot move combustion air in and exhaust out the way it needs to, so it shuts down and displays 10.

11 — No ignition — the heater ran its ignition sequence and no flame was established, so it shut down and flashed 11 on the temperature controller Code 11 is Rinnai's no-ignition fault, and it applies across the whole tankless range this site covers: Non-SENSEI models, SENSEI models and the SENSEI RX/CX series all define 11 the same way in Rinnai's Troubleshooting Tankless Water Heater Diagnostic Codes manual.

17 — Venturi blockage — debris is obstructing the venturi on the gas valve/fan assembly (SENSEI and SENSEI RX/CX only) Code 17 is one of Rinnai's narrowest codes: it exists only on SENSEI and SENSEI RX/CX models, and it means exactly what its title says — venturi blockage.

12 — Flame failure — the burner lit, then the flame was lost during operation and the heater shut down Code 12 is the running partner of code 11, and the distinction is the whole diagnosis.

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Facts on this page were checked against manufacturer documentation: www.rinnai.us · media.rinnai.us · www.rinnai.ca · media.rinnai.us · www.manualshelf.com · www.rinnai.us