Rinnai Water Heater Error LC – Causes & Fix
Mineral scale is coating the heat exchanger – the Rinnai needs a descaling flush; the digit counts overrides.
Safety first · Consult a professional
Before flushing, turn off the heater's power and close its gas supply valve, and let it cool – water in the unit can be scalding. Never open gas fittings. If you ever smell gas, get out, don't touch switches, and call 911 or your gas utility from outside; if a CO alarm sounds, go outside and call 911.
- Gas appliance: no DIY repairs. If you smell gas, leave the building, do not use switches and call your gas utility’s emergency line from outside.
- Carbon monoxide is odorless. Keep a CO alarm near fuel-burning appliances; if it sounds, get everyone outside and call 911.
- Hot surfaces and hot water can cause burns. Let the appliance cool down first.
At a glance
Severity: Low
- Meaning
- Scale Build-Up in Heat Exchanger (LC0–LC9)
- Typical repair cost
- $60–$3,000
- DIY difficulty
- Moderate
LC (shown as LC or LC0 to LC9) means the Rinnai has detected scale building up inside its heat exchanger and wants it flushed before overheating damages it. On older models it's a warning: the heater keeps running, the display alternates between LC and the set temperature and the controller beeps. Newer models lock out; pressing the controller's on/off button five times buys about 70 hours of operation, and the digit counts those overrides – LC2, for example, means it has already been reset for three 70-hour periods without a flush. Flushing is ordinary owner maintenance described in Rinnai's manuals: power and gas off, isolation valves closed, then a pump circulates undiluted white vinegar (Rinnai suggests 4 gallons) or an approved descaler through the unit. Hard water that isn't treated will bring LC back quickly.
How it looks on the display
Model families & display variants
| Model family | Shows | Note |
|---|---|---|
| Older non-SENSEI models (e.g. V-series, earlier RL/RU) | LC | Warning only: the heater keeps running while LC alternates with the temperature and the controller beeps. |
| Newer models with scale lockout | LC0–LC9 | Digit counts 70-hour overrides without a flush; the heater locks out again at the end of each period. |
| Maintenance code history | 00 | In the stored code history Rinnai substitutes '00' for LC. |
Symptoms
- Display alternates between LC (or LC0–LC9) and the temperature setting
- Controller keeps beeping even though hot water still flows (older models)
- Newer models stop heating until the override or a flush
- Water temperature swings or takes longer to get hot
- White, crusty scale on faucet aerators and shower heads – a sign of hard water
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Hard water with no softener or scale-inhibiting treatment
- Flushing interval missed – Rinnai recommends flushing at least once a year
- High set temperature or continuous recirculation speeding up scale formation
- Scale-reduction cartridge or pre-filter exhausted
- Previous flush incomplete – too short, valve partly open, inlet filter left dirty
- Counter not cleared after a flush on a newer lockout model
Fix it step by step
Shut down power, gas and water
Turn the heater off at the controller and cut power at the plug or breaker. Close the gas valve to the heater, then close the cold and hot isolation valves (handles perpendicular to the pipe are off). Let the unit cool.
Tools: Gloves
Relieve pressure and connect the flush kit
Put a bucket under the service ports and slowly remove the purge-port caps – water will escape. Connect one hose from a small submersible pump to the cold-side service port and a second hose from the hot-side port back into the bucket.
Tools: Towels & shallow pan, Pliers
Circulate the descaling solution
Pour about 4 gallons of undiluted white vinegar (or a Rinnai-approved, food-safe descaler) into the bucket, open both service ports and run the pump for roughly 45 minutes to an hour. Longer circulation helps when the LC count is high.
Tools: Towels & shallow pan
Rinse with fresh water
Empty the bucket, close the cold service port and open the cold isolation valve so clean water pushes through the unit and out the hot-side hose for several minutes until it runs clear.
Tools: Towels & shallow pan
Clean the inlet water filter
Close the valves again, unscrew the inlet filter at the cold connection, rinse or brush off debris and screw it back in hand-tight. Refit the purge caps.
Tools: Soft brush, Pliers
Restore service carefully
Open the isolation valves, run a hot tap until the air sputtering stops, then reopen the gas valve and restore power. Check the service ports for drips before walking away.
Tools: Towels & shallow pan
How to reset LC
- Older models: after flushing, switch power off and back on – the LC warning clears.
- Newer lockout models awaiting a flush: press the on/off button on the controller five times to run for about 70 hours (the LC digit increases with each override).
- After a flush on newer models, the LC counter is cleared at the control board – a step Rinnai reserves for service technicians, so ask your installer if LC stays on.
- If LC returns within weeks of a proper flush, have the water hardness tested.
Where the parts are
- 1Flue
- 2Fan
- 3Heat exchanger
- 4Ignition electrode
- 5Gas valve
- 6Circulation pump
- 7Expansion tank
- 8Pressure gauge
- 9Temperature sensor
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| DIY flush: pump, hoses and vinegar or descaler | Yes | $60–$150 |
| Professional descaling flush | No | $150–$350 |
| Add isolation (service) valve kit if none installed | No | $150–$350 |
| Scale-inhibitor cartridge filter installed | No | $100–$300 |
| Whole-house water softener | No | $1,000–$3,000 |
Rough US estimates; technician call-out fees vary by region. Not a quote.
When to call a technician
Call your installer or a Rinnai-trained plumber if the heater has no isolation valves, if LC stays on after a correct flush (the counter may need clearing at the board), or if you notice leaks, overheating codes or poor hot water even after descaling. Gas and electrical parts stay closed during a flush – if the procedure would require opening them, it's not an owner job.
Frequently asked questions
Can I keep using my Rinnai while it shows LC?
Older models keep running, but scale insulates the heat exchanger and can overheat it, so flush soon. Newer models only run in 70-hour blocks after the five-press override, which is meant to bridge the time until the flush, not replace it.
How often should I flush a Rinnai tankless heater?
Rinnai recommends at least once a year. With hard water or high set temperatures, every six months is common, and the LC code is the heater telling you the interval is too long.
Vinegar or a commercial descaler?
Rinnai's consumer guidance uses undiluted white vinegar, about 4 gallons. If you use a descaler, choose one approved for potable water systems and for tankless heaters, and always rinse thoroughly afterward.
What does the number after LC mean?
It counts how many times the lockout has been overridden without a proper flush. The higher the number, the longer the heat exchanger has been running scaled – don't let it climb.
Will a water softener stop LC codes?
A softener or a scale-inhibiting filter slows scale dramatically, so LC comes back far less often. You'll still want periodic flushing, but the intervals get much longer.
Sources
- Official Rinnai user documentation and support pages (facts only; wording is our own)
- Methodology: How We Build Code Pages · Data Sources We Use · Editorial Policy