Rinnai says to flush the heat exchanger on a schedule set for your water. On SENSEI models, the Service Soon reminder can be set to every 6 months, 1 year or 2 years (installer-selected). Rinnai’s SENSEI manual caps total hardness at 200 ppm and says that in hard-water areas the water must be treated and may require a more frequent flushing schedule. Most Utah water is hard enough that a Rinnai here needs both: some kind of treatment and a regular flush.
Which Rinnai code is on your display?
| Code | Rinnai’s meaning | Likely cause | Safe check for you | When to call |
|---|---|---|---|---|
| LC | there is scale build-up in the heat exchanger and it needs to be flushed to prevent damage | Hard water scale on an RE Series unit | Note how long since the last flush | Book a flush soon, before damage |
| SS | it is time for service and the heat exchanger should be flushed | The SENSEI service timer has come due | Check your flush records | Book a flush within the next few weeks |
| 14 | the heat exchanger has overheated | Scale blocking heat transfer, or low flow | Confirm the isolation valves are fully open | Call; Rinnai says to look for scale hot spots |
| 16 | high outgoing temperature (a safety shutdown because the heater is too hot) | Scale, or a temperature set very high | Note the set temperature | Call if it returns |
| 10 | an air supply or exhaust blockage | Blocked vent, intake or condensate trap | Look for snow or debris at the outside vent, without touching it | Call; venting is technician work |
| 11 | no ignition (the heater is not turning on) | Gas supply or ignition parts | See whether other gas appliances work | Call |
| 12 | no flame | Gas supply, venting or a dirty burner | Note when it happens | Call |
| 79 | a water leak has been detected | A leaking fitting or failed part | Turn off the water supply to the heater | Call now, as Rinnai’s manual directs |
Our error code hub lines these up against Navien, Noritz and Rheem.
LC: scale build-up warning
LC is the classic Rinnai flush code from the RE Series manual. It means there is scale build-up in the heat exchanger and it needs to be flushed to prevent damage. Rinnai Technical Bulletin 75 points out that LC is the only code that lets the unit keep running. You will see the display flip between LC and your temperature setting while hot water still comes out.
That makes LC easy to ignore. Don’t. The heat exchanger is the bundle of thin tubes where the burner heats your water. Calcium and magnesium in hard water come out of solution when heated and stick to those tube walls as scale. Rinnai’s SENSEI manual adds that scale build-up is accelerated by high temperature settings, so a heater set hot in a hard-water city scales fastest.
SS: Service Soon on SENSEI models
Newer SENSEI heaters use SS instead of LC. Rinnai’s manual says SS means it is time for service and the heat exchanger should be flushed. It works from the reminder interval the installer chose, so it is a calendar alert more than a measurement. If your installer picked the longest setting and your water is hard, ask a technician to shorten it after the flush.
What about LC0, LC1 and the rest?
Many people search for “Rinnai LC0” through “LC9”, or for “LC#”. Those labels aren’t defined in any Rinnai installation manual, warranty or technical bulletin we could find. Some websites give them meanings anyway. This page doesn’t, because a guess could send you the wrong way.
What Rinnai does document is plain: LC on RE Series models, SS on SENSEI models, and both mean flush the heat exchanger. If your display shows a number next to LC, write it down exactly and give it to the technician with your model number.
Codes 14 and 16: overheating
Code 14 means the heat exchanger has overheated. Rinnai’s manual tells technicians to check for hot spots that point to a blockage from scale. Code 16 is high outgoing temperature (a safety shutdown because the heater is too hot). Both can come from scale. A coated tube can’t pass heat into the water, so the metal and the outgoing water run hotter than the controls allow. If either code keeps returning, a flush is usually part of the fix, but a technician should rule out other causes first.
Rinnai’s limit against Utah water
Rinnai states its limit in milligrams per liter (mg/L), which for water is the same as parts per million (ppm). Here is how Utah reports compare with Rinnai’s 200 ppm:
- Orem: the city’s latest report lists hardness of 120–428 mg/L across its system. Orem blends treated surface water with springs and wells, so homes on the high end are well past Rinnai’s limit.
- Spanish Fork: the city listed 231–349 mg/L, above the limit even at the low end.
- West Jordan: the 2026 report averages 182.7 mg/L, just under.
- Saratoga Springs: the city averages 142 mg/L, which it calls moderately hard.
Being under the limit doesn’t mean scale-free. The USGS calls anything more than 180 mg/L “very hard”, and West Jordan’s average is in that band. Our Utah hard water guide explains the scale.
Seeing a code? Call to book a descale
Rinnai’s warranty language on scale
Rinnai is blunt. The SENSEI manual states that failing to flush will damage the heat exchanger, and that damage caused by lime build-up is not covered. The SENSEI limited warranty excludes failures from improper maintenance such as scale build-up and from poor water quality. There is no grain figure in the warranty itself; scale damage is simply out. Our warranty and scale guide covers what records to keep.
What a Rinnai flush involves
Rinnai’s SENSEI manual describes a vinegar flush. The aim here is to help you know what a proper job looks like, not to replace the manual. Gas, venting and electrical work stay with a technician, and a mistake can affect your warranty.
The setup depends on isolation valves, the shut-off valves on the hot and cold lines under the heater. Rinnai says isolation valves are strongly recommended, because they let a pump loop through the heat exchanger without draining the house. If yours are missing, our isolation valve guide explains the fix.
With the valves closed, a pump pushes four gallons of undiluted, food-grade white vinegar through the heater at four gallons per minute. It keeps circulating for at least 60 minutes. Fresh water then rinses the heater for 5 minutes. The procedure also calls for the technician to remove and clean the in-line filter at the cold water inlet. That screen traps loose scale, and a clogged one can cut flow on its own.
Seeing a code? Call to book a descale
Rinnai Scale Control and other prevention
Rinnai sells a Scale Control System that treats incoming water with a replaceable cartridge. Rinnai says most homes need a new cartridge every 10 to 12 months, depending on size and water use. A softener is the other common route. Either one slows scale but doesn’t end the need to flush. Compare them on our scale prevention page.
Seeing LC or SS? Call to book a descale. A local tankless technician does the flush and descale. Angi’s data puts tankless flushing at $150–$250; the technician quotes the actual price.