The honest answer is that it depends on your water, not on a calendar — but the manufacturers publish their own intervals, and one of them lets you set the reminder yourself. Here is how to pick a number and stop guessing.

There is no universal interval

“Once a year” is the number everyone repeats, and for a lot of homes it is wrong in both directions — wasteful on soft water, far too slow on hard.

What actually drives it is how much dissolved mineral passes through the heat exchanger, which is a function of your water and how much hot water you use. A household of five on well water at 15 grains per gallon is a completely different machine from a couple on municipal soft water, even if the heater is identical.

Start with your water hardness

This is the number the interval hangs on, and it is free to find:

Municipal supply: your utility publishes an annual water quality report. Hardness is usually given in grains per gallon (gpg) or parts per million (ppm). Divide ppm by 17.1 to get gpg.
Private well: a test kit is a few dollars, and worth doing once.

Then pick from this, and treat it as a starting point you adjust based on what comes out of the unit:

Hardness	Interval
Under 3 gpg (soft)	Every 2 years
3-7 gpg (moderate)	Yearly
7-10 gpg (hard)	Every 6-9 months
Over 10 gpg (very hard)	Every 6 months, and look at treating the water

Let the unit keep the schedule

Rinnai’s SS service reminder is not a fault — it is a timer set at installation, and Rinnai publishes the options: off, six months, one year, or two years. Almost every unit in the field is on whatever the installer left it at, which is rarely a decision about your water.

Matching that setting to your hardness is a five-minute conversation with whoever services the unit, and it converts “I should probably flush it sometime” into something that tells you.

Navien does the equivalent with E760, a flushing and service alarm.

When the unit overrides the schedule

Flush now, whatever the calendar says, if:

A scale code appears. Rinnai’s LC and LC0-LC9 are an actual scale detection rather than a timer, and Rinnai notes they are the only codes that let the unit keep running — which is why they get ignored. The digit counts how many times the flush has been postponed.
It rumbles or kettles. See tankless water heater making noise.
Output has quietly dropped. Scale insulates, so the unit works harder for less hot water before it ever throws a code.
You get an overheat or high-exhaust-temperature code. Rinnai’s troubleshooting for code 54 sends the technician to look for hot spots on the heat exchanger caused by scale.

What the manufacturers say about hard water

Both are blunt, and it is worth quoting the substance because it affects more than the schedule. Rinnai’s instruction alongside its scale codes is that hard water must be treated to prevent scale build-up or damage to the heat exchanger will occur.

That is not a maintenance tip, it is a warranty condition — Navien’s warranty excludes damage caused by hard water deposits, lime or mineral build-up outright. See what your warranty expects of you.

If your water is above 10 gpg, softening or a scale-inhibiting cartridge is usually cheaper over the life of the heater than flushing it three times a year, and much cheaper than a heat exchanger.

What to use

Rinnai’s own text for code 54 specifies four gallons of vinegar, following the flushing instructions in the owner’s manual. Food-grade white vinegar and purpose-sold citric acid descalers are both used in practice; the flush procedure is the same either way, and the thing that matters more than the choice is doing it before the unit starts complaining.

Never use anything harsher. A hydrochloric or sulphamic descaler intended for a tank heater will attack the heat exchanger you are trying to save.

How to descale a tankless water heater

How Often to Flush a Tankless Water Heater