The right interval depends on two things: what your heater’s maker asks for, and how hard your water is. Only Navien puts a plain yearly minimum in writing. The other brands either let the installer pick a reminder, or say to flush “periodically” where water is hard. This guide sets those instructions side by side, then compares them with the hardness figures Utah cities publish.
A flush, also called descaling, means pumping a mild acid such as white vinegar or a commercial descaler through the heat exchanger. The heat exchanger is the set of narrow tubes where the burner heats your water. Scale is the chalky mineral crust that builds up inside those tubes.
What each manufacturer says
| Brand | Interval or advice | Reminder on the unit | Where it comes from |
|---|---|---|---|
| Navien | at least once a year; with untreated hard water, once a year (twice a year for very hard water in commercial use) | Descaling reminder can be set to no alarm, 6, 12 or 24 months; shows E760 when due | NPE-2 manuals and Navien’s descaling article |
| Rinnai | Service Soon reminder: every 6 months, 1 year or 2 years (installer-selected) | SS on SENSEI units; LC on RE Series units | SENSEI and RE Series manuals |
| Noritz | Says periodic flushing in hard-water areas is necessary | Optional C1# service reminder | EZ owner’s guide |
| Rheem | Says to flush the heat exchanger regularly | Maintenance notice after 500, 750 or 1,000 hours of operation (code 88); off by default | RTGH use and care manual |
| Takagi | Have the unit checked once a year; FAQ mentions periodic flushing by a plumber in exceptionally hard water | No scale reminder found | 110/310/510 manual and Takagi FAQ |
| A.O. Smith | Standard models: no interval found. The company says its scale technology eliminates routine descaling (X3 models) | Not documented in our sources | A.O. Smith announcement |
| Bosch | Descale periodically, where water is highly mineralized | No interval published | Bosch descaling bulletins |
Two things stand out. First, the reminder choices run from half a year to two years. Makers expect the right interval to change with your water, so they leave room to adjust it.
Second, a reminder is only as good as the setting someone chose at install time. If the installer picked the longest setting, or switched the reminder off, the display won’t warn you. Rheem’s notice is off unless someone turns it on. So a quiet display does not prove your heater is clean.
What HomeGuide says about hard water
HomeGuide’s cost guide advises that homes with hard water consider flushing every 6 to 9 months instead of once a year. That is a cost guide’s advice, not a manufacturer’s rule. Still, it lines up with the shortest settings Rinnai and Navien offer. It also matches Rinnai’s own note that in hard-water areas the water must be treated and may require a more frequent flushing schedule.
Utah city hardness against the brand limits
Every maker sets a maximum hardness for the water going into its heaters. Hardness is measured in grains per gallon (gpg) or in mg/L, which is the same as parts per million (ppm). The brands fall into three groups:
- Takagi and A.O. Smith: 7 gpg for single-family homes.
- Bosch: 100 ppm, which is lower than the Takagi line.
- Navien and Noritz: 12 gpg. Rinnai and Rheem state the same line as 200 ppm.
Here is how the figures Utah cities publish compare. Because Bosch’s limit is lower than Takagi’s, every city in this table is above the Bosch line as well.
| City | Published hardness | Who publishes it | Above the Takagi and A.O. Smith limit? | Above the Navien, Noritz, Rinnai and Rheem limit? |
|---|---|---|---|---|
| American Fork | 12.38 gpg | City FAQ (a softener setting) | Yes | Yes, slightly |
| Saratoga Springs | 8.3 gpg average | City culinary water page | Yes | No |
| Highland | 14 gpg | City water hardness page | Yes | Yes |
| Orem | 7–25 gpg | Orem water quality report (2025 data) | At or above | The upper part of the range |
| Provo | 10–15 gpg | Provo 2025 water report | Yes | The upper part of the range |
| Spanish Fork | 13.4–20.24 gpg | City page (archived copy) | Yes | Yes |
| South Jordan | 7–10 gpg | City water page | At or above | No |
| Herriman | 12–15 gpg for the district water it buys; local wells 25–60 gpg | Herriman City | Yes | Mostly yes; the wells far exceed it |
| Riverton | about 10.7 gpg (district average) | Jordan Valley Water Conservancy District | Yes | No |
| West Jordan | about 10.7 gpg average | West Jordan 2026 report | Yes | No |
| Draper | about 10.8 gpg average (WaterPro area) | WaterPro report | Yes | No |
| Lehi | Not published | Lehi’s 2025 report has no hardness row | Unknown | Unknown |
A few notes on reading it:
- Riverton buys all its water from the Jordan Valley Water Conservancy District (JVWCD), so its figure is the district’s average, not a Riverton measurement.
- Herriman’s figure is what the city says about the JVWCD water it buys. JVWCD’s own website says its water ranges 1–12 gpg.
- Draper’s figure comes from WaterPro, which serves only part of Draper. Treat it as a rough guide.
- Lehi blends its own wells and springs with Central Utah Water Conservancy District water. The district’s treatment-plant water averaged 8.2 gpg in its 2021 report, but that is wholesale water, not a Lehi tap reading.
For sources, report years and the USGS hardness classes, see Utah hard water and tankless heaters.
Matching your interval to your water
No manufacturer publishes a chart that turns gpg into months. What they do say is enough to sort most homes into one of three rows.
| Your situation | What the makers and guides point to |
|---|---|
| Water above your brand’s limit, no softener | The makers expect treatment. Navien’s installation manual says water above 12 gpg requires a softener or PeakFlow, and Rinnai says the water must be treated. Until then, use the short end, such as HomeGuide’s every 6 to 9 months. |
| Water below your brand’s limit, no softener | Navien’s at least once a year baseline, or the reminder your installer set, whichever comes first. |
| Softener or scale-prevention system in place | Scale builds far more slowly. Keep a yearly check, and keep the treatment system serviced so it keeps working. |
If your city reports a range rather than one number, plan around the high end. Orem’s report runs from 7–25 gpg, and Herriman switches to district-only water every year in mid-October. Your water can change with the season or the part of town you live in.
Being above the limit matters for more than scale. Several warranties exclude damage from water above the stated hardness. See tankless warranties and scale for the exact wording.
What speeds up scale
High temperature settings
Hotter water drops more of its minerals as scale. Rinnai’s SENSEI manual says scale build-up is accelerated by high temperature settings. Bosch’s descaling bulletin goes further and says a softener is strongly recommended above 140 F outlet temperature. If you keep the setpoint high for a large tub or a dishwasher, expect scale sooner.
Heavy hot water use
Scale forms while the burner is heating water. More showers, more laundry and more people mean more hours of heating. Navien’s descaling article lists heavier hot water use, along with higher mineral content and higher temperatures, as reasons to descale more often than at least once a year. Rheem’s reminder counts burner hours instead of months for the same reason: it is set to 500, 750 or 1,000 hours of operation.
Signs it is due before the calendar says so
Your heater may tell you before your schedule does. Bosch lists lower flow rates, A7 and E9 codes and boiling sounds as signs of scale. Rinnai’s RE Series LC code means there is scale build-up in the heat exchanger and it needs to be flushed to prevent damage. Overheating codes, such as Navien E016, can also point to scale. The error code guide explains each brand’s codes, and no hot water covers the symptoms that a flush won’t fix.
When to call a technician
Call a technician if you don’t know what reminder setting your installer chose, if your heater has no isolation valves, or if a scale or overheating code keeps coming back. A technician can check the setting, flush the unit and tell you whether your water calls for a shorter interval or a softener. Flush My Tankless books the flush and descale with a local tankless technician. For what that usually costs, see tankless flush cost.