Tankless warranties can run for many years on the heat exchanger, which can make them sound like a safety net. For scale, they mostly aren’t. The fine print in each brand’s manual or warranty treats scale as a maintenance problem, not a defect. Below is what each brand says, where the hardness limits come in, and how to keep a claim from being denied.
This guide summarizes manufacturer documents. Terms differ by model and by the date your heater was made, so read the warranty that came with yours.
What each warranty says about scale
| Brand | What the document says | Document |
|---|---|---|
| Rinnai | “damage caused by lime build-up is not covered.” The limited warranty also excludes failures from improper maintenance such as scale build-up and damage from poor water quality | SENSEI manual; SENSEI residential limited warranty |
| Navien | Under local adverse conditions, the limited warranty says hard water deposits and lime or mineral build-up are excluded | NPE-2 installation manual |
| Noritz | “problems resulting from scale formation are not covered.” | EZ owner’s guide |
| Rheem | Says damage caused by lime, mineral build-up, or scale is excluded, including heat exchanger failures | RGT/RGTH limited warranty |
| Takagi | Damage from hard water above 7 gpg (120 ppm) is excluded | Residential limited warranty |
| A.O. Smith (non-X3) | Damage from hard water above 7 gpg (120 ppm) is excluded | Warranty in the A.O. Smith-hosted 110U/310U/510U manual |
| Bosch | “damage from scale/mineral deposits is not covered.” | Therm descaling bulletin |
The pattern is the same across all seven. Scale is your job to prevent, and the warranty steps back if it isn’t prevented. Rinnai’s manual is the most direct: it says failure to flush will damage the heat exchanger, and that lime damage is not covered.
Hardness limits written into the coverage
Several brands go a step further and tie coverage to water quality. If your water is harder than the line, damage that results from it may be excluded even if you flush on time.
| Brand | Hardness line | How it affects coverage |
|---|---|---|
| Takagi | 7 gpg for single-family homes | Damage from water above it is excluded |
| A.O. Smith (non-X3) | 7 gpg for single-family homes | Damage from water above it is excluded |
| Noritz | 12 gpg | The warranty says hardness above 12 grains per gallon voids coverage for resulting damage; the owner’s guide says to treat water above it to keep full coverage |
| Navien | 12 gpg | The manual says water above 12 gpg requires a softener or PeakFlow |
| Rheem | 200 ppm | Damage from installing where water falls outside Rheem’s chart is excluded |
| Rinnai | 200 ppm in the manual’s water guidelines | The warranty names no grain figure but excludes damage from poor water quality |
| Bosch | 100 ppm | Bosch recommends a conditioner or softener above it |
This matters in Utah. Highland publishes 14 gpg, American Fork 12.38 gpg, and Provo 10–15 gpg. All three are above the Takagi and A.O. Smith line, and the first two are above Navien’s and Noritz’s. Check your own city in our Utah hard water guide.
Noritz’s reminder clause
Noritz EZ heaters can show code C1#, which the owner’s guide describes as a service reminder for periodic maintenance, such as flushing scale from the heat exchanger. The same guide adds a sharp condition: damage after an ignored C1# flush reminder is not covered.
In plain terms, once your Noritz tells you it needs a flush, the clock is running. Clearing the reminder without flushing doesn’t help, and could make a later claim harder. If you see C1#, book a flush and keep the record. The Noritz page covers its other codes.
Navien’s warning about DIY descaling
Navien’s descaling article encourages regular flushing, but adds a caution: incorrect descaling could void the warranty. It recommends hiring a professional for that reason.
Navien’s wording is about damage from doing it wrong. If a homeowner’s flush goes wrong, for example with the wrong solution or a skipped rinse, the resulting damage may not be covered. Our DIY vs professional guide compares the brand procedures in detail. See the Navien page for its E760 reminder and overheating codes.
The exceptions: X3 and PeakFlow
A few products change the scale picture:
- A.O. Smith X3. The company says its X3 condensing models eliminates routine descaling (X3 models) and carry a limited warranty with no exclusions for scale-related failures (X3 models).
- Takagi X3. Takagi’s X3 warranty provides 15 years of heat exchanger coverage including scale failure.
- Navien PeakFlow. Navien offers one additional year of labor warranty when a new Navien heater is installed with a PeakFlow A or PeakFlow C scale-prevention system. That extends labor coverage. It is not a promise to cover scale damage.
If you’re replacing a heater in a hard-water city, these are worth asking about. See scale prevention for how PeakFlow and similar systems work.
How to protect your coverage
Keep a service record
A dated record is your best evidence that you did your part. For each flush, write down or keep:
- the date and who did the work,
- the solution used and how long it circulated,
- any code on the display before the flush,
- whether the inlet filter was cleaned,
- the invoice or receipt.
Keep these with your warranty papers and the heater’s model and serial number. If a technician does the flush, ask for a written record that lists these items.
Follow the interval or the reminder
Use your manufacturer’s interval, or the reminder on the display, whichever comes first. Navien recommends at least once a year. Rinnai’s Service Soon reminder can be set to every 6 months, 1 year or 2 years (installer-selected). See how often to flush for every brand.
Treat water that is above the limit
If your city’s hardness is above your brand’s line, a flush alone may not keep you covered. A softener or an approved scale-prevention system brings the water inside the line. Keep the installation receipt and service records for that system too.
Keep your water evidence
Save a copy of your city’s water report, or a hardness test from your own tap. If you have a softener, a test of the softened water shows what is actually going into the heater.
Don’t clear codes without acting
Reminder codes such as Rinnai’s SS, Navien’s E760, Noritz’s C1# and Rheem’s code 88 are there to protect the heat exchanger. Clearing one without a flush leaves no record and, on a Noritz, may leave damage uncovered.
When to call a technician
If your heater is showing a reminder, if you’re unsure whether your water is above your brand’s line, or if you’ve lost track of when it was last flushed, a technician can flush it and leave a dated record. Book a flush and descale or a full annual maintenance visit with Flush My Tankless.
Prefer to have it done? Call to book a flush.