Hard Water and a Tankless Water Heater: What Scale Does, and Whether You Need a Softener
On soft water, no. On hard water the manufacturers expect the water to be treated, and a softener is one of the options they name: Navien’s manual recommends a water softener or its own PeakFlow system from 4 grains per gallon and requires one of the two above 12 grains (200 mg/L) [5], and Rinnai’s says water known to be hard must be treated [4]. The reason is scale. Heated hard water leaves calcium carbonate behind [1], and in a tankless heater that happens inside the heat exchanger, where Rinnai traces overheating hot spots to blockage by scale [4]. Damage from scale is excluded in all three warranties we read, from Rinnai, Navien and Rheem [4][5][8]. How hard your water is in Clearwater or anywhere in Tampa Bay depends on your utility; our hardness article gives each utility’s own figure.
What does hard water actually do inside a tankless water heater?
It leaves scale inside the heat exchanger — the part where the heat goes into your water. When a hot tap opens, cold water flows through the heat exchanger and a gas burner or an electric element heats it on the way through [3]. When hard water is heated, calcium carbonate comes out as solid deposits [1], and Rinnai’s manual speaks of scale in the heat exchanger directly [4].
Hardness is the calcium and magnesium dissolved in the water [1]. The University of Nebraska–Lincoln Extension describes what heat does with it: some comes out as calcium and magnesium carbonate, and the scale that builds up can slow the heating process and increase energy use [2]. The U.S. Geological Survey adds that scale can shorten the life of equipment, raise the cost of heating water and clog pipes [1].
In a tankless heater, scale on the heat exchanger does two things at once: it sits between the hot metal and the water, and it takes up room the water flows through. Rinnai’s troubleshooting table shows where that ends. For its heat-exchanger overheat code, the checks include looking for hot spots on the heat exchanger surface, which indicate blockage due to scale build-up [4].
It is not a cosmetic problem. Rinnai’s manual says that failure to flush the appliance will cause damage to the heat exchanger, that damage caused by lime build-up is not covered by the warranty, and that scale build-up is caused by hard water and can be accelerated if the heater is set at a high temperature [4].
What are the signs of scale buildup in a tankless water heater?
The manuals name them as fault codes and lost performance rather than as something you can see: an overheating heat exchanger, temperature-sensor faults, and, as Rinnai puts it, a heater whose performance is affected by scale build-up [4][6]. A service reminder on the display is something else — a timer set at installation, not a measurement of scale [4][5].
None of these codes proves scale on its own. The same tables list other checks beside it, from cleaning the inlet filter to testing the overheat switch [4][6]. Note the code and the model number before you call: the manual for that model is what the fix is read against.
The codes and reminders in the manuals we read:
- Rinnai code 14, heat exchanger overheat: the checks include hot spots on the heat exchanger that indicate blockage by scale [4]
- Navien E016, overheating of the heat exchanger, and E030, exhaust overheating: cleaning the cold-water inlet filter and flushing the heat exchanger are the listed actions [6]
- Rinnai inlet, outgoing and heat-exchanger sensor codes: cleaning scale off the sensor is among the listed remedies [4]
- Rinnai SS: a time-based service indicator set during installation [4]; Navien’s descaling reminder can be set to 6, 12 or 24 months and is off by default [5]
How hard is too hard for a tankless water heater?
In the three manufacturers’ documents we read, the line is 200 mg/L of total hardness — 12 grains per gallon, as Navien writes it. Rinnai and Navien set it as the maximum in their installation manuals [4][5], and Rheem’s warranty does not cover damage from installing its heater where water quality falls outside a table with the same 200 mg/L figure [8].
Where your water sits against that line depends on the utility that supplies it, not the city in your address. Some of the ranges in our Tampa Bay hardness article reach past 200 mg/L and others are soft; the article gives each utility’s own figure, and a test at your tap settles it for your house [2].
Navien grades hardness in bands of its own, which match the USGS classes up to 120 mg/L and differ above it [1][5]. For the heater, the manual for your model is the scale that counts.
Hardness is not the only line. Most of the other limits in these tables come, the documents say, from the EPA’s National Secondary Drinking Water Regulations [4][5][8]:
- Total hardness: up to 200 mg/L [4][5][8]
- pH: 6.5 to 8.5 [4][5][8]
- Total dissolved solids: up to 500 mg/L [4][5][8]
- Chlorides: up to 250 mg/L [4][5][8]
- Iron: up to 0.3 mg/L · manganese: up to 0.05 mg/L [4][5][8]
- Copper: up to 1.0 mg/L · zinc: up to 5 mg/L · aluminum: up to 0.2 mg/L (Navien and Rheem write 0.05 to 0.2) [4][5][8]
- Dissolved carbon dioxide: up to 15 mg/L (Rinnai, Rheem) · sulfate: up to 250 mg/L and chlorine: up to 4 mg/L (Navien) [4][5][8]
Do I need a water softener with a tankless water heater?
Not on soft water. On hard water the manufacturers expect treatment, and a softener is one of the options they name. Navien’s manual lays it out by hardness, in the list below, and adds two notes: flushing is recommended if a treatment device is not installed, and local codes should be checked for any restrictions on water softeners [5].
Rinnai’s manual is shorter. Where the water is known to be hard or to cause scale, it must be treated and may need a more frequent flushing schedule; another page of the same manual says treated and/or flushed regularly [4]. It does not name a softener. The product Rinnai offers is a ScaleCutter system that feeds a blend of control compounds into the cold water supply [4].
Those are two different kinds of equipment. A softener is an ion-exchange unit that removes nearly all the calcium and magnesium from the water [2]. The manufacturers’ own options are scale-prevention systems: Rinnai’s feeds compounds into the water [4], and Navien calls PeakFlow a scale prevention system [7]. The University of Nebraska–Lincoln Extension notes that many techniques are marketed for hardness and that ion exchange is still the one most used in homes [2].
A softener changes the water for the whole house, not only for the heater: the same extension guide lists spotted glassware, films on shower doors and dingy, harsh-feeling laundry among hard water’s effects, and notes that softening adds about 8 mg/L of sodium for each grain of hardness [2].
Navien’s hardness bands [5]:
- Soft, 0–4 grains per gallon (0–60 mg/L): no treatment, no flushing [5]
- Moderately hard, 4–7 gpg (61–120 mg/L): water softener or Navien PeakFlow recommended; flush once a year [5]
- Hard, 7–9 gpg (121–160 mg/L): softener or PeakFlow recommended; flush once a year in a home, twice a year in commercial use [5]
- Very hard, 9–12 gpg (161–200 mg/L): softener or PeakFlow highly recommended; the same flushing [5]
- Extremely hard, above 12 gpg (200 mg/L): softener or PeakFlow required [5]
Can I flush a tankless heater instead of installing a softener?
Below 12 grains per gallon, Navien’s manual recommends treatment but does not require it, and recommends flushing once a year in a home where no treatment device is installed [5]. Above 12 grains it requires a softener or PeakFlow [5]. Rinnai asks for both on hard water: the water treated, and a flushing schedule that may need to be more frequent [4].
Either way, the heater has to be flushable. Rinnai strongly recommends isolation valves so the heat exchanger can be flushed [4], and the U.S. Department of Energy’s consumer guidance said periodic maintenance can significantly extend a water heater’s life and minimize loss of efficiency, pointing owners to the manual for the specifics [3]. On the tankless heaters we install, an isolation valve kit under the unit is what makes a flush possible without cutting pipe.
How a flush is done — the valves, the pump and the solution — is in our separate article on flushing a tankless water heater, linked below.
Does a tankless warranty require a water softener?
Not by name. None of the three warranties we read says “no softener, no warranty”, but all three exclude damage from scale or poor water quality. Rinnai’s manual says damage caused by lime build-up is not covered by the water heater’s warranty, and its limited warranty excludes improper maintenance, naming scale build-up, and any damage caused by poor water quality [4].
Navien’s limited warranty shall be void on, among other things, damage caused by local adverse conditions including hard water deposits and lime or mineral build-up, and its manual adds that damage caused by poor water quality is not covered [5]. Two more lines sit beside these in Navien’s documents: the hardness table makes a softener or PeakFlow required above 12 grains per gallon, and the warranty is void for failure to install in strict compliance with the Installation Manual procedures [5].
Rheem’s limited warranty does not cover failures resulting from installation where water quality falls outside its table — total hardness up to 200 mg/L is one of the limits — or damage, malfunctions or failures caused by lime, mineral build-up or scale [8].
Terms differ by brand, model and year, so the warranty that came with your heater is the one that applies. Keep the installation date, any water test and a note of each flush with it.
One manufacturer rewards treatment. Navien adds a year to the labor warranty — two years instead of one — on these conditions [7]:
- A new Navien tankless heater: NPE-A2, NPE-S2, NHW-AI, NHW-AE, NHW-SU or NHW-SE [7]
- A new PeakFlow A or PeakFlow C scale prevention system; PeakFlow S does not count [7]
- Both installed at the same time and registered online within 60 days of installation [7]
Can a softener sit in the same room as a gas tankless heater?
Yes, if the heater’s combustion air is kept clean of the salt. Navien’s installation manual lists sodium chloride used for water softening among the corrosive contaminants for combustion air, tells installers to remove contaminants or move the air inlet and vent terminations to clean areas, recommends direct venting with air from outside, and says the warranty does not cover damage caused by air contaminants [5].
Rinnai’s list of corrosive compounds does not name softener salt, but its manual says the air around the heater, the venting and the terminations must be free of compounds that cause corrosion, and that damage from corrosive compounds in the air is not covered by warranty [4]. So where the brine tank stands and where the heater draws its air are worth settling before either goes on the wall.
What this means in Clearwater, Largo and across Tampa Bay
Hardness here is set by the utility that supplies you, and on the utilities’ own figures it runs from soft to very hard; our article on Tampa Bay hardness gives each one’s number. Put that number beside the table in your heater’s manual. Below 4 grains per gallon, Navien asks for no treatment [5]; above 12, it requires a softener or PeakFlow [5]; and Rinnai’s manual asks for treated water wherever the water is known to be hard [4].
The heater in the photograph is one of our installations in Largo: a gas tankless unit on copper piping, with a whole-house filter housing in the same utility room. We install both tankless heaters and whole-house water treatment, so the heater and the water can be planned together: what the manual for your model asks for, what a test at your tap shows, and whether a softener, the manufacturer’s own scale-prevention option or a flushing schedule fits the house.
Send your heater’s model number and the name of your water utility — we will tell you what its manual asks for and what your water needs, in Clearwater, Largo and across Tampa Bay.
Call 727-470-7126FAQ
Does hard water damage a tankless water heater?
It can. Heated hard water leaves calcium carbonate scale [1], and in a tankless heater it builds up in the heat exchanger. Rinnai says failure to flush will cause damage to the heat exchanger and traces overheating hot spots to blockage by scale; damage from lime build-up is not covered by its warranty [4].
Do I need a water softener with a tankless water heater?
Not on soft water. Navien’s manual recommends a softener or its PeakFlow system from 4 grains per gallon and requires one of the two above 12 grains (200 mg/L) [5]; Rinnai’s says water known to be hard must be treated, and the product it offers feeds control compounds into the water [4]. Check your utility’s figure against your heater’s manual.
Do I need a water softener to keep my tankless warranty?
None of the Rinnai, Navien or Rheem warranties we read asks for a softener by name, but all three exclude scale damage [4][5][8]. Rheem’s does not cover failures where water quality is outside its table, which includes total hardness over 200 mg/L [8], and Navien’s manual requires a softener or PeakFlow above 12 grains per gallon [5]. Navien also adds a year of labor coverage with a new PeakFlow A or C [7].
What are the signs of scale buildup in a tankless water heater?
In the manuals, fault codes: a heat-exchanger overheat (Rinnai code 14, where the checks include hot spots from scale blockage; Navien E016, where flushing is a listed action) and sensor faults where Rinnai’s remedy includes cleaning scale off the sensor [4][6]. A service reminder is a timer set at installation, not a reading of scale [4][5].
How many grains of hardness is too much for a tankless water heater?
The Rinnai, Navien and Rheem documents we read all use 200 mg/L of total hardness as the limit — 12 grains per gallon, as Navien writes it [4][5][8]. Navien starts recommending a softener or PeakFlow well below that, from 4 grains per gallon [5].
Is a salt-free water conditioner the same as a water softener?
No. A softener removes nearly all the calcium and magnesium by ion exchange [2]. The other treatment the heater manufacturers name is their own scale-prevention equipment: Navien’s PeakFlow, listed beside a softener, and Rinnai’s ScaleCutter, which feeds control compounds into the water [4][5][7]. For any other product, check whether your heater’s manual accepts it.
Does a higher temperature setting cause more scale in a tankless heater?
Rinnai says so: scale build-up is caused by hard water and can be accelerated if the heater is set at a high temperature [4]. Its warranty coverage is also reduced to 8 years or 12,000 operation hours, whichever comes first, if the temperature settings exceed 160°F (71°C) [4].
Can I put in a tankless water heater on hard water without a softener?
Below 12 grains per gallon, Navien’s manual recommends a softener or PeakFlow but does not require one, and recommends flushing once a year in a home without a treatment device; above 12 grains it requires one of the two [5]. Rinnai says water known to be hard must be treated [4]. Check your heater’s manual against a test of your water.
Sources
We review and cite. We do not reproduce a paper or a manual — follow the links for the originals.
- [1]Hardness of Water — U.S. Geological Survey, Water Science School · 2026Government or standards body · hardness is dissolved calcium and magnesium; heated hard water forms calcium carbonate deposits that can reduce equipment life, raise heating costs, lower electric water heater efficiency and clog pipes; classes 0–60 soft, 61–120 moderately hard, 121–180 hard, >180 mg/L very hard; page dated 2026-08-24; read 2026-10-11
- [2]Drinking Water Treatment: Water Softening (Ion Exchange) (G1491) — Sharon O. Skipton, Bruce I. Dvorak · University of Nebraska–Lincoln Extension · revised 2014University extension · heat drives calcium and magnesium carbonate out as scale, which can slow heating and increase energy use; ion exchange removes nearly all calcium and magnesium; many techniques are marketed, ion exchange the most common in homes; about 8 mg/L sodium per gpg; hard-water effects on glassware, shower doors, laundry; read 2026-10-11
- [3]Tankless or Demand-Type Water Heaters (Energy Saver) — U.S. Department of Energy · archived 2026-06-30Government or standards body · cold water flows through a heat exchanger and a gas burner or electric element heats it; periodic maintenance can significantly extend a water heater’s life and minimize loss of efficiency; read the owner’s manual for maintenance. The page has returned 404 on energy.gov since July 2026 (Internet Archive captures from 2026-07-03), so it is cited from the archived copy of 2026-06-30; the page says nothing about hard water; read 2026-10-11
- [4]Rinnai Tankless Water Heater Installation and Operation Manual — SE+ Series RU130–RU199 (100000467(05)) — Rinnai America Corporation · 2019Manufacturer's manual · Water Quality Guidelines table (total hardness up to 200 mg/L; pH 6.5–8.5; TDS 500; chlorides 250; iron 0.3; manganese 0.05; copper 1.0; zinc 5; aluminum 0.2; CO2 15 mg/L); hard water must be treated and may need more frequent flushing (elsewhere: treated and/or flushed regularly); scale accelerated by high temperature; code 14 hot spots from scale blockage; sensor remedies include cleaning scale; SS time-based reminder; failure to flush will damage the heat exchanger; isolation valves strongly recommended; ScaleCutter feeds control compounds; air free of corrosive compounds; warranty excludes scale build-up and poor water quality, coverage reduced above 160°F; read 2026-10-11
- [5]Installation Manual — NPE Condensing Water Heaters (NPE-A2/NPE-S2), Version 2.0, 20049624B — Navien, Inc. · 2026Manufacturer's manual · §3.1 About Water Quality: maximum allowable levels (total hardness up to 200 mg/L, 12 gpg) and treatment/flush table by hardness band (softener or PeakFlow recommended from 4 gpg, highly recommended 9–12, required above 12; flushing recommended if no treatment device); check local codes on softeners; poor-water-quality damage not covered; descaling reminder No Alarm/6/12/24 months, default No Alarm; corrosive contaminants for combustion air include sodium chloride for water softening; warranty void list includes hard water deposits, lime or mineral build-up; read 2026-10-11
- [6]User’s Information Manual — NPE Condensing Water Heaters, Version 1.0, 20049624A — Navien, Inc. · 2020Manufacturer's manual · §5.2 error codes: E016 overheating of heat exchanger and E030 exhaust overheating (clean cold-water inlet filter, flush heat exchanger); E760 flushing/service alarm; read 2026-10-11
- [7]Navien Tankless Water Heater with PeakFlow — Extra One-Year Labor Warranty (1925-1) — Navien, Inc. · 2024Manufacturer's manual · residential labor warranty 1 + 1 years when a new Navien tankless heater is installed with a new PeakFlow A or C, at the same time, registered online within 60 days; PeakFlow S not included; NPE-A2, NPE-S2, NHW-AI, NHW-AE, NHW-SU, NHW-SE; a warranty sheet, not a manual; read 2026-10-11
- [8]Limited Warranty — Rheem, Ruud, Richmond Residential Tankless Gas Water Heaters (AP21265 Rev 03) — Rheem Sales Company, Inc. · 2023Manufacturer's manual · exclusions: installation where water quality is outside the Recommended Water Quality Levels table (total hardness up to 200 mg/L; pH 6.5–8.5; TDS 500; CO2 15; aluminum 0.05–0.2; chlorides 250; copper 1.0; iron 0.3; manganese 0.05; zinc 5 mg/L); damages, malfunctions or failures caused by lime, mineral build-up or scale; a warranty certificate, not a manual; read 2026-10-11
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