Do You Need a Water Softener for a Tankless Water Heater?
Quick answer: if your water is hard, you need either a softener or scale protection, and the heater makers say so. Rinnai’s manual says hard water must be treated, sets a limit of 200 mg/L (about 11.7 gpg) of total hardness, and doesn’t cover heat exchanger damage from water quality under warranty. Noritz’s warranty excludes damage from hardness above 12 gpg. A whole-house softener protects the heater and everything else, as long as it can handle your peak flow. A scale-prevention cartridge on the heater’s cold line costs less but doesn’t soften the water. Either way, flush the heater as its manual directs.
Our picks:
- Best whole-house softener: SpringWell salt-based softener
- Best efficiency-rated softener: Whirlpool WHES48
- Best softener on Amazon: AFWFilters Fleck 5600SXT 48,000-grain
- Best salt-free option for a Navien: Navien PeakFlow A
- Best for a Rinnai on a budget: Rinnai Scale Cutter
- For flushing any tankless heater: Chromex flush kit and descaler
How we chose: we haven’t installed or tested these ourselves. We started from what the heater makers’ manuals and warranties require, then compared flow ratings, limits and upkeep from the makers’ own pages. Where a detail comes only from a seller’s listing, we say so.
Why hard water is a problem for tankless heaters
Hard water leaves scale on the heat exchanger. Rinnai’s manual says scale build-up is caused by hard water and can be accelerated if the heater is set to a high temperature. When it starts, a Rinnai shows a diagnostic code (“LC”) telling you to flush the heat exchanger, and Rinnai warns that failing to flush will damage it. North Dakota State University Extension notes that scale slows heating and takes more energy to heat water.
What the tankless makers say about hardness
| Brand | Hardness limit | What it says |
|---|---|---|
| Rinnai | Total hardness up to 200 mg/L (about 11.7 gpg) | In an area with hard water, “the water must be treated.” Replacing the heat exchanger because of water quality isn’t covered by the warranty, and installers are told to inform you if a water softening system isn’t installed. |
| Noritz | 12 gpg (200 mg/L) | The warranty excludes damage from water with hardness above 12 gpg, or from untreated or poorly treated well water. |
| Navien | Not found in the documents we checked | Sells its own PeakFlow scale-prevention systems for tankless heaters. |
From Rinnai’s RUS/RUCS installation and operation manual, Noritz’s tankless limited warranty and Navien’s PeakFlow A manual, checked October 2026. Check the water quality section of your own model’s manual.
Rinnai’s manual also limits iron to 0.3 mg/L, manganese to 0.05 mg/L and chlorides to 250 mg/L, so on well water, test for those too. Not sure of your hardness? See how to test your water.
Softener or scale prevention: which is right?
| Option | Protects | Softens? | Things to know |
|---|---|---|---|
| Salt-based softener | The heater and the whole house | Yes | Needs a drain, an outlet and salt; must handle your peak flow |
| Crystal-forming (TAC) system, such as Navien PeakFlow or Watts OneFlow | The heater | No | No salt, drain or power. Navien says PeakFlow isn’t for recirculating systems or tank heaters |
| Polyphosphate cartridge, such as Rinnai Scale Cutter | The heater | No | Replace the cartridge regularly; Navien says PeakFlow can’t be used with polyphosphate |
| Flushing only | Removes scale after it forms | No | Needed anyway when the heater asks for it |
Of these options, only a softener removes hardness, so it also protects fixtures, the dishwasher and the washing machine. If you only care about the heater, or you have no drain for a softener, a scale-prevention system on the heater’s cold-water line is the simpler fix. In an independent study published by WateReuse, the crystal-forming method (TAC) cut scale by more than 88%; see our salt-free guide for details and Watts’ tankless OneFlow models.
Size the softener for your peak flow
A tankless heater heats water as it flows, and every gallon passes through the softener first. Rheem’s softener manual says to always install the softener between the water inlet and the water heater. If the softener’s rated flow is below what your home uses at peak, pressure drops when several taps run. Add up what runs at once: EPA puts a conventional showerhead at 2.5 gallons per minute (gpm), or 2.0 gpm for WaterSense models, and a standard faucet at 2.2 gpm. Two showers and a faucet at once comes to about 7.2 gpm, close to the 7.5 gpm maximum of a Rinnai RUS75e. Then compare that with the softener’s service flow:
| Softener | Rated service flow |
|---|---|
| Rheem Preferred Compact RHS18 | 7.0 gpm |
| Whirlpool WHES48 | 11.9 gpm |
| AFWFilters Fleck 5600SXT 48,000-grain | 12 gpm (seller listing) |
| SpringWell SS1 / SS4 / SS+ | 11 / 13 / 20 gpm |
From each maker’s spec sheet or manual, and the seller’s listing for the Fleck.
For capacity (how many grains between regenerations), use our size calculator. Remember that a single-tank softener sends hard water to the heater while it regenerates, so let it regenerate at night; see regeneration.
Tankless hard water and flow check
Check your water against your heater maker’s limit, and see whether a softener can keep up with your peak hot water use.
Enter your water hardness to check it.
How it works: mg/L ÷ 17.1 = gpg. Limits are Rinnai’s 200 mg/L from its manual and Noritz’s 12 gpg from its warranty. “Hard” starts above 120 mg/L (about 7 gpg) on the U.S. Geological Survey’s scale. Peak flow = showers × showerhead flow + faucets × 2.2 gpm. A heater’s maximum flow is usually rated at a small temperature rise, so in winter it delivers less.
Best water softeners and scale protection for tankless heaters
SpringWell salt-based water softener
Sized by bathroom count, with flow ratings high enough for most homes’ peak hot water use. It softens everything, including the water going into the heater.
- Sizes and flow: SS1 for 1–3 bathrooms (11 gpm, 32,000 grains); SS4 for 4–6 (13 gpm, 48,000 grains); SS+ for 7+ (20 gpm, 80,000 grains)
- Warranty: lifetime on covered parts such as tanks and valves; 5 years on the electronic head (see SpringWell’s terms)
Good to know: it needs a drain and an outlet near where your water comes in. On city water with a lot of chlorine, consider SpringWell’s filter and softener combo; see our best water softeners guide for the full review.
See it at SpringWellWhirlpool WHES48
A 48,000-grain cabinet softener with a published NSF/ANSI 44 efficiency rating and a rated service flow of 11.9 gpm, enough for two showers and a faucet at once.
- Flow and resin: 11.9 gpm rated service flow; 1.33 cubic feet of resin
- Limits: up to 160 gpg hardness and 12 ppm clear-water iron
- Size: 15 in wide, 20½ in deep, 48 in tall
Good to know: Whirlpool’s page links to Lowe’s to buy it; we didn’t find it on Amazon. More detail in our high-efficiency softeners guide.
See it at WhirlpoolAFWFilters Fleck 5600SXT 48,000-grain softener
A two-tank softener on Pentair’s Fleck 5600SXT metered valve, popular with do-it-yourself installers.
- Flow: 12 gpm (seller listing)
- Tanks: 10 × 54-inch resin tank with 1.5 cubic feet of resin, 15-inch brine tank; 62 in tall overall (seller listing)
Good to know: program the hardness and capacity correctly after installation; see our settings guide.
Check price on AmazonNavien PeakFlow A
Navien’s own scale-prevention system for its tankless heaters. It needs no salt, drain or electricity. Navien is clear that it isn’t a water softener: it prevents scale but doesn’t reduce hardness.
- Limits: up to 75 gpg hardness; pH 6.5–8.5; iron up to 0.3 mg/L; chlorine up to 2 ppm
- Flow: 6 gpm continuous, 10 gpm peak
- Certification: certified by IAPMO R&T to NSF/ANSI/CAN 61
Good to know: Navien says not to use it with tank-type heaters, in recirculating or closed-loop systems, or alongside polyphosphate or other scale inhibitors. If your home has a hot water recirculation pump, check with Navien first.
See it at NavienRinnai Scale Cutter (103000038)
A small cartridge system for the heater’s cold-water line. Rinnai’s manual describes this kind of scale control as blending control compounds into the cold water supply.
- How it works: keeps scale-causing minerals in solution and coats wetted surfaces with a thin protective layer (listing)
- Upkeep: replacement cartridges (103000039) are sold separately
Good to know: it doesn’t soften water, and the details here come from the listing; check the cartridge life for your water. Don’t combine it with a TAC system like PeakFlow.
Check price on AmazonChromex tankless flush kit and descaler
Even with treatment, you’ll need to flush the heat exchanger when the heater asks. Rinnai’s manual describes the job: circulate 4 gallons of undiluted white vinegar through the heater with a pump for at least an hour, then rinse.
- Kit: 3.5-gallon bucket with lid, 1/6 HP pump and two steel hoses; add vinegar or descaler (listing)
- Descaler: Chromex’s 32 oz descaler is listed as certified to NSF/ANSI/CAN 60; not for aluminum or magnesium parts (listing)
Good to know: the kit’s bucket holds less than the 4 gallons Rinnai specifies, so follow your heater’s manual and use a larger bucket if needed. You’ll also need isolation valves on the heater; installers often fit them, and Rinnai tells installers to inform you if they aren’t there.
Check the kit on Amazon Check the descaler on AmazonWater softeners and tank water heaters
A regular tank water heater benefits too: NDSU notes that scale slows heating and wastes energy, and a softener installed before the heater keeps it out. Two tips from Whirlpool’s and Rheem’s softener manuals: install the softener before the water heater, and avoid using hot water while the softener regenerates, or the heater refills with hard water.
Just want soft water for the washing machine?
- A whole-house softener covers the washer along with everything else.
- A portable softener on the washer’s cold tap needs no plumbing changes; see our small water softeners guide for renters.
- Laundry products sold as “water softeners”, like Calgon Liquid Water Softener, are detergent boosters added to each load; its listing says it adds cleaning power to detergent. They don’t soften the water in your pipes or heater.
What about boilers?
Heating boilers are different from water heaters. Rheem’s high-efficiency combi boiler manual sets the hardness of the water in the heating loop between 1 and 9 grains, and Worcester Bosch says, for its oil boilers, that the central heating circuit should be filled with hard water and a chemical inhibitor, even though a softener helps the hot water side. If you have a softener, fill the heating loop as your boiler’s manual says, which may mean filling through the softener’s bypass. Rinnai’s tankless warranty also excludes heaters installed in a closed loop for space heating. One more tip: Rheem’s combi manual lists sodium and potassium chloride used for water softening among the corrosive contaminants to avoid, so don’t store softener salt next to a gas boiler or water heater.
Frequently asked questions
Do tankless water softeners work?
Products sold as “tankless water softeners” are usually salt-free conditioners or electronic descalers, not softeners: every true softener has a resin tank. They can reduce scale, but they don’t remove hardness. In the WateReuse study, TAC cut scale by more than 88% and electronic devices by about 50%; see our salt-free guide.
Will a water softener void my tankless warranty?
Not with the makers we checked. Rinnai’s manual expects treatment on hard water and tells installers to let you know if a softening system isn’t installed, and Noritz’s warranty excludes damage from water above 12 gpg. Check your own warranty for its exact terms.
How often should I flush a tankless water heater?
Whenever the heater shows its scale code (“LC” on Rinnai) and as your manual directs. Rinnai also says the heater must be inspected annually by a licensed professional. Hard water without treatment means more frequent flushing.
What hardness needs treatment for a tankless heater?
Rinnai’s limit is 200 mg/L (about 11.7 gpg) and Noritz’s warranty limit is 12 gpg, but Rinnai says water in hard-water areas must be treated regardless. The U.S. Geological Survey calls water above about 7 gpg hard.
Does softened water cause problems in a tankless heater?
Not according to the makers we checked: Rinnai’s manual lists a softening system among the things an installer should tell you about if it’s missing. Softened water does still need to meet the manual’s other limits, such as chlorides.
Related guides
- What size water softener do I need? (calculator)
- Best water softeners (all types)
- Best salt-free water softeners
- How to install a water softener
Sources
- Rinnai, RUS/RUCS (KCM series) installation and operation manual, U328-0700X04, from the RUS75eN product page: water quality limits, scale, flushing, flow and installer checklist.
- Rinnai, SENSEI residential limited warranty: scale, water quality and closed-loop exclusions.
- Noritz, tankless limited warranty: 12 gpg hardness exclusion.
- Navien, PeakFlow A installation manual.
- Whirlpool, WHES48 manual; Rheem, RHS18 manual; SpringWell salt-based softener specs.
- Rheem, high-efficiency combi boiler installation and operating manual: heating-water chemistry and corrosive contaminants.
- Worcester Bosch, Can I use artificially softened water in the heating circuit?
- U.S. EPA WaterSense, Showerheads and Bathroom Faucets; U.S. Geological Survey, Hardness of Water.
- NDSU Extension, Water Softening (Ion Exchange); WateReuse, project 08-06.
- Amazon listings for the AFWFilters, Rinnai Scale Cutter, Chromex and Calgon products, checked October 2026.