THE SHORT VERSION
- Why: hard-water minerals coat the heat exchanger, which cuts output and flow, triggers error codes and eventually damages the exchanger. Every major maker excludes scale damage from the warranty.
- How often: at least once a year (Rinnai, Navien, Rheem). Rheem says about every 6 months in hard water; Noritz says yearly unless the water is treated, and treatment is required above about 10–12 grains per gallon.
- How: power and gas off, close the hot and cold shutoffs, then pump the solution in through the cold service port and back out the hot one for 45–60 minutes, and rinse with fresh water.
- With what: Rinnai’s manual specifies 4 gallons of undiluted food-grade white vinegar; Navien and Noritz also accept approved descaling products. Use what your manual names.
- Cost: HomeAdvisor puts a professional tankless flush at $150–$250, and a DIY pump kit at $100–$120 plus vinegar. Rinnai, Navien and Noritz recommend a professional.
Scale builds up where the water gets hottest
A gas tankless heater heats water as it passes through a compact heat exchanger over a powerful burner; the units in the study below were 199,000 BTU/h. Calcium and magnesium in hard water come out as scale on those hot surfaces, the same way a kettle furs up. Navien compares it to descaling a coffee maker and says the deposits “can erode and clog the walls inside your heat exchanger, reducing efficiency, reducing flow.”
How fast it happens is measurable. In a lab study by Battelle for the Water Quality Association (a trade group whose members sell softeners), 199,000 BTU/h tankless units on very hard, unsoftened water fell from 80% to 72% efficiency and had to be delimed after the equivalent of 1.6 years of use. Deliming brought them back to about 77%. Softened units held their efficiency. The background is in hard water and your water heater.
Signs your heater is due:
- A scale code on the display. Rinnai’s “LC” code means the unit “is beginning to lime/scale up and must be flushed.” Other brands have their own codes; see tankless error codes.
- A service reminder. Rinnai’s “SS” (Service Soon) reminder can be set to every 6 months, 1 year or 2 years. Noritz’s newer models show a service reminder code that clears after a correct flush.
- Lower flow or temperature swings at fixtures that used to work fine.
- It’s been a year, and you’re on moderately hard or harder water.
How often each manufacturer says to descale
| Maker | Standard interval | Hard water |
|---|---|---|
| Rinnai | Flush “at least once a year” (FAQ); service reminder selectable at 6 months, 1 year or 2 years | Water must be treated and may need more frequent flushing; limit 200 mg/L |
| Navien | “At least once a year” | More often with higher mineral content, higher temperature setting and heavier use |
| Rheem | Yearly maintenance, including a heat exchanger flush | “About every six months” |
| Noritz | None needed for soft or slightly hard water (0–3 gpg) | Yearly if untreated at 3–10 gpg; treatment required above 10 gpg in its residential table; damage above 12 gpg not covered |
Sources: Rinnai residential FAQ and condensing tankless manual; Navien, “How to Descale Tankless Water Heaters” (2023); Rheem maintenance article (Dec. 2025); Noritz NRC111 installation manual (2011) and NRCR training guide (2024). 1 grain per gallon = 17.1 mg/L. Your own model’s manual has the final word.
To place yourself in that table you need your hardness number. City water customers can ask the utility; well owners need a lab test. USGS classes water above 120 mg/L (about 7 gpg) as hard and above 180 mg/L (about 10.5 gpg) as very hard.
What the job needs: valves, a pump and solution
Descaling depends on isolation valves with service ports on the hot and cold lines under the heater. Each valve can shut the heater off from the house plumbing, and its side port takes a washing-machine-style hose. Rinnai “strongly recommends” them for flushing, and Noritz’s procedure assumes them. If your heater was installed without them, the first descaling visit should include adding them, which is plumbing work.
Equipment named in the manufacturer procedures
- A bucket of 5 gallons or more (Navien)
- A small submersible circulation pump; Rinnai’s procedure runs it at about 4 gallons per minute
- Two or three hoses with hose-thread ends to fit the service ports
- Descaling solution: Rinnai specifies 4 gallons of undiluted, food-grade white vinegar; Noritz uses 1 gallon of a calcium-lime-rust product mixed with 1 gallon of water, or vinegar; Navien allows an approved descaler or white vinegar
- Towels, and a drain or sink for the rinse water
Vinegar or a commercial descaler
Both dissolve calcium carbonate, which is what scale is. A.O. Smith’s deliming bulletin calls for food-grade phosphoric acid or household cider vinegar, and USGS notes vinegar’s acidity is what dissolves mineral deposits. The practical rule is simpler: use the solution your heater’s manual names, at the strength it names. Rinnai’s procedure is written around undiluted white vinegar; Noritz’s around a diluted calcium-lime-rust product; Navien says to follow the chemical maker’s directions. Don’t substitute stronger acids or mix products, and don’t reuse the scale-laden solution for the rinse.
The isolation-valve flush, step by step
This sequence follows the procedures in Rinnai’s condensing tankless manual, Navien’s NPE manual and Noritz’s descale procedure. Valve labels and model-specific steps differ, so read your own manual first. It assumes the heater has a gas shutoff and isolation valves already installed; it never involves opening gas lines or venting.
Turn the heater off and close the gas
Turn off the controller (and unplug or switch off power if your manual says so), then close the gas valve below the heater by turning its handle crosswise to the pipe.
Close the hot and cold shutoffs
This isolates the heater from the house. The service ports stay closed for now.
Connect the hoses
Pump outlet to the cold-side service port. A second hose from the hot-side service port back into the bucket. Put the pump inlet in the bucket.
Add the solution and open the service ports
Pour in the solution your manual specifies (Rinnai: 4 gallons of undiluted white vinegar). Open both service ports.
Circulate
Run the pump for at least 1 hour (Rinnai, Noritz) or at least 45 minutes (Navien). Some newer Rinnai models have a descaling mode set from the circuit board that counts down the 60 minutes.
Rinse with fresh water
Move the return hose to a drain. Close the cold service port and open the cold shutoff so fresh water flushes through and out the hot port: 5 minutes in Rinnai’s and Navien’s manuals, at least 10 minutes in Navien’s online guide, 3 minutes or more for Noritz.
Clean the inlet filter
With the cold shutoff closed again, remove the small screen at the cold inlet, rinse out debris, and refit it (Rinnai, Navien).
Restore service
Close the service ports, open both shutoffs, disconnect hoses, open the gas valve and turn the heater back on. Check the fittings for drips. Rinnai says to reset the LC code after flushing.
Model-specific details matter. Noritz’s older units, for example, must be run at 140°F before the flush so an internal bypass valve closes, and Rinnai’s condensing units have their own rinse sequence. If your manual has steps that aren’t listed here, follow the manual.
What can cost you warranty coverage
| Maker | What the documents say |
|---|---|
| Rinnai | “Damage caused by lime/scale build-up is not covered”; “improper maintenance (such as but not limited to scale build-up …)” is excluded; total hardness limit 200 mg/L |
| Rheem | Excludes failures “caused by lime, mineral build-up, or scale”; hardness up to 200 mg/L in its water-quality conditions |
| Navien | Excludes “hard water deposits, lime or mineral build-up”; “improper maintenance can void your warranty” |
| Noritz | Damage from water above 12 gpg (200 mg/L) not covered; unauthorized use of its pro descale procedure “may result in voiding the warranty” |
Sources: Rinnai condensing tankless manual; Rheem RTGH warranty; Navien NPE operation manual (2016) and descaling guide (2023); Noritz installation manual and descale procedure. Check the certificate for your model.
Put simply: descaling is the owner’s responsibility, and scale damage is the owner’s cost. Keep receipts or a log of each descaling and any water treatment. It helps if you ever need to show the heater was maintained. More on what voids coverage in water heater warranties.
What descaling costs, pro or DIY
Source: HomeAdvisor, “How Much Does It Cost to Flush a Water Heater?” (updated June 19, 2026). HomeAdvisor is a contractor-referral site; these are its national ranges, not quotes.
Worked example, using HomeAdvisor’s ranges: a DIY kit at $110 plus 4 gallons of vinegar at $4.50 is about $128 the first year, then about $18 a year. A professional visit at $150–$250 a year costs more. Rheem’s yearly maintenance list also covers cleaning the inlet filter, inspecting the burner and checking the condensate line and pressure relief valve, so ask whether a pro visit includes them. If valves have to be added first, that is extra plumbing labor.
If you’re weighing tankless against a tank, count this yearly cost. It’s one reason the tankless water heater guide calls descaling the price of admission in hard water, and one way water treatment can pay for itself.
Common questions
How often should a tankless water heater be descaled?
At least once a year according to Rinnai, Navien and Rheem. Rheem suggests about every six months in hard water, and Noritz says yearly unless a treatment device is installed. Very soft water may need it less often; follow your manual.
Can I use vinegar to descale a tankless water heater?
Yes, if your manual allows it. Rinnai’s procedure specifies 4 gallons of undiluted food-grade white vinegar, and Navien and Noritz list vinegar as an option alongside approved descaling products.
How long does descaling take?
The solution circulates for 45 minutes (Navien) to an hour (Rinnai, Noritz), plus setup, a rinse of 3–10 minutes and cleaning the inlet filter.
What happens if you never descale a tankless water heater?
Scale narrows the heat exchanger, so flow and efficiency drop and error codes appear. Eventually the exchanger can overheat and fail, and scale damage isn’t covered by Rinnai, Rheem, Navien or Noritz warranties.
Do I need to descale if I have a water softener?
Less often, but don’t stop checking. Noritz’s table drops the yearly flush requirement when a treatment device is installed, while Rinnai and Navien still describe at least yearly service, which also covers the inlet filter and burner.
My tankless heater has no service valves. Can it be descaled?
Not with the standard method. A plumber can add isolation valves with service ports, which Rinnai strongly recommends, and then descale it. Plan for that labor on the first visit.
Does descaling void my warranty?
Descaling as the manual describes doesn’t. Damage caused by doing it wrong can, Navien warns, and scale damage from not descaling is excluded by every major maker.
Sources
Sources checked October 5, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- CX/CXP condensing tankless installation and operation manual (flushing the heat exchanger, water quality, warranty) ↗ · Rinnai, hosted by Lowe’s
- Residential tankless water heater FAQ ↗ · Rinnai
- How to descale tankless water heaters (May 2, 2023) ↗ · Navien
- NPE condensing water heaters operation manual (version 5.1, 2016) ↗ · Navien
- How to keep your tank and tankless water heaters running at peak performance (Dec. 22, 2025) ↗ · Rheem
- Residential tankless gas water heater limited warranty (RTGH) ↗ · Rheem
- How do I descale/flush the heat exchanger (knowledge base) ↗ · Noritz
- Descale procedure, Group C models (Rev. 06/16) ↗ · Noritz
- NRC111 condensing tankless installation manual (water treatment tables) ↗ · Noritz, hosted by SupplyHouse
- NRCR training guide (2024) ↗ · Noritz
- Technical Bulletin 13: Mineral build-up ↗ · A.O. Smith
- Hardness of water ↗ · U.S. Geological Survey
- Battelle finds water softeners aid energy efficiency, appliance life (2010; study funded by the Water Quality Association) ↗ · Contractor magazine
- How much does it cost to flush a water heater? (updated June 19, 2026) ↗ · HomeAdvisor (contractor-referral site)