Hard water is the main maintenance issue for a tankless water heater: scale builds up in the heat exchanger, cuts flow and can void the warranty. Noritz’s warranty, for example, excludes damage from water harder than 12 grains per gallon (200 mg/L). In mineral-rich areas, utilities such as the City of Anaheim advise descaling a tankless unit once a year with undiluted white vinegar.
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Below is why tankless units are so sensitive to hardness, what manufacturers’ warranties say, how the descaling flush works, and how to tell whether your water needs softening first.
Why tankless heaters scale up faster
A tank heater warms a large volume of water slowly, and the scale that forms mostly settles at the bottom of the tank as sediment. A tankless heater does the opposite: it heats water as it flows, through narrow passages in a heat exchanger, in seconds. When hard water is heated, dissolved calcium bicarbonate turns into solid calcium carbonate, and in a tankless unit that happens right on the hot surfaces that do the heating.
The City of Anaheim describes the result for its own mineral-rich water: calcium deposits in tankless units that can cause clogs, reduced flow and debris in the hot water lines. A layer of scale also insulates the heat exchanger, so the burner or element has to work harder to reach the set temperature.
What the warranties say
| Manufacturer | Document | What it says about hardness and scale |
|---|---|---|
| Noritz | Limited warranty, tankless (form SBA8743) | Excludes damage from “water with high PH levels or hardness levels in excess of 12 grains per gallon (200 mg/L)”, and from untreated or poorly treated well water. |
| Rinnai | Residential warranty, SENSEI models (6/2024) | Excludes “any damage caused by poor water quality” and improper maintenance “such as but not limited to scale build-up”. |
Read your own model’s warranty and owner’s guide: the wording and any hardness limit vary by brand and model. The pattern, though, is consistent. Scale damage is treated as a maintenance problem, so the owner carries the cost.
Is your water over the line?
12 gpg is 205 mg/L by the exact conversion (Noritz rounds it to 200). Here’s how some published supplies compare:
| Water | Hardness | Against 12 gpg |
|---|---|---|
| Austin, TX | 5.4 gpg average (4.1–7.4) | Below |
| Las Vegas, NV | 16 gpg (280 mg/L) | Above |
| San Antonio, TX | 15–20 gpg typical | Above |
Being under a warranty limit doesn’t mean no scale. Water of 8 or 10 gpg still scales a heat exchanger; it just takes longer. The water hardness scale shows where your number sits.
How to descale a tankless water heater
The City of Anaheim’s procedure for a gas tankless unit, in outline:
- Turn off the power to the unit and close the gas supply.
- Close the hot and cold water isolation valves. Many installations have service valves with small purge ports for this job.
- Connect a submersible pump in a bucket of at least 4 gallons of undiluted, food-grade white vinegar to the cold-side port, and a return hose from the hot-side port back into the bucket.
- Open the purge ports and circulate the vinegar through the heat exchanger for about 2 hours.
- Flush with clean water until the vinegar is gone, close the ports, open the isolation valves and check for leaks and air before restoring power and gas.
Electric tankless units differ: the city’s guidance for those involves removing and soaking the heating elements, and checking with a voltage tester that power is off before touching anything. Your owner’s guide takes priority over these general steps. If your installation has no isolation valves with purge ports, or you aren’t comfortable with gas and electrical shutoffs, this is a job for a plumber.
How much vinegar does the job?
White vinegar labelled 5% acidity holds about 50 g of acetic acid per litre, enough in theory to dissolve about 42 g of limescale per litre. Four US gallons (15.1 litres) could dissolve up to roughly 630 g of scale, which leaves a wide margin for a yearly flush. Because the vinegar circulates, it reaches every passage rather than sitting in one spot. If it comes back cloudy and stops fizzing quickly, the unit was heavily scaled; replace it with fresh vinegar and run it again.
How often to descale
Anaheim recommends once a year for tankless units in its high-mineral area. Some owner’s guides set their own interval. Sensible adjustments:
- Very hard water, no softener: follow the manufacturer’s interval and check flow and temperature stability in between. On water like Las Vegas’s, the unit may need it more than once a year.
- Moderately hard water: once a year is a reasonable default.
- Softened water: scale forms very slowly. Follow the manual, which may still call for periodic maintenance.
Prevention: softener or conditioner?
A water softener removes calcium and magnesium before the water reaches the heater, which stops scale at the source. It’s the most complete fix and protects every other fixture too. Size it to your hardness with our sizing guide.
A salt-free conditioner doesn’t remove the minerals. Wessex Water describes such devices as changing the minerals so they are less likely to form scale when heated. Check whether your heater’s manufacturer accepts them as water treatment before relying on one.
Not sure you need treatment at all? Our do I need a water softener checklist walks through it.
Signs your tankless heater is scaling up
- Hot water that swings between hot and cold during a shower.
- Lower hot water flow than when the unit was new.
- The unit taking longer to reach temperature, or shutting down at high demand.
- A maintenance or error code that the manual links to scale or overheating.
- White flakes in the hot water at taps and shower heads.
Frequently asked questions
Can you use a tankless water heater with hard water?
Yes, but it needs regular descaling, and above some manufacturers’ limits (12 gpg for Noritz) scale damage isn’t covered by the warranty. A softener or the manufacturer’s scale protection is the safer route on very hard water.
How often should I flush a tankless water heater?
At least once a year in hard water areas, according to the City of Anaheim, or as often as the owner’s guide states if that’s more frequent.
Is vinegar safe for a tankless water heater?
Undiluted white vinegar is the descaling solution in Anaheim’s procedure. Some manufacturers sell their own descaler; if your manual names one, use it.
Does a water softener void a tankless warranty?
Not in the warranties above; they exclude damage from hard water, not from treated water. Check your model’s owner’s guide for any limits on softened water.
What hardness is too high for a tankless water heater?
It depends on the brand. Noritz names 12 gpg (200 mg/L) as the point above which hardness damage isn’t covered. Rinnai’s 2024 residential warranty excludes scale build-up without naming a figure.
