How often should I descale a tankless water heater in Phoenix?

Use your model's maintenance schedule, with more frequent descaling when its guidance and your water conditions call for it. City water testing reports total hardness of roughly 172 to 302 mg/L (City of Phoenix, 2025 Water Quality Report), which works out to about 10 to 17.6 grains per gallon. On the USGS scale, anything above 180 mg/L counts as very hard, and much of Phoenix sits right at or past that line.

The more minerals in the water, the faster scale forms inside the unit. A heater receiving untreated hard water can accumulate more scale than one receiving softened water, depending on temperature and use. So the interval is not one-size-fits-all. It tracks your actual water.

If you have a whole-house water softener doing its job, it reduces the calcium and magnesium reaching the heater and can reduce scale formation. It does not automatically set a longer descaling interval. Follow the heater manufacturer's guidance for softened feed water, confirm the softener is working and keep the heater's other maintenance tasks on schedule.

How much hot water you use matters too. A large family running multiple showers and laundry loads pushes more gallons through the heat exchanger, which lays down scale faster than a one or two person household. Higher set temperatures speed it up as well, since minerals drop out of solution faster in hotter water. For more on local water chemistry, see our page on how hard Phoenix water is.

Use service findings to review the schedule with your technician. Heavy scale or recurring flow problems can justify more frequent descaling. Clean-looking flush water alone is not a reason to extend service beyond the manufacturer's guidance. A technician can consider those findings alongside the measured inlet hardness and your household's use.

The damage happens inside a narrow heat-exchanger coil, where even a thin layer of scale chokes both heat transfer and water flow. A tankless heater has no storage tank. Water races through a tight, twisting passage and gets heated on the fly. That design is efficient, but it leaves almost no room for buildup.

When hard water heats up, calcium and magnesium fall out of solution and stick to the hot metal walls as scale. The coil starts to narrow. You feel it first as temperature swings, where the water goes hot then cold mid-shower because the unit cannot hold a steady output. Then comes reduced flow, since the passage is partly blocked. Eventually the control board throws an error code and may shut the unit down to protect itself.

The PNNL report PNNL-22921 reviews Battelle's accelerated tests using water at about 444 mg/L hardness, above the City of Phoenix's reported range. Gas tankless efficiency fell from 80 percent to 72 percent over 1.6 equivalent years of service before a clogged sensor or control stopped operation and deliming was needed. Efficiency recovered to 77 percent after deliming. Those are simulated service results under severe test conditions, not a lifespan forecast or a maintenance interval for your model.

Scale does not only waste energy. It makes the burner or element work longer to hit the same temperature, which means more gas or power and more wear. Left alone, the unit ages years faster than it should.

There is a safety angle as well. As the coil narrows, the unit can overheat in spots, and the control board responds by cutting output or locking out. That is why a heater that was fine last winter can start short-cycling or refusing to fire by midsummer. The water did not change overnight. The scale simply crossed the threshold where the unit can no longer cope. Catching it with a routine flush is far cheaper than replacing a heat exchanger, which is one of the priciest parts in the unit.

You descale by circulating a mild acid through the unit's service valves for about 45 to 60 minutes, then rinsing it clean. Tankless heaters are built for this. Most have a pair of isolation or service valves on the cold and hot lines that let you loop a pump and bucket through the heat exchanger without disconnecting the plumbing.

The basic steps look like this:

• Cut the power or gas to the unit and shut the isolation valves so no household water enters.
• Connect a small submersible pump in a bucket to the cold service valve, and run a hose from the hot service valve back into the same bucket.
• Fill the bucket with food-grade white vinegar or a commercial descaler. Many techs use three to four gallons of vinegar.
• Open the service valves and circulate the solution for 45 to 60 minutes so the acid dissolves the scale.
• Drain the solution, close the loop, and flush clean water through to rinse out any residue and loosened scale.
• Restore the valves to normal, restore power or gas, and clean the inlet water filter screen while you are at it.

Food-grade vinegar is the common choice because it is cheap, safe, and effective on calcium scale. A purpose-made descaler can work faster. Either way, always follow your unit's manual, since the exact valve layout and approved cleaners vary by brand. This is a different job from draining a tank-style heater, which we cover in our guide on flushing a water heater in Phoenix.

Scale is only one of the ways a tankless heater gives trouble. Our page on common tankless water heater problems covers filters, low flow and error codes, and why a tankless heater goes cold mid-shower covers the cold water sandwich.

Tankless makers treat descaling as routine maintenance, not an optional extra, and many tie warranty coverage to it. Navien, one of the larger condensing tankless brands, recommends flushing the unit on a regular schedule and warns that hard water shortens that interval. Navien advises homeowners to descale at least once a year, and more often as mineral content, water temperature, and usage rise.

In its maintenance guidance, Navien notes that the descaling frequency "depends on the water quality in your area." With 10 to 17.6 grains per gallon coming out of the tap, more frequent service may be appropriate, but that range alone does not establish a six-month schedule for every heater. Check your model's manual and your actual inlet hardness.

Following the manufacturer's schedule does more than protect performance. It often protects the warranty. Brands can deny scale-related claims if there is no record of maintenance, so keeping a simple log of flush dates is worth the trouble. a DOE Building America case study describes domestic hot water as the second-largest energy end use in U.S. buildings, exceeded only by space conditioning, so keeping the unit clean also keeps that bill in check.

If the hot water gets unpredictable or the unit starts complaining, scale is usually the cause and the flush is overdue. You rarely get a calendar reminder, so the unit tells you in other ways. Watch for these three signs:

• Fluctuating temperature. Water that swings hot to cold during a shower is a classic scale symptom. The narrowed coil cannot hold steady output.
• Lower flow. A weaker hot-water stream, especially at a tub or shower, points to buildup restricting the passage.
• An error code. Many tankless units display a fault code tied to scale, overheating, or flow problems. That is the unit asking for service. If yours shows a code, our guides to Rinnai error code 10, Rinnai error code 11, Rinnai error code 12 and Navien error code E003 explain what each means and which ones are not scale at all.

Other clues line up with hard water generally, like white crust around fixtures and spotty glassware. Those mean your water is laying down scale everywhere, the heater included.

On DIY versus a pro, a homeowner who is comfortable with tools can descale a tankless unit with the equipment and cleaner approved in its manual. If your unit is throwing error codes, if you cannot find the service valves, or if flow stays weak after a flush, a plumber should investigate. A pro can also confirm whether a whole-house softener makes sense for reducing scale in your home, while keeping heater maintenance aligned with the model's requirements. HQ Plumbing & Air services tankless units across metro Phoenix and can set you up on a maintenance schedule that matches your water.

Some heaters warn you when a flush is due. Rinnai's LC code is one, explained on our page on the Rinnai LC code, and Navien's E016 overheating code also lists a flush among its fixes, covered on our page on Navien error codes E012 and E016.

Noritz's own service reminder code 05 works the same way, and the codes it shows for combustion problems are on our page on Noritz error codes 11, 90 and 99.

For how tankless units compare with tank heaters under hard water, see whether hard water will ruin a hot water heater.

Rheem's tankless heaters show a T111 reminder when a flush is due, explained on our page on Rheem tankless error codes.

If you are still choosing a unit, read should I get a tankless water heater if I have hard water. Navien's flush alarm is code E760, and its overheating codes E016 and E030 point at the same buildup. See our list of Navien tankless error codes.

Rinnai's high exhaust temperature shutdown is another sign a heat exchanger needs attention; see our page on Rinnai error code 54.

Descaling is one part of tankless water-heater maintenance. The model may also require water-screen, air-filter, vent, condensate and combustion checks.