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Tankless water heater maintenance: flushing and descaling

By the VettedHeater team · Last updated October 23, 2026 · How we verify

Flush and descale a tankless water heater on the schedule in your owner's manual. Rinnai says to flush at least once a year (Rinnai, "How do I flush clean my Rinnai tankless water heater?"), and Navien recommends descaling at least once a year, more often with harder water, higher temperature settings and heavier use (Navien, "How to Descale Your Tankless Water Heater"). San Diego's water is hard: the City's 2025 water quality report shows average total hardness of about 14 grains per gallon at its treatment plants, and says most of its imported water is a blend from the Colorado River and State Water Project (City of San Diego Public Utilities, 2025 Annual Drinking Water Quality Report).

How often to flush a tankless water heater

BrandWhat the manufacturer says
RinnaiFlush mineral deposits at least once a year (Rinnai, "How do I flush clean my Rinnai tankless water heater?"). In hard water areas the water must be treated and may need a more frequent flushing schedule (Rinnai, SENSEI RX Residential Installation and Operation Manual (100000839), Water Quality section).
NavienDescale at least once a year; more often with more minerals, higher set temperatures and more hot water use (Navien, "How to Descale Your Tankless Water Heater").
Rheem, Noritz and othersUse the interval in your model's manual (Rheem support, Noritz).

San Diego water hardness and why it matters

Hard water leaves scale in the heat exchanger. Rinnai says scale build-up is caused by hard water and can be sped up by a high temperature setting (Rinnai, SENSEI RX Residential Installation and Operation Manual (100000839), Water Quality section).

Hardness by water supplier

City of San Diego source (2025)Average total hardnessRange
Alvarado plant237 ppm (13.8 gr/gal)203–270 ppm
Miramar plant242 ppm (14.1 gr/gal)200–285 ppm
Otay plant243 ppm (14.2 gr/gal)201–271 ppm
Purchased treated water145 ppm (8.5 gr/gal)54.4–232 ppm

Source: (City of San Diego Public Utilities, 2025 Annual Drinking Water Quality Report). The report's map shows which plant serves your address. For comparison, the water quality table in Rinnai's SENSEI RX manual lists total hardness up to 200 mg/L (Rinnai, SENSEI RX Residential Installation and Operation Manual (100000839), Water Quality section). Outside the City of San Diego, check your own water district's annual report; the City lists its reports here (City of San Diego, Water Quality Reports).

Do water softeners or scale filters help?

Rinnai says that where water is hard the water must be treated, and it offers a scale-prevention conditioning system for its heaters (Rinnai, SENSEI RX Residential Installation and Operation Manual (100000839), Water Quality section). Treatment doesn't replace the flush schedule in your manual.

How to flush and descale

This is the outline from Rinnai's published procedure (Rinnai, "How do I flush clean my Rinnai tankless water heater?"). Follow your own model's manual if it differs.

  1. Turn off power to the unit, and shut off the gas (or the breaker on electric units).
  2. Close the cold, hot and main water valves at the heater.
  3. Slowly remove the purge port caps on the cold and hot service valves; there may be pressure.
  4. Attach hoses to the service valves; some setups use a small pump in a bucket to circulate the solution.
  5. Open the purge port valves.
  6. Circulate 4 gallons of undiluted white vinegar. Rinnai says never to use chemical solutions of any kind.
  7. Run the flush per your manual; Rinnai says it may take up to 45 minutes.
  8. Close the purge ports, remove the hoses, replace the caps, reopen the water valves and restore gas and power.

Navien's guidance differs slightly: it allows an approved tankless descaling solution or household white vinegar, circulated for at least 45 minutes, then a fresh-water flush of at least 10 minutes (Navien, "How to Descale Your Tankless Water Heater"). Navien recommends hiring a professional and says incorrect descaling that damages the heater could void the warranty (Navien, "How to Descale Your Tankless Water Heater").

Rinnai and Navien notes

Some Rinnai models can show a Service Soon (SS) code when it's time to flush (Rinnai, SENSEI RX Residential Installation and Operation Manual (100000839), Water Quality section). Navien publishes written descaling instructions on its downloads and support pages (Navien, Support).

Other maintenance

DIY vs pro maintenance

Rinnai says flushing should be done by a professional installer (Rinnai, Residential FAQ (maintenance)), and Navien also recommends a professional (Navien, "How to Descale Your Tankless Water Heater"). A pro visit can include the descale, inlet filter cleaning and a check of venting and error history. We don't quote a price because we haven't found a reliable published San Diego figure. Pros who list tankless repair are on our tankless repair page.

Signs you need repair, not a flush

Error codes that come back after a flush, a unit that won't ignite, leaks, or a gas smell aren't maintenance problems. See tankless water heater repair in San Diego. If you're replacing a worn-out unit instead, see tankless installation in San Diego and what to look for in a new unit. Maintenance is one of the differences covered in tankless vs tank.

Licensed pros who list tankless maintenance

More pros who list tankless repair and service are on our repair page.

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Related tankless guides

FAQs

How often should a tankless water heater be flushed?

Follow your manual. Rinnai says at least once a year, and Navien recommends descaling at least once a year, more often with hard water.

Can I use vinegar to descale a tankless water heater?

Rinnai specifies 4 gallons of undiluted white vinegar. Navien allows white vinegar or an approved tankless descaling solution. Check your model manual.

What happens if you never flush a tankless water heater?

Scale builds up in the heat exchanger. Rinnai says mineral build-up can erode the heating chamber walls, and it can reduce flow and efficiency.

Is San Diego water hard?

Yes. The City of San Diego 2025 water quality report shows average total hardness of about 14 grains per gallon (237 to 243 ppm) at its three treatment plants.

Does a water softener mean I can skip flushing?

No. Rinnai says hard water must be treated and may still need more frequent flushing. Keep the schedule in your manual.

Does skipping maintenance void my warranty?

Check your manufacturer terms. Rinnai says component replacement due to water quality damage is not covered, and Navien says incorrect descaling that causes damage could void it.

Sources