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How often should you replace a tankless water heater?

Updated July 2026

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A tankless (on-demand) water heater lasts about 20 years, nearly double the 8–12 years a tank gives you, but that number holds only if you descale the heat exchanger every year in hard water. Skip that one habit and lime scale insulates and corrodes the exchanger, trips error-code shutdowns, and can cut the useful life in half. Replacement runs $2,500–4,500 installed, so the annual flush is the difference between one unit and two.

What changes the interval

How often should you replace a tankless water heater? By situation, Updated July 2026
Your situation Interval Why
Descaled yearly, soft or softened water Push past 20 years A clean heat exchanger is exactly what earns the long life
Hard water, never descaled As little as 10–12 years Scale insulates the exchanger and corrodes it years early
City water on a working softener Around 20 years The softener keeps scale off the exchanger for you
Well water with iron or grit 12–15 years Sediment and iron foul the exchanger faster than lime alone
Error codes (like LC or 1L) already showing Descale now, then reassess Scale is throttling flow and firing rate right now
Undersized unit run at full fire Nearer 15 years Constant maximum firing ages the exchanger and blower

The cost of skipping it

What skipping it costs: scale on the heat exchanger cuts efficiency, triggers error shutdowns mid-shower, and shortens a 20-year life toward 10. A heat-exchanger repair runs $500–1,000, and a full replacement is $2,500–4,500 installed, versus roughly $150–250 for a plumber to descale or about $40 in food-grade vinegar and a pump kit if you do it yourself. Many manufacturers also make the annual descale a warranty condition, so skipping it can void your coverage on top of the wear.

How to do it (60–90 minutes · DIY with a pump kit, or $150–250 pro)

  1. Cut power (breaker) or set a gas unit to off, and close the hot and cold isolation valves on the service kit.
  2. Connect a submersible pump in a bucket to the cold service port, and a return hose from the hot port back to the bucket.
  3. Fill the bucket with 2–3 gallons of food-grade white vinegar or a descaler solution.
  4. Open the service valves and run the pump to circulate the solution through the exchanger for 45–60 minutes.
  5. Drain the vinegar, then flush clean water through for a few minutes to rinse.
  6. Close the service ports, reopen the hot and cold valves, clear any air at a faucet, and restore power or gas.
  7. While there, rinse the inlet water-screen filter, and check that your water pressure is in range with an annual pressure check.

Knowing the interval is the easy half. OnOtto reminds you at the right interval, and it won't let you snooze it into next year. Tasks persist until they're done, and completing tankless replacements schedules the next one automatically.

Put this on autopilot, free for 30 days

Frequently asked questions

Is a 20-year lifespan really double a tank heater?

Roughly, yes. A standard tank averages 8–12 years because the steel tank itself corrodes from the inside. A tankless unit has no tank to rust, just a heat exchanger and serviceable parts, so a well-descaled unit commonly reaches 20 years and some push further. The catch is that neglect erases the advantage fast: an unserviced tankless in hard water can die at 10–12, no better than the tank it replaced.

How do I know if my tankless unit needs descaling now?

Watch for a slow drop in hot-water flow, water that runs hot then cold (the exchanger struggling), longer waits for temperature to stabilize, or an error code on the display. In a hard-water area, assume it needs it yearly even without symptoms. If your water is above about 7 grains of hardness and you have never flushed the unit, it is overdue.

Is a tankless water heater worth the higher price?

It depends on use. Tankless units cost more up front (often $1,000–2,000 more installed) but last longer, never run out of hot water, and cut standby energy loss. The math works best for households with steady, moderate demand and a commitment to the annual descale. If you will skip maintenance, a well-flushed tank can be the more economical choice.

What warranty do the heat exchangers carry?

Most residential tankless heat exchangers carry a 12–15 year warranty, with shorter coverage on parts and labor. Read the fine print: manufacturers commonly require documented annual descaling and a minimum water-quality standard to honor a claim. Keeping receipts or logging each flush in a maintenance app protects that coverage, which is worth thousands if the exchanger fails.

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