A gas tankless water heater usually lasts 15 to 20 years, and sometimes longer, because there is no tank to rust through. Scale buildup in the heat exchanger is what shortens its life.
A tankless unit heats water as it passes through a copper or stainless heat exchanger. There is no stored water sitting against steel, so the failure that kills tank heaters, a rusted-through tank, does not apply. What wears out instead is the heat exchanger, which collects mineral scale on the inside, and the electronics, fan and gas valve that control the burner.
The U.S. Department of Energy puts the expected life above 20 years. Utility guides tend to say 15 to 20. The difference is mostly maintenance: a unit that is descaled every year in hard-water areas can run two decades; one that is never flushed in a hard-water town can be throwing error codes in eight.
Parts are replaceable in a way that tank parts are not. A failed igniter, flow sensor or control board on a 12-year-old tankless heater is normally worth fixing. A leaking heat exchanger usually is not.
Find the serial number and decode it: Rheem · Ruud · Richmond · Rinnai · Navien · Noritz · Takagi · Bosch (tankless water heater) · Paloma · HTP / Westinghouse / American Standard (water heaters) · EcoSmart / Eemax · Stiebel Eltron
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