How Long Should a Tankless Water Heater Last?

The range, and what moves you within it

Tankless units are generally expected to outlast tank heaters by a wide margin, which is much of the reason people buy them. That expectation assumes maintenance. An unmaintained tankless unit on hard water can fail well inside the life of the tank heater it replaced.

So the useful question is not how long they last. It is what decides where in the range yours lands.

Water hardness

This is the dominant factor here. Mineral scale builds on the heat exchanger every time the unit fires. A scaled exchanger runs hotter for the same output, and heat stress is what eventually cracks components. Two identical units, one on softened water and one on untreated valley water, will not reach the same age.

Whether it was sized correctly

A unit that is undersized for the house runs at maximum output constantly. One sized correctly modulates and spends most of its life working gently. Undersizing is usually decided at installation and cannot be corrected later without replacing the unit.

Gas supply and venting

A tankless burner needs both a gas line that can deliver its full rated demand and a vent that can clear the products of combustion. Undersized gas pipe is a common shortcut on a retrofit, and it makes the unit work at the edge of its tolerance for its whole life.

Installation quality

Isolation valves fitted at installation make descaling a routine job. Without them, servicing means cutting into the pipework, so in practice it does not get done. Whether a unit was installed with service in mind often decides whether it is ever serviced.

What to do about it

Have it descaled on a schedule that reflects your water rather than the manual's generic interval, keep the inlet screen clean, and consider softening the supply. Those three things move a unit toward the top of its range instead of the bottom.