How long does a tankless water heater last?
Updated August 2026
A tankless (on-demand) water heater lasts 15 to 20 years, nearly double the 8 to 12 you get from a conventional tank. The reason is simple: there is no reservoir of hot water sitting there corroding a steel tank from the inside, and most of the parts that do wear, from the igniter to the flow sensor, can be replaced instead of scrapping the whole unit. What decides whether yours reaches 20 or stalls at 12 is scale. In hard water, minerals bake onto the heat exchanger, the single most expensive part, so an annual descaling flush is the maintenance that protects your investment.
Lifespan by type and situation
| Type or situation | Typical lifespan | Why |
|---|---|---|
| Gas tankless | 15–20 years | The common type; most parts are replaceable, unlike a sealed tank |
| Electric tankless | 15–20 years | Fewer combustion parts; the heating elements are serviceable |
| Descaled yearly (soft or treated water) | 20+ years | A scale-free heat exchanger is the key to the top of the range |
| Hard water, never descaled | Cut to 10–12 years | Scale clogs the heat exchanger, the costliest part to replace |
| Air and water inlet filters neglected | Shortened | Restricted flow makes the unit overheat and error out |
| Tank water heater (for comparison) | 8–12 years | Half the life; a tankless is the longer bet if you maintain it |
Signs yours is near the end
- Hot water output drops off, or the temperature swings hot and cold mid-shower even at a steady flow
- The display throws error or fault codes and locks out, especially scale-related codes, more and more often
- You wait noticeably longer for hot water to arrive than the unit used to need
- A rumbling, whistling, or crackling from inside as water forces past a bed of scale on the heat exchanger
- Rusty or discolored hot water, or a leak anywhere on the unit or its fittings
- Repairs start stacking up once the unit is past 15 years, with parts getting harder to source
How to add years
Descaling is the difference between 12 years and 20. In hard water, minerals cake the heat exchanger, so flush the unit with vinegar or a descaling solution once a year, twice if your water is very hard. Cleaning the inlet air and water filters keeps flow up so the unit does not overheat and lock out. A whole-house water softener slows scale on this and every other fixture at once. Because most tankless parts can be swapped, a mid-life repair is usually worth it, but once errors pile up past 15 years, compare the running costs to a new unit. Skip the yearly flush and you meet the cost of scale early. A maintenance checklist keeps the flush on schedule.
What it costs to replace: plan on $2,500–4,500 installed. Replacing on your own schedule, before it fails, is almost always cheaper than the emergency version. The home maintenance cost calculator helps you set the money aside before it comes due.
OnOtto tracks your tankless descaling age and service history, and warns you before end of life instead of after the flood.
Track your home's systems, free for 30 daysFrequently asked questions
Do tankless water heaters really last twice as long as a tank?
They can, and often do. A conventional tank averages 8 to 12 years because the stored water slowly corrodes the steel tank until it leaks. A tankless unit heats water only as it flows through, so there is no tank to rust out, and it commonly reaches 15 to 20 years. The important qualifier is maintenance: that longer life assumes you descale it yearly in hard water. A tankless heater that is never flushed can scale up and fail at 10 to 12, no better than a tank, so the extra lifespan is earned, not automatic.
How important is annual descaling on a tankless heater?
Critical in hard-water areas, and the single most valuable thing you can do. As water heats, dissolved minerals precipitate out and cling to the walls of the heat exchanger, the pricey copper or stainless part at the heart of the unit. A yearly flush with vinegar or a descaler dissolves that scale before it insulates the exchanger, forces the burner to work harder, and eventually clogs the passages. Miss it year after year and you can turn a 20-year unit into a 12-year one, then face a heat-exchanger repair that costs a large share of a whole new heater.
Are tankless water heaters worth the higher upfront cost?
For many households, yes, on two counts. First, the unit lasts nearly twice as long, so a $2,500 to $4,500 tankless amortized over 20 years can beat replacing a cheaper tank twice. Second, because it heats water only on demand rather than keeping a full tank hot around the clock, it trims standby energy losses. The payback is slower where energy is cheap or hot-water use is light, and the install can cost more if gas lines or venting must be upsized. If you plan to stay in the home long term, the math usually favors tankless.
What is the most expensive part to fail on a tankless unit?
The heat exchanger, which is why descaling matters so much. It is the metal core where the burner or elements transfer heat into the flowing water, and replacing it can cost a large fraction of a whole new unit, sometimes enough that you simply buy a new heater instead. Scale buildup is its chief enemy: it insulates the metal, drives overheating, and can crack or clog it over time. Igniters, flow sensors, and fans are far cheaper and routinely replaced. Protect the heat exchanger with a yearly flush and the rest of the unit is mostly serviceable.
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