How Long Do Tankless Water Heaters Last?

1. The Short Answer: 20 Years or More, If the Water and the Owner Allow It

The 20-year figure has respectable sources. The Department of Energy's Energy Saver guidance says most tankless water heaters have a life expectancy of more than 20 years and have easily replaceable parts that can extend that, while storage water heaters last 10 to 15 years. The National Association of Home Builders' widely cited component life study also lists tankless heaters at 20 years or more. Neither is a promise, and neither describes a heater that has been neglected.

Why tankless heaters can outlast tanks is straightforward. A tank heater stores 40 or 50 gallons of hot water against a glass-lined steel shell, and once the lining and anode rod are used up, the tank rusts through and leaks. There is no repairing that. A tankless heater stores almost nothing, so there is no tank to corrode, and most of what wears out on it, such as igniters, sensors, fans and circuit boards, can be replaced one part at a time.

Why many do not reach 20 years is just as straightforward. Everything depends on the heat exchanger, the coil of copper or stainless steel that the water flows through. In hard water it scales up, runs hot, and eventually cracks or leaks. A heater on soft or moderate water that is descaled when needed can reasonably be expected to see 20 years. One on very hard water that is never flushed can be in serious trouble within the first several years, and the warranty will not pay for it.

So the honest answer is a range that you control more than the manufacturer does. Expect roughly 15 to 20 years or more from a quality unit that is installed correctly and maintained, and plan as if you will reach the low end of that if your water is hard. If you are still deciding whether the lifespan justifies the upfront cost, our tankless pros and cons guide weighs it against a tank, and our best tankless water heaters guide notes which units are built for the long haul.

2. What Actually Wears Out in a Tankless Heater

The heat exchanger is the part that decides whether a tankless heater is worth repairing. It is the most expensive component, the one that warranties cover longest, and the one that hard water attacks. Non-condensing units have one, usually copper. Condensing units add a stainless steel secondary exchanger in the exhaust path to recover extra heat. A leak or crack in either is usually the end of the heater once it is out of warranty, because the part and labor together approach the price of a new unit.

Everything else is a service part. The igniter and flame rod sit in the burner and can carbon up or wear; dirty or failing ones cause ignition and flame-failure codes. The flow sensor, which tells the heater water is moving, can stick or fail. The combustion fan motor runs every time the burner fires. Thermistors that read water temperature drift or fail. The control board can be damaged by surges or moisture. These parts typically cost $150 to $600 to replace with labor on a gas unit, and a 12-year-old heater with a solid heat exchanger is usually worth that repair.

Electric tankless heaters have fewer parts and no combustion system, so there is less to wear. Their failure points are the heating elements, which can burn out, especially if the unit is powered with air in the lines, plus the flow sensor, the control board and the relays or triacs that switch the elements. Many of these are replaceable, and an electric unit's lifespan is, again, largely a question of scale on the elements and how much electrical abuse it takes.

The practical lesson is that a tankless heater's lifespan is really its heat exchanger's lifespan. Parts will need replacing along the way, and that is normal, not a sign the heater is failing. Budget for one or two service calls over two decades and do everything you can to protect the exchanger.

3. What Shortens a Tankless Heater's Life

Hard water is the biggest factor by a long way. As water heats, calcium and magnesium carbonate drop out and coat the inside of the heat exchanger. The coating insulates the water from the heat, so the exchanger runs hotter for the same output, which accelerates both more scaling and metal fatigue. Rinnai's published water-quality limit puts total hardness at up to about 200 mg/L, roughly 12 grains per gallon, and above that range scale builds fast. Test your water; many households are surprised by the number.

Not flushing is the second factor, and it compounds the first. Descaling removes the deposit before it does damage, and the right interval depends on hardness: some homes on soft water barely need it, while hard-water homes need it every year or more often. Our guide on how to flush a tankless water heater sets out the schedule and the method. If you find yourself descaling more than once a year, a water softener upstream is cheaper than an early heat exchanger, and our best water softener systems guide covers the options.

Venting and combustion problems wear a heater out from the other side. A vent that is partly blocked, too long or poorly sloped makes the burner run out of balance, sooting the heat exchanger and stressing the fan. On a condensing unit, a condensate drain that backs up or freezes can damage the unit, and Rinnai's warranty specifically excludes condensate damage and improper venting. Corrosive air matters too: a heater that draws combustion air near laundry chemicals, chlorine pool supplies or refrigerant can corrode its burner and exchanger, and warranties exclude corrosive atmospheres.

Operating choices matter as well. Rinnai cuts its residential heat exchanger coverage from 15 years to 8 if the setpoint exceeds 160 degrees, because hotter water scales faster. Running a heater in a loop with a pump that circulates continuously, rather than one controlled by the heater, an aquastat or an on-demand trigger, puts far more hours on it: Navien drops its NPE-A heat exchanger warranty to 5 years for uncontrolled recirculation, and Rinnai cuts its Value series coverage to 3 years in a continuous loop. Freezing, in an unheated garage or an outdoor unit during a power cut, can split an exchanger in one night.

Installation quality sits underneath all of this. An undersized gas line that keeps the burner starved, a missing set of isolation valves that makes flushing a chore nobody does, or a heater sized so small that it runs flat out every day will all shorten its life. Many installers now fit isolation valves as standard, and Rinnai extends the heat exchanger warranty on its non-condensing Value series from 10 to 12 years when an isolation valve kit is installed.

4. Heat Exchanger Warranties Compared

A warranty is not a lifespan estimate, but it shows what the manufacturer is willing to back. Residential tankless warranties are split into three parts: the heat exchanger, everything else, and labor, and the numbers vary by brand and series. The figures below come from the manufacturers' published residential warranties for current lines; always check the document for your exact model, because terms change between series and over time.

The fine print matters more than the headline number. Navien excludes damage caused by hard water deposits and lime or mineral build-up, and failure to perform regular maintenance. Rheem's residential tankless warranty excludes heat exchanger failures caused by lime, scale or mineral build-up. Rinnai lists improper maintenance, explicitly including scale build-up, freeze damage and vent blockage, along with poor water quality. In other words, the most common way a tankless heat exchanger dies is the one cause no manufacturer covers.

Other conditions catch owners out. Rinnai's labor coverage depends on registering the product within 90 days, and its coverage ends if the unit is moved to a new location. Navien's warranty requires installation by a licensed installer and states that purchases through the internet or other e-commerce channels void it. Rheem's longest terms apply only in a single-family dwelling. Keep the receipt, register the unit, and keep a record of every flush.

Notice also that Rinnai's 15 years is capped at 12,000 operating hours, meaning hours the burner is actually firing. A household that runs the burner about two hours a day takes roughly 16 years to reach that, so for most homes the year limit arrives first, but a large, busy household could hit the hours cap earlier. Many units let you view total operating hours on the display or through a service menu, which is worth knowing before a claim.

5. Signs a Tankless Water Heater Is Failing

Some symptoms are routine and fixable; the ones that signal the end are those that point at the heat exchanger. The clearest is a scale code or temperature instability that returns soon after a thorough descale. If a flush used to buy a year and now buys a month, the exchanger is either badly scaled in a way flushing no longer clears or damaged. Overheat codes that appear even at modest flow rates point the same way.

Water where it should not be is the other serious sign. Drips from inside the cabinet, corrosion or white mineral crust on the exchanger or its fittings, or water staining below the unit can indicate a leak. On a condensing heater, liquid coming from the condensate line when the burner has not been running is a known indicator of a leaking heat exchanger in Rinnai's own troubleshooting guidance, because water is escaping into the combustion side rather than condensing there.

Combustion symptoms deserve attention too. Soot around the vent or the cabinet, a yellow rather than blue flame through the view window, a smell of exhaust, or repeated air and exhaust codes after the vent has been cleared all point to a burner or exchanger problem that also has safety implications. Turn the heater off at its gas valve and call a technician, and make sure a carbon monoxide alarm is working nearby.

Gradual decline is the subtler pattern: hot water that takes noticeably longer to arrive, a maximum flow that has fallen so that two showers no longer run together comfortably, and a rising number of error codes and service calls. None of these alone condemns a heater, and several have simple causes covered in our guide to a tankless water heater not heating. Together, on a heater past 12 or 15 years, they are the signal to start pricing a replacement before it fails in January.

6. Repair or Replace: How to Decide

Start with the warranty. If the heater is inside its heat exchanger or parts coverage, call the installer or the manufacturer before paying anyone, because unauthorized repairs can void coverage. Have the model number, serial number, installation date and your maintenance records ready, and expect the manufacturer to examine whether scale, venting or installation caused the failure. Labor coverage is usually shorter than parts coverage, so a covered part can still come with a labor bill.

Out of warranty, a service-part failure on a heater with a healthy heat exchanger is usually worth repairing. An igniter, flame rod, flow sensor, thermistor or fan generally runs $150 to $600 installed, a fraction of the several thousand dollars a replacement tankless heater costs installed. That holds even on a heater in its teens, provided the exchanger is not leaking and descaling still restores normal performance.

A heat exchanger failure out of warranty usually means replacement. By the time the part, labor and the likelihood of other worn parts are added up, the repair commonly approaches the price of a new unit, which comes with a fresh warranty and better efficiency. The same logic applies to a heater past 15 years that needs a control board and has other parts showing their age: spending heavily on old hardware is a poor bet.

The good news about replacement is that the second tankless install is usually cheaper than the first. The gas line, electrical supply and water connections are already in place, and if the new heater can use the existing vent, which is most likely with the same brand and type, the job can be close to a straight swap. If the original was non-condensing with stainless venting, this is the moment to weigh switching; our condensing vs non-condensing comparison walks through that decision, and our tankless water heater cost guide covers replacement pricing.

7. How to Make a Tankless Water Heater Last 20 Years

Know your water. Test hardness with an inexpensive test kit or check your utility's water quality report, and set a descaling schedule from the result rather than from a generic rule. Soft-water homes may need to descale only occasionally; hard-water homes need it at least yearly. If the number is high, treat the water before it reaches the heater, either with a whole-house softener or a scale-reduction system rated for tankless heaters.

Make maintenance easy enough that it happens. Isolation valves on the heater turn a descale into a one-hour job with a bucket and a small pump instead of a plumbing project. Clean the inlet water filter screen when you flush. Look at the vent terminations outside each autumn and after heavy snow, and on a condensing unit check the condensate line, trap and neutralizer media. Have a technician inspect the burner, flame rod and gas pressure every few years.

Run it sensibly. Keep the setpoint at around 120 degrees unless you have a specific reason to go higher, both for scald safety and because hotter water scales faster. If you want quicker hot water, use an on-demand recirculation system triggered by a button or motion sensor rather than a pump that circulates around the clock, which both manufacturers penalize in their warranties. Protect any heater in an unheated space from freezing, including during power cuts.

Finally, protect the paperwork. Register the heater with the manufacturer as soon as it is installed, keep the receipt and the installer's details, and log every flush and service visit with the date. If the heat exchanger fails in year nine, those records are the difference between a covered claim and a denied one. Do all of this and 20 years is a realistic expectation rather than a marketing line.