How long does a tankless water heater last?
Nothing in a tankless heater can rust through the way a tank does, so the end usually comes from the heat exchanger or from a pile-up of part failures that stop making sense to fix. In its December 2024 rule for gas-fired instantaneous water heaters, the Department of Energy assumes an average life of 20 years and a median of about 18.3 years, while noting that field data on tankless lifetimes is limited. Other charts disagree widely, from more than 20 years in a 2007 builders' study down to 10 years for instant or on-demand heaters on one inspector chart. Care explains a lot of that spread. A unit that scales up and never gets serviced can fail in the low teens. One with clean water and regular descaling can reach DOE's average.
What wears out
The heat exchanger and scale
Water passes through narrow passages in the heat exchanger and is heated in seconds. Hard water leaves mineral scale in those passages, which cuts flow, makes the burner work harder and can overheat the exchanger. A.O. Smith notes that tankless units don't need flushing like tanks but often need descaling. A heat exchanger that cracks or leaks is the most expensive tankless repair and often the one that ends its life.
Igniter, flame sensor and gas valve
Every hot water draw starts an ignition cycle, so ignition parts see heavy use. Error codes for ignition failure or flame loss often trace to these parts. Testing and replacing them is a job for a licensed plumber or gas technician.
Fans, sensors and the control board
Tankless heaters depend on a combustion fan, flow sensors, temperature sensors and a circuit board. Any one can fail and shut the unit down. Most are replaceable, and parts availability for your model can decide whether a repair makes sense at all.
Venting and condensate
Condensing tankless models produce acidic condensate that drains away through a line or neutralizer. A blocked drain or damaged vent can shut the heater down or cause corrosion. A technician should inspect venting during service.
Signs it's near the end
- Error codes that keep coming back after a reset.
- Water temperature that swings hot and cold during a shower.
- Lower flow at the tap than when the unit was new.
- Longer waits for hot water, or the unit failing to fire on small draws.
- Water dripping from inside the cabinet or corrosion on the case.
- An age past roughly 15 to 20 years, the range DOE's numbers center on.
What helps it last
- Have the unit descaled on the schedule in your owner's manual, and more often if you have hard water.
- Clean the inlet water filter screen if your manual shows it as an owner task.
- Keep the area around the unit and the outdoor vent termination clear of debris, snow and plants.
- Write down any error codes and look them up in the manual before calling, so the technician knows what to expect.
- Consider a softener or scale-reducing filter in hard water areas.
- Schedule a yearly service visit for combustion and venting checks by a licensed technician.
Repair or replace?
Because tankless heaters have a long expected life, repairs make sense for longer than they do on tanks. A fan, sensor or igniter on a unit under 12 or so years old is usually worth fixing. It gets harder with a heat exchanger failure, a control board on an older model, or several repairs in a row. Try cost per remaining year. Divide the repair cost by the years the unit can realistically keep running, and compare it with the replacement cost shown above divided by a new unit's expected life of around 18 to 20 years. If the repair costs more per year, replace. The 50 percent rule gets you to the same place faster. When one repair passes about half the cost of a new installed unit and the heater is in the second half of its life, replacement usually wins. Check warranty first. Tankless heat exchangers often carry longer warranties than other parts, and some makers, such as Rinnai, extend coverage when the unit is registered.
Common symptoms
- Why does my hot water run out so fast
- Why is my hot water rusty
- Why is my water heater leaking from the bottom
- Why is my water heater not making hot water
- Why won't my water heater pilot stay lit
- Why is my water heater making a popping noise
- Why is my water heater relief valve leaking
- Why does my hot water smell like rotten eggs
Common questions
How long do gas tankless water heaters last on average?
DOE's 2024 analysis assumes an average of 20 years with a median of about 18.3 years, and notes limited field data. A 2007 builders' study said 20-plus years, while one inspector chart lists 10.
Why does a tankless heater need descaling?
Minerals in hard water coat the small passages in the heat exchanger. That reduces flow and efficiency and can overheat the exchanger. Descaling removes the buildup.
Is a cracked tankless heat exchanger worth replacing?
It depends on warranty and age. Under warranty, the part may be covered and only labor is charged. Out of warranty on an older unit, the repair is often a large share of a new heater's cost.
Do tankless water heaters really last longer than tanks?
On DOE's numbers, yes: about 20 years for gas tankless versus about 14.5 for gas tanks. The advantage depends on regular descaling and service.
What are signs a tankless water heater is failing?
Recurring error codes, temperature swings, reduced flow, leaks inside the cabinet and failure to ignite on small draws are the most common signs.