Should you repair or replace your tankless water heater?
Updated August 2026
← Every repair-or-replace call
Descale the unit before condemning it, then repair anything under 12 years old for $150–600 and replace only past 15 or when the heat exchanger itself leaks. Tankless heaters outlive tanks by a wide margin, 15–20 years against 8–12, so a mid-life unit throwing an error code is usually reporting scale, a dirty flame rod, or a tired flow sensor. A new tankless installed runs $2,500–4,500 once venting and gas line work are counted, which makes nearly any component repair the better buy up to the 12-year mark. A leaking exchanger is the exception, since fitting one approaches the price of the appliance.
The short version
Scale is the story of tankless ownership. Water passes through a narrow copper or stainless exchanger at high heat, and in hard water the mineral film builds fast enough to trigger flow and temperature faults that read like a dying appliance. That is why the first move on almost any error code is a descaling flush, about $40 in supplies yourself or $150–250 from a plumber, before anyone talks about replacement. The age brackets after that are simple. Under 12 years, replace the failed flow sensor, igniter, flame rod, or fan and expect years more from a unit designed for 15 to 20. Past 15, apply the 50% rule against $2,500–4,500 installed. A leaking heat exchanger ends the calculation on its own.
Your situation, decided
| Situation | Verdict | Why |
|---|---|---|
| Error code, hard water, never descaled | Repair (descale) | A $40 flush kit or a $150–250 service clears most codes |
| Flow sensor, igniter, or flame rod failed under 12 years | Repair | A $150–600 part on a heater with years of life left |
| Control board failed at 13–15 years | Judgment call | Weigh a $400–800 board against a $2,500–4,500 replacement |
| Heat exchanger leaking | Replace | Part plus labor approaches the price of a new unit |
| Past 15 years with repeat faults | Replace | The 50% rule at the end of a 15–20 year service life |
What each path costs
Repairs typically run $150–600; a replacement is $2,500–4,500 installed. Tankless heaters last 15–20 years, roughly double a tank, but only with descaling. Repair under 12 years; past 15, or with a leaking exchanger, replace. Price your own number on the home maintenance cost calculator before you commit either way.
The repair-or-replace call is easy when you know the unit's age and service history. OnOtto keeps both, plus the maintenance that delays this decision for years.
Start your home's service log, freeFrequently asked questions
How often does a tankless water heater need descaling?
Once a year is the default, every six months above roughly 10 grains of hardness, and every two years on softened water. The service circulates vinegar or a descaling solution through the exchanger for 45 to 60 minutes using a small pump and two hoses. A DIY kit costs $40 to $150 and a plumber charges $150–250. Skipping it is not only an efficiency question: many manufacturers make annual descaling a warranty condition, and scale sits behind most error codes.
Why does my tankless heater go cold then hot?
Two different faults feel the same at the tap. The cold water sandwich is normal on any tankless, a slug of cooled pipe water between draws, and a small buffer tank is the only real cure. Separately, a low-flow fixture that drops under the burner activation threshold, often 0.4 to 0.75 gallons per minute, makes the unit cut out mid-shower. Scale on the exchanger and a blocked inlet screen or air filter produce the same swings.
Is switching from a tank to tankless worth it?
At replacement time, often yes. The U.S. Department of Energy puts the efficiency gain at 24 to 34 percent for homes using 41 gallons of hot water a day or less, and a tankless unit lasts 15–20 years against 8–12 for a tank. Installation is the catch: $2,500–4,500 versus $1,200–2,500, because upsized gas lines and new venting are common. If your current tank is healthy, wait. If it just failed, price both, since the lifespan math favors tankless in a long-term home.