How to Tell If a Tankless Water Heater Is Bad

Graphic: summary of this guide, made by our team from the cited sources.

Service problem or failing unit?

Sign	What it usually means	Who handles it
Water dripping inside the cabinet or a leak code	Failed fitting, seal or heat exchanger	Shut off water, call a licensed pro
Ignition or flame code that keeps returning	Gas supply, igniter, flame rod or board fault	Gas-qualified technician
Overheat or scale code (LC, 14, E016, E030)	Scale in the heat exchanger	Flush now; repeat codes after flushing are a warning
Yellow or lifting burner flame	Combustion problem	Technician; check CO alarms
Rust or corrosion on the cabinet	Corrosive air, often near the coast	Inspection, then repair or replace
Lukewarm water with several taps on	Demand above capacity, not a failure	Stagger loads or resize
SS or E760 reminder	Routine flush is due	Book service

Start with the code on the display

Modern tankless units diagnose themselves, so the display is the best first clue. Write the code down before resetting anything, so you can tell the technician exactly what the unit reported.

Rinnai code	Navien code	What the manual says it means
11	E003	No ignition; check the gas supply first
12	E012	No flame or flame loss
14, 16	E016, E030	Thermal fuse, high outgoing temperature, heat exchanger or exhaust overheating
LC	E760	Scale build-up (Rinnai) or flushing and service alarm (Navien)
10	E110	Air supply or exhaust blockage, abnormal air pressure
32, 33	E407	Water temperature sensor (thermistor) fault
70	E515	Control board fault
79	none in this table	Water leak detected

Sources for the table: the Rinnai RE manual and the error code chart in Navien's NPE-2 user manual (Navien NPE-2 manual). Codes differ by model and generation, so confirm against the manual for your exact unit.

A single code after a power cut or a gas outage is not a verdict. The same code coming back week after week, after the owner-level checks in the manual have been done, is.

Sign 1: Water inside the cabinet

A tankless heater should be dry inside. Drips from the bottom of the cabinet, mineral crust around fittings or a puddle under the unit mean something under pressure is leaking. Rinnai's code 79, water leak detected, comes with a single instruction: turn off the water supply and contact a licensed professional. A loose fitting or a seal is a modest repair. A leak from the heat exchanger itself is the kind that pushes an older unit toward replacement.

Make sure the water is really coming from the heater. Water from a relief valve discharge pipe is a different problem, covered in our guide to a water heater relief valve that keeps dripping.

Sign 2: Ignition and flame failures that keep returning

When the burner will not light or loses its flame, the manuals start with the cheap causes. Navien's first action for E003, ignition failure, is to make sure the main gas supply valve is open. Rinnai's list for code 11, no ignition, begins with checking that gas is on at the heater, the meter or the propane cylinder and that a propane tank actually has gas in it. Further down, the same lists move into the parts that wear out: the igniter, the flame rod and its carbon build-up, the gas solenoid valves and the control board.

So the pattern to watch for is this: gas confirmed on, unit reset, and the code still returns. That is a component fault. Usually it is fixable, but it is a job for a technician with the model's diagnostic data, not for trial and error.

Sign 3: Overheat and scale codes

Scale is the main long-term threat to a tankless heat exchanger in hard water. The Building America Solution Center says scale building up in heat exchangers, especially in hard water areas, reduces capacity and efficiency and in extreme situations can cause complete water heater failure (Building America Solution Center). The codes give advance warning. On Rinnai units, LC means scale in the heat exchanger that must be flushed to prevent damage, and the code 14 checklist includes hot spots on the heat exchanger that may be caused by scale. Navien's actions for E016, heat exchanger overheating, and E030, exhaust overheating (the unit shuts down when the flue passes 149°F), both include flushing the heat exchanger.

A flush that clears the code buys time. A unit that throws overheat codes again soon after a proper flush, or keeps running lukewarm afterward, may have scale damage that flushing cannot reverse. Navien itself calls flushing "a somewhat complicated procedure" and warns that improper maintenance can void the warranty, so book a professional if you have not done one before.

Sign 4: A yellow or unstable flame

On a gas unit, the flame is a health check. Rinnai's maintenance section says the burner must burn with a clear, blue, stable flame that covers the burner surface, and it shows a yellow flame as unsatisfactory. Viewing the flame means taking off the front panel, and the manual assigns repairs and maintenance to a trained and qualified professional, so this is something to ask about at the annual service rather than open up yourself. A technician who reports a yellow flame has found a combustion problem that needs fixing before the unit runs normally again.

Sign 5: Corrosion on the cabinet

Outdoor and coastal units take a beating. Rinnai notes that installations in or near coastal areas may need extra maintenance because of corrosive airborne salt, and that if corrosion is observed on the body of the water heater, it should be inspected to ensure proper operation and, if necessary, repaired or replaced. Pay closest attention to rust around the vent, the gas connection and the water fittings.

Sign 6: Blocked air or exhaust

Venting codes such as Rinnai's 10 (air supply or exhaust blockage) and Navien's E110 (abnormal air pressure) often have simple causes. Navien's actions are to make sure the exhaust pipe is free of obstructions and to clean the intake air filter, and Rinnai tells owners to keep the area around the flue terminal clear of snow and ice. If a clear vent and a clean filter do not stop the code, the fan or the vent installation needs a technician.

Carbon monoxide: the sign you cannot see

A gas tankless heater with a combustion or venting fault can produce carbon monoxide, which the CDC describes as an odorless, colorless gas, with no warning sign on the unit itself. The U.S. Consumer Product Safety Commission says about 170 people in the United States die every year from CO produced by non-automotive consumer products, including malfunctioning water heaters, and that early CO poisoning feels like the flu without the fever: headache, fatigue, shortness of breath, nausea and dizziness (CPSC). If you have those symptoms, CPSC says to get outside to fresh air immediately and call the fire department from a neighbor's home.

For prevention, CPSC recommends CO alarms that meet the UL 2034 standard, and the CDC advises having the water heater and other gas appliances serviced by a qualified technician every year and replacing CO detectors per the manufacturer's instructions or every 5 years (CDC).

Things that look like failure but are not

• Lukewarm water when two or three fixtures run. That is usually capacity or winter inlet temperature. Our guide on why a tankless water heater runs out of hot water walks through the checks.
• A few seconds of cold mid-shower. The cold water sandwich effect after the tap is turned off and on again is a known trait of tankless units.
• No hot water from a trickle. Each unit has a minimum flow that fires the burner; Navien's checklist asks whether the tap draws at least 0.5 gallons per minute.
• A service reminder. Rinnai's SS and Navien's E760 mean a flush is due, not that the unit is failing.

How old is too old?

ENERGY STAR's demand water heater bulletin puts the life expectancy of these units at 20 years, much longer than conventional storage heaters (ENERGY STAR), and the Solution Center gives up to 20 years for tankless against 10 to 15 for tank-type heaters. That is a ceiling for a well-kept unit, not a promise; hard water and missed flushes pull it down.

Graphic: made by our team from the Building America Solution Center, ENERGY STAR and the Navien NPE-2 user manual warranty table.

Your installation invoice is the best record of age, and it matters for repairs: Navien's warranty periods run from the date of original installation, and Navien can ask for proof of that date. If you are not sure what kind of heater you have, our guide on how to tell if you have a tankless water heater shows what to look for.

Repair or replace: check the warranty first

The warranty often decides the math. Navien's residential NPE warranty runs 1 year on labor, 5 years on parts and 15 years on the heat exchanger for standard or controlled recirculation installs, dropping to 3 years on parts and 5 on the heat exchanger when an external pump circulates continuously. The exclusions matter as much as the years. Navien's list includes damage from hard water deposits and lime or mineral build-up, gas flow problems, failure to perform regular maintenance, installation by a non-licensed installer and products bought through the internet. Rinnai's manual says heat exchanger replacement due to water quality damage, lime build-up damage and freeze damage are not covered.

A practical way to decide:

1. Heat exchanger sound, single failed part. Repair. A fault traced to a sensor, flame rod, igniter or control board is a single-part repair.
2. Heat exchanger still under warranty. Call the original installer first; Navien routes warranty claims through the installer and asks for proof of purchase.
3. Heat exchanger leaking or scaled out, out of warranty, unit approaching 20 years. Replacement is usually the better use of money, and it is the moment to fix whatever caused the scale, such as water treatment and a flushing schedule.
4. Any combustion or CO concern. Shut the unit down until a qualified technician has checked it, whatever its age.