Tankless Water Heater Maintenance: What to Do and How Often

Tankless water heaters promise on-demand hot water and a 20-to-25-year service life. They deliver on both — but only if you maintain them. The biggest mistake new homeowners make with tankless units is assuming "no tank = no maintenance." The heat exchanger, inlet filters, and condensate system all need regular attention, and in hard water areas, scale buildup is the number-one cause of premature failure.

Here's the complete maintenance schedule for gas and electric tankless water heaters.

Annual Task: Descale the Heat Exchanger (Hard Water Areas)

The heat exchanger is the core of a tankless unit — a series of narrow coils or channels where cold water passes through while being rapidly heated by the burner or heating elements. In areas with hard water (calcium and magnesium concentrations above 180 ppm, or roughly 10 grains per gallon), minerals deposit on the interior walls of these channels and restrict water flow.

Scale buildup causes:

• Reduced hot water output and pressure
• Increased energy consumption (more heat needed to push through scale)
• Overheating and sensor trips
• Premature heat exchanger failure — a $400 to $800 replacement part

How to descale (flushing procedure):

You'll need: a submersible pump, two hoses with the correct fittings for your unit's service ports, a 5-gallon bucket, and white vinegar or a commercial descaling solution.

1. Turn off the power (electric) or set gas to "OFF"
2. Close the cold water inlet and hot water outlet service valves (most modern tankless units have dedicated service valves with flush ports built in)
3. Connect the pump hose to the cold water service port inlet and the return hose to the hot water service port outlet — both hoses running into the bucket
4. Fill the bucket with 2 to 3 gallons of undiluted white vinegar or manufacturer-recommended descaler
5. Run the pump continuously for 45 to 60 minutes — the vinegar circulates through the heat exchanger, dissolving mineral scale
6. Flush with clean water: disconnect the pump, connect a fresh water hose to the inlet port, and run clean water through for 10 minutes to rinse
7. Restore normal operation

How often: Annually in hard water areas (above 10 grains per gallon, or check your municipal water report). Every two years in moderate water (7 to 10 grains per gallon). A water softener or inline scale inhibitor reduces the frequency, but doesn't eliminate the need entirely.

If you don't know your water hardness, purchase a simple home test strip kit (under $10) or request your municipality's annual water quality report.

Every 6 to 12 Months: Clean the Inlet Water Filter Screen

Most tankless units have a small stainless mesh filter screen at the cold water inlet that catches sediment and debris before it enters the heat exchanger. This screen is simple to clean but easy to forget.

1. Shut off the cold water supply valve
2. Relieve pressure by opening a hot water faucet
3. Unscrew the inlet connection fitting — the screen is inside, usually press-fit into the valve body
4. Rinse the screen under running water and use a soft brush to clear any debris
5. Reinstall and restore water supply

A partially clogged inlet screen reduces flow rate and can trigger low-flow fault codes, making the unit appear to malfunction when it just needs cleaning.

Annually: Inspect the Vent and Exhaust System (Gas Units)

Gas tankless water heaters vent combustion gases through a flue pipe — either a single-pipe or concentric (pipe-within-a-pipe) system. Inspect the full vent run each year:

• Check vent connections for corrosion, separation, or gaps. Combustion gases escaping into the house are a carbon monoxide hazard.
• Clear the exterior vent terminal of debris, bird nests, and insect screens — a blocked exhaust triggers safety shutoff codes and prevents heating.
• Inspect for condensate blockage on condensing units: condensing tankless heaters produce acidic water vapor that collects in a drain trap. If the trap is blocked or the drain line is kinked, the unit will shut down or leak.

Annually: Professional Service for Gas Units

Gas tankless water heaters should receive a professional tune-up every year. A qualified technician will:

• Test burner combustion efficiency and adjust the gas-to-air ratio
• Inspect and clean the burner assembly
• Test the igniter and flame sensor
• Check for gas leaks at all connections
• Verify proper venting draft
• Review error code history and diagnose any recurring fault patterns

A professional service call typically costs $100 to $200. Skipping annual service on a gas appliance is a safety risk, not just a maintenance risk.

Common Fault Codes and What They Mean

Modern tankless units display error codes on an LED panel when something is wrong. Common codes include:

Consult your unit's manual for brand-specific codes. Rinnai, Navien, Rheem, and Noritz all use different code systems.

How Long Should a Tankless Heater Last?

With annual descaling and regular professional service, a gas condensing tankless water heater typically reaches 20 to 25 years. Without descaling, heat exchanger failure commonly occurs at 8 to 12 years — the same lifespan as a conventional tank heater you paid three to four times more to replace.

Electric tankless units in low-to-moderate hardness water areas are lower maintenance: flush the inlet filter, verify the heating elements annually for scale buildup, and they'll last 15 to 20 years without major intervention.

The First-Year Homeowner Maintenance Calendar includes the tankless descaling schedule alongside every other seasonal maintenance task in your home. It's built so you can handle water heater care, HVAC, gutters, and appliances from a single organized calendar rather than tracking each system separately.