This tankless water heater maintenance guide covers the four annual tasks that keep the unit running. The one maintenance task that determines whether a tankless water heater lasts 12 years or 25 years is descaling — flushing the heat exchanger with a vinegar or chemical descaling solution to dissolve the mineral deposits that accumulate from hard water. A tankless water heater that is never descaled will fail within 5 to 8 years in hard-water areas. The heat exchanger will clog with calcium carbonate scale. The restricted flow will cause the unit to overheat and shut down. The heat exchanger replacement costs $500 to $1,200. The annual descaling — a 45-minute DIY procedure using a $20 pump and $15 in vinegar or descaling solution — prevents the failure. The descaling costs $35 per year in materials. The heat exchanger replacement costs $800. The descaling is the cheapest insurance in plumbing. The insurance costs $35 per year. The failure costs $800. The insurance is worth the premium.
Tankless water heaters are marketed as low-maintenance and long-lasting. The marketing is accurate — if the maintenance is performed. The maintenance is descaling. A tanked water heater requires annual flushing to remove sediment. A tankless water heater requires annual descaling to remove mineral scale. The maintenance is different. The frequency is the same. The consequence of skipping it is the same — premature failure. Here is the complete maintenance schedule for a tankless water heater.
Annual Task: Descale the Heat Exchanger
Descaling is the process of circulating a mild acid, white vinegar or a commercial descaling solution, through the heat exchanger to dissolve calcium carbonate scale. The scale forms when hard water is heated, the minerals precipitate out of the water and bond to the inside of the heat exchanger. The scale acts as an insulator between the burner and the water. The burner runs longer to heat the water through the scale layer. The efficiency drops. The outlet temperature fluctuates. The unit eventually overheats and shuts down with an error code. The descaling removes the scale. The efficiency returns. The error code clears. The descaling is the annual maintenance that makes the tankless water heater last.
The descaling procedure requires a submersible utility pump, $60 to $120 purchased once, two 5-gallon buckets, two washing machine hoses, and 4 gallons of white vinegar or a gallon of commercial descaling solution. Turn off the power to the water heater, electric disconnect or gas shutoff. Close the isolation valves on the hot and cold water lines, these are the valves that were installed with the unit, one red-hot, one blue-cold. Connect the hoses from the pump in the bucket of vinegar to the cold water service port and from the hot water service port back into the bucket. Open the service port valves. Run the pump for 45 to 60 minutes. The vinegar circulates through the heat exchanger, dissolving the scale. After 45 minutes, disconnect the pump, close the service port valves, open the isolation valves, turn the power back on, and run hot water from a faucet to flush any remaining vinegar from the system. The descaling is complete. The heat exchanger is clean. The unit is ready for another year of operation.
If the isolation valves with service ports were not installed when the unit was installed, some contractors skip them to save $50, a plumber must install them before descaling is possible. The service port valves cost $50 to $100 for the valves and $200 to $400 for the installation. The valves are the access point for descaling. Without them, the heat exchanger cannot be flushed without cutting into the plumbing. The valves are not optional. The valves are the maintenance access. Install them if they are not present. The installation costs $300 once. The descaling costs $35 per year thereafter. The heat exchanger replacement costs $800. The installation of the service port valves is the cost of making the unit maintainable. The unmaintained unit will fail. The maintainable unit will last 25 years.
The U.S. Department of Energy recommends annual descaling for tankless water heaters, particularly in areas with hard water. The DOE notes that mineral scale buildup reduces the efficiency of the heat exchanger by 5% to 10% per year if not removed. The efficiency loss compounds. A unit that starts at 95% efficiency drops to 85% after one year of scale accumulation, 75% after two years, and continues to decline until the scale restricts flow enough to trigger a shutdown. The annual descaling resets the efficiency to 95%. The reset is the maintenance. The maintenance is the efficiency. The efficiency is the energy savings that the tankless unit was purchased to deliver.
Annual Task: Clean the Air Intake Filter
A gas tankless water heater has an air intake filter, a fine mesh screen that prevents dust, lint, and debris from entering the combustion chamber. A clogged air filter restricts the combustion air supply. The burner runs lean, too little air for the amount of gas. The incomplete combustion produces carbon monoxide. The unit senses the inadequate air supply and shuts down with an error code. The air filter cleaning takes 5 minutes. Turn off the unit. Locate the air intake filter, typically on the top or the front of the unit. Remove the filter. Rinse it under running water. Shake it dry. Reinstall it. Turn the unit back on. The filter is clean. The combustion air is unrestricted. The unit runs efficiently. The filter cleaning is the simplest maintenance task and the most commonly overlooked. The filter is hidden inside the unit. The homeowner does not know it exists. The filter is in the manual. The manual is the instruction. Read it.
Annual Task: Inspect the Venting System
A gas tankless water heater vents combustion gases to the exterior through a dedicated vent pipe, typically PVC, CPVC, or polypropylene. The vent pipe must be intact, properly sloped, and free of obstructions. A blocked or disconnected vent pipe allows carbon monoxide to enter the living space. The CO is odorless and colorless. The vent inspection is a visual check: follow the vent pipe from the unit to the exterior termination. Look for disconnected joints, cracks, holes, or signs of melting or discoloration. A disconnected joint allows combustion gases to escape into the mechanical room. A cracked or melted pipe indicates improper vent material, PVC exposed to temperatures above its rating. The vent pipe that is compromised must be repaired or replaced by a qualified technician. The vent inspection takes 5 minutes. The vent inspection is the visual check. The visual check is the difference between a vent that safely exhausts combustion gases and a vent that silently fills the house with carbon monoxide. The difference is invisible. The vent inspection makes it visible.
Annual Task: Clean or Replace the Inline Water Filter
Many tankless water heaters have an inline water filter, a small mesh screen on the cold water inlet that traps sediment and debris before it enters the heat exchanger. A clogged water filter restricts the flow of water into the unit. The restricted flow triggers a low-flow error code and the unit shuts down. The water filter cleaning takes 5 minutes. Turn off the cold water isolation valve. Locate the inline filter, typically inside the cold water inlet connection. Remove the filter. Rinse it under running water. Reinstall it. Turn the water back on. The filter is clean. The flow is unrestricted. The unit operates normally. The filter cleaning is the fix for a low-flow error code. The code appears on the display. The filter is the cause. The cleaning is the fix. The fix is free.
Frequently Asked Questions
How often should I descale my tankless water heater?
Descale annually in areas with hard water, more than 7 grains per gallon of hardness. Descaling every two years in areas with moderately hard water, 3 to 7 grains. Descaling every three years in areas with soft water. The descaling frequency is determined by the water hardness. A water test, $10 to $20 from a hardware store, provides the hardness number. The number determines the frequency.
Can I descale a tankless water heater myself?
Yes, if isolation valves with service ports are installed on the unit. The descaling procedure takes 45 to 60 minutes and requires a submersible pump, two buckets, two washing machine hoses, and 4 gallons of white vinegar. The pump costs $60 to $120 purchased once. The vinegar costs $15 per year. The procedure is within the reach of a confident DIYer who is comfortable turning off power and water and connecting hoses. If you are not comfortable, a plumber charges $150 to $300 for the annual descaling service.
How much does professional tankless water heater maintenance cost?
Professional annual maintenance costs $150 to $300 and includes descaling, air filter cleaning, vent inspection, water filter cleaning, and a performance check. The cost is the labor. The materials, vinegar or descaling solution, cost the plumber $15 to $30. The professional maintenance is the convenient option. The DIY maintenance is the money-saving option. Both achieve the same result: a clean heat exchanger and a unit that operates at full efficiency.
What happens if I never descale my tankless water heater?
The heat exchanger will clog with calcium scale. The unit will produce fluctuating hot water temperatures, eventually shut down with an error code, and require a heat exchanger replacement, $500 to $1,200, if the scale has caused permanent damage. In hard-water areas, an unserviced tankless water heater will fail within 5 to 8 years. A properly maintained unit will last 20 to 25 years. The difference is the descaling.
Do I need a water softener if I have a tankless water heater?
Not required, but a water softener extends the descaling interval and protects the heat exchanger from scale formation. A water softener costs $400 to $2,500 installed and reduces the calcium and magnesium in the water before it reaches the water heater. The softener is the prevention. The descaling is the cure. The prevention is better than the cure. The cure works. The prevention works better.
What is the lifespan of a tankless water heater with proper maintenance?
A properly maintained tankless water heater lasts 20 to 25 years. A tanked water heater lasts 10 to 15 years. The tankless unit lasts nearly twice as long. The maintenance is the difference between a tankless unit that fails at year 8 and a tankless unit that lasts the full 25 years. The maintenance is the descaling, the filter cleaning, and the vent inspection. The maintenance is annual. The maintenance is $35 in DIY materials or $200 in professional service. The maintenance is the reason the tankless unit outlasts the tanked unit. Skip the maintenance and the tankless unit fails before the tanked unit would have. The maintenance is the contract. The contract is annual. The contract delivers 25 years of hot water. The breach of contract delivers 8 years. The choice is the annual maintenance. The choice is the $35 and 45 minutes. The choice is the difference between 25 years and 8.
The Maintenance That Makes the Tankless Last
Descale the heat exchanger annually with a pump, two buckets, and 4 gallons of vinegar. Clean the air intake filter annually. Inspect the vent pipe annually. Clean the inline water filter annually. The maintenance takes 60 to 90 minutes per year. The materials cost $35 per year in vinegar and pump electricity. The maintenance is the difference between a tankless water heater that fails at year 8 and one that provides hot water for 25 years. The difference is 17 years of hot water. The 17 years are the return on the $35 annual investment. The investment is the descaling. The return is the hot water. The hot water is the reason the tankless unit was purchased. The maintenance is the reason the unit continues to deliver hot water. The maintenance is annual. The hot water is daily. The daily hot water is the return on the annual maintenance. The return is 365 days per year. The investment is one day per year. The ratio is 365 to 1. The ratio is the reason to descale. The descaling is the 1. The hot water is the 365. The 365 is worth the 1. Ever stood in the shower on a winter morning, the water hot and endless because the tankless unit was doing what it was designed to do, and thought “this is why I bought a tankless water heater”? The water was hot. The water would never run cold. The water was the return on the purchase. The purchase was $3,000. The return was infinite hot water. The return is sustained by the descaling. The descaling is the $35 annual payment to the heat exchanger. The payment is small. The return is large. The payment is annual. The return is daily. The daily hot shower in January is the reminder of why the $35 was well spent. The $35 is the cost of the shower. The shower is free after that.
