How to Descale and Clean a Tankless Water Heater Annually

Cleaning a tankless water heater means flushing the unit with a descaling solution once a year to dissolve mineral scale before it damages the heat exchanger. Homeowners who tackle this themselves can add years to the appliance's life and avoid a service call that often costs more than the supplies.

In Brief

• Flush the system annually, or every 6 to 12 months if you have hard water or heavy hot water use
• The job takes about an hour to an hour and a half from shutoff to restart
• A basic flush kit, pump, and vinegar or descaling solution run well under $100
• Skipping regular descaling can void manufacturer warranties and shorten the unit's lifespan
• Always shut off gas or electrical power before opening any service ports

Why Mineral Buildup Is the Real Threat

Tankless units run water through a narrow heat exchanger rather than storing it in a tank, and that narrow pathway is exactly why scale becomes a problem. Brandon Hughes, owner of Hughes Plumbing Services Inc. in Newbury Park, California, points out that calcium and magnesium in hard water gradually coat the interior surfaces, especially in regions where mineral content runs high.

Left unchecked, that buildup restricts flow and blocks heat transfer. Jordan Benjamin, owner and president of Done Rite Services in Tucson, Arizona, says the symptoms show up gradually: weaker hot water output, dropping pressure, error codes on the display, and a creeping rise in energy bills. In the worst cases, he says, the unit simply stops working. A well maintained system, by contrast, can run for 20 years or longer, and most manufacturers require documented descaling to keep the warranty intact.

How Often You Actually Need to Flush the System

Both plumbers land on the same baseline: once a year. Hughes suggests moving to a 6 to 12 month schedule if your home has notably hard water or if your household burns through a lot of hot water. Benjamin's advice is simpler still: check the owner's manual for the manufacturer's recommended interval and stick to it without deviation.

Waiting for symptoms is the wrong strategy, according to Benjamin. By the time you notice reduced hot water flow, scale has likely been accumulating for years and internal parts may already be compromised. His practical fix is to tie the task to something you already do on a schedule, like cleaning gutters or booking annual HVAC service, so it doesn't slip your mind.

Step by Step: Flushing a Tankless Water Heater

Before starting, confirm the unit has a flush kit installed. Look for what resembles a hose bib on both the hot and cold water lines. If you don't see one, call a plumber rather than attempting to force a connection.

• Tankless water heater flush kit
• Submersible utility pump
• Two washing machine or service hoses
• One 5 gallon bucket
• White distilled vinegar or a manufacturer approved descaling solution
• Adjustable wrench or channel lock pliers
• Safety gloves
• Safety glasses
• Towels or rags

Safety warning: This job involves gas, electricity, and water lines together, so shut off the gas supply or disconnect electrical power completely before touching any valves, and let the unit cool if it has been running.

1. Close the cold water supply and hot water outlet isolation valves after confirming gas or power is off.
2. Remove the inlet screen or prefilter following the manufacturer's instructions, rinse it clean under running water, and reinstall it.
3. Open the service ports and drain the water trapped inside the heat exchanger.
4. Set a submersible pump inside a 5 gallon bucket filled with descaling solution. Connect one hose from the pump to the cold water service port, and a second hose from the hot water service port back into the bucket, forming a closed loop.
5. Run the pump for 45 to 60 minutes to dissolve mineral scale built up inside the heat exchanger.
6. Empty the bucket, refill it with clean water, and circulate fresh water through the unit for several minutes to flush out leftover solution and loosened debris.
7. Remove the hoses, close the service ports, reopen the water supply valves, and restore gas or electrical power. Run a nearby hot water faucet to clear trapped air, then check all connections for leaks.

What Difficulty, Time, and Cost Look Like

Wear safety gloves and glasses throughout, since descaling solutions and vinegar can irritate skin and eyes on contact. If you notice persistent leaks after reassembly, error codes that won't clear, or suspect the flush kit was never installed correctly, that's the point to bring in a licensed plumber rather than push further on your own. For most households, though, this is a task that fits neatly into the same seasonal rhythm as clearing gutters or servicing an HVAC system, and doing it on schedule is far cheaper than replacing a heat exchanger that's been quietly scaling over for years.