How to Flush a Tankless Water Heater: The Maintenance Task That Keeps Your System Running Like New

Mineral deposits are the silent assassins of tankless water heaters. Every gallon of water flowing through your system carries microscopic hitchhikers—calcium, magnesium, and other minerals that gradually accumulate inside the heat exchanger like barnacles on a ship's hull. Left unchecked, this buildup transforms your efficient heating system into an energy-guzzling, underperforming shadow of its former self. Yet surprisingly few homeowners realize that flushing their tankless water heater isn't just recommended maintenance—it's the difference between a system that lasts two decades and one that fails after seven years.

I learned this lesson the hard way when my own tankless unit started making sounds like a coffee percolator during a particularly cold January morning. The technician who came out didn't mince words: "When's the last time you flushed this thing?" My blank stare told him everything he needed to know.

Understanding Why Flushing Matters More Than You Think

Inside every tankless water heater lies a heat exchanger—a sophisticated piece of engineering that rapidly heats water as it passes through. Picture a radiator in reverse, with water flowing through narrow passages while intense heat transfers from gas flames or electric elements. These passages are precision-engineered for maximum efficiency, but they're also perfect collection points for mineral scale.

The chemistry is straightforward but relentless. When water heats up, dissolved minerals become less soluble and precipitate out, forming crusty deposits. In traditional tank heaters, these minerals mostly settle to the bottom. But in tankless systems, they cling to the heat exchanger's surfaces like stubborn graffiti.

What really opened my eyes was watching a cutaway demonstration at a plumbing supply house. The scaled-up heat exchanger looked like it had been dipped in concrete. The passages meant to be the width of a pencil were reduced to pinholes. No wonder these units lose efficiency—they're literally choking on minerals.

Recognizing When Your System Needs Attention

Your tankless water heater won't send you a calendar reminder, but it will definitely communicate its distress through various symptoms. Temperature fluctuations become the first red flag—one moment you're enjoying a comfortable shower, the next you're doing the hot-cold dance. This happens because scale buildup creates hot spots and restricts water flow, making it impossible for the system to maintain consistent temperatures.

Strange noises rank as the second most common indicator. That coffee percolator sound I mentioned? It's actually water trying to squeeze through restricted passages, creating turbulence and vibration. Some people describe it as popping, crackling, or even a rumbling sound. Whatever the specific noise, it's your heater crying for help.

Reduced hot water output sneaks up on you gradually. You might not notice it day to day, but suddenly you realize you can't run the dishwasher and take a shower simultaneously anymore. Or maybe the kitchen faucet takes forever to deliver hot water when it used to be almost instant.

Error codes on your unit's display panel provide the most direct communication. Different manufacturers use different codes, but many relate to flow restrictions or overheating—both symptoms of scale buildup. My Rinnai unit, for instance, threw a Code 11 (no ignition) because scale had restricted flow so severely that the minimum flow rate couldn't trigger the ignition sequence.

Gathering Your Arsenal of Flushing Supplies

Before diving into the actual flushing process, you'll need to assemble some specific equipment. This isn't a job where you can improvise with whatever's lying around the garage—trust me, I tried that approach initially and ended up making three trips to the hardware store.

The cornerstone of your flushing setup is a submersible pump. Not just any pump will do; you need one capable of circulating at least 3-4 gallons per minute. I've had excellent results with a 1/6 horsepower utility pump that cost about forty dollars. Some people try to use aquarium pumps or fountain pumps—don't bother. They lack the necessary flow rate to effectively dislodge scale.

You'll need two washing machine hoses or similar high-quality hoses with female garden hose connections. Why washing machine hoses? They're designed to handle both hot water and pressure, plus they come with built-in rubber washers that create reliable seals. Standard garden hoses work in a pinch, but they're more prone to kinking and may not seal as well.

A five-gallon bucket serves as your circulation reservoir. Some pros use larger containers, but five gallons provides enough volume for effective circulation while remaining manageable. Make sure it's clean—any debris in the bucket will end up circulating through your heater.

For the descaling solution, white vinegar remains the go-to choice for most homeowners. You'll need about four gallons for a proper flush. Commercial descaling solutions work faster and more aggressively, but vinegar is gentler on seals and gaskets while still being effective against mineral deposits. Plus, it's food-safe and won't leave harmful residues.

Don't forget safety equipment. Rubber gloves protect your hands from the acidic solution, and safety glasses prevent splashes from reaching your eyes. The vinegar won't cause severe burns, but it's uncomfortable if it gets where it shouldn't.

The Pre-Flush Preparation Ritual

Preparation prevents poor performance—an old military saying that applies perfectly to water heater maintenance. Start by locating your water heater's service valves. These should be installed on both the cold water inlet and hot water outlet. If you don't have service valves, stop right here and call a plumber to install them. Attempting to flush without proper valves is like trying to change your oil without a drain plug.

Turn off the power to your water heater completely. For gas units, this means turning the gas valve to the off position and unplugging the electrical cord. Electric units require flipping the appropriate circuit breaker. This isn't just about safety—running the unit dry, even briefly, can cause catastrophic damage to the heat exchanger.

Close the main water supply valve feeding your tankless heater. Then open a hot water faucet somewhere in your house—preferably at the lowest point—to relieve pressure in the system. You'll hear a brief rush of water followed by a gurgling sound as air enters the lines.

Now comes a step many DIY guides skip: removing the inlet filter screen. Every tankless water heater has one, usually located where the cold water enters the unit. This screen catches debris before it can enter the heat exchanger. During normal operation, it gradually accumulates sediment. If you flush without cleaning this screen first, you're essentially washing a dirty car with muddy water.

The screen typically unscrews or pulls out after removing a small cover plate. Rinse it under running water, using an old toothbrush to dislodge stubborn debris. I once found a screen so clogged it looked like a lint trap from a dryer. No wonder the homeowner complained about low water pressure.

Executing the Flush Like a Professional

With preparation complete, it's time for the main event. Remove the caps from your service valves and attach your hoses. The cold water service valve connects to your pump's outlet, while the hot water service valve feeds back to your bucket. This creates a closed loop where the descaling solution circulates through the heat exchanger repeatedly.

Pour your vinegar into the bucket, submerge the pump completely, and ensure the return hose is also in the bucket. Open both service valves fully—this is crucial. Partially open valves restrict flow and reduce flushing effectiveness.

When you plug in the pump, you should see immediate circulation. The vinegar will likely turn cloudy or discolored within minutes as it begins dissolving scale. This is exactly what you want to see. If the flow seems weak or irregular, check for kinks in the hoses or partially closed valves.

Let the system circulate for at least 45 minutes. Some manufacturers recommend an hour, others say 45 minutes suffices. I typically run mine for a full hour because patience here pays dividends in system longevity. During this time, the acidic vinegar slowly dissolves mineral deposits, breaking them down into particles small enough to flush away.

About halfway through, I like to give the bucket a gentle stir and check the pump's intake for any debris accumulation. Sometimes larger scale chunks break loose and can clog the pump inlet. A quick check prevents circulation problems.

The Critical Post-Flush Procedures

After circulation time expires, unplug the pump and close both service valves. This next part requires careful attention to sequence. Disconnect the hose from the cold water service valve first, then remove the pump and empty the bucket. Rinse it thoroughly—you'll need it again shortly.

Here's where many DIYers make a crucial mistake: they reconnect everything and call it done. But your heat exchanger is still full of vinegar and dissolved minerals. Skipping the rinse cycle is like washing your hair with shampoo but skipping the rinse—you haven't finished the job.

Reattach the hose to the cold water service valve, but leave the hot water valve connected to drain into your bucket. Open the cold water service valve slowly, allowing fresh water to flow through the heat exchanger and out into the bucket. This pushes out the vinegar solution and any remaining debris.

Let fresh water run for at least five minutes. The first minute or two will smell strongly of vinegar, but this should dissipate as fresh water displaces the descaling solution. I once rushed this step and ended up with vinegar-flavored coffee the next morning—not recommended.

After rinsing, close the cold water service valve and remove both hoses. Replace the service valve caps, ensuring they're snug but not overtightened. Reinstall the inlet filter screen you cleaned earlier.

Now you can restore water and power to the unit. Open the main water supply valve slowly to prevent water hammer. Turn on a few hot water faucets in your house to purge air from the lines. You'll hear sputtering and spitting initially—this is normal. Let them run until you get steady flow.

Finally, restore power or gas to your water heater. Run hot water at a faucet near the unit and verify proper operation. The water should heat quickly and maintain steady temperature. Listen for any unusual noises—properly flushed units run almost silently.

Troubleshooting Common Flushing Challenges

Even with careful preparation, flushing doesn't always go smoothly. Weak circulation ranks as the most common issue. If your pump seems to be working but flow is minimal, scale buildup might be severe enough to restrict flow even during flushing. Try reversing the flow direction by swapping the hose connections. This sometimes dislodges stubborn blockages.

Persistent error codes after flushing usually indicate either incomplete descaling or other issues unrelated to mineral buildup. Some units have specific reset procedures after maintenance—consult your owner's manual. My neighbor's Navien unit required a specific button sequence to clear maintenance codes.

Leaks at hose connections typically stem from worn washers or cross-threaded connections. Keep spare washers handy, and thread connections carefully. If a service valve leaks even with the cap tightened, the valve's internal seals may need replacement—a job best left to professionals.

Temperature fluctuations persisting after flushing might indicate problems beyond simple scale buildup. The unit's thermistor (temperature sensor) could be coated with scale or failing. Some units have multiple temperature sensors that all need to be clean for proper operation.

Establishing Your Maintenance Schedule

Flushing frequency depends entirely on your water quality. Homes with hard water (over 7 grains per gallon) need annual flushing at minimum. Some areas with extremely hard water benefit from flushing every six months. I learned this after moving from Seattle (notoriously soft water) to Phoenix (liquid limestone). What worked in Seattle failed miserably in Phoenix.

Well water presents special challenges. Beyond hardness, well water often contains iron, manganese, and other minerals that create particularly stubborn deposits. These homes might need flushing every 3-4 months, plus specialized descaling solutions beyond simple vinegar.

You can gauge your needed frequency by checking the vinegar's color after flushing. Dark, cloudy vinegar indicates heavy scale removal—you might need more frequent maintenance. Lightly tinted vinegar suggests your current schedule works well.

Some newer tankless models include maintenance reminders based on usage patterns. While helpful, these generic reminders don't account for your specific water conditions. Trust your observations over preset timers.

Professional Versus DIY Considerations

While flushing is definitely a DIY-capable task, certain situations warrant professional intervention. If your unit has never been flushed and it's over five years old, professional service might be wise. Severe scale buildup sometimes requires commercial-strength descalers and specialized equipment.

Units still under warranty present another consideration. Some manufacturers void warranties if you perform your own maintenance. Check your warranty terms carefully. The few hundred dollars for professional service beats voiding a warranty worth thousands.

Professional plumbers also inspect components you might miss. They check gas pressures, venting integrity, and electrical connections. They can spot developing problems before they become expensive repairs. Think of it like taking your car to a mechanic versus just changing the oil yourself—both have their place.

That said, regular DIY flushing between professional services makes sense for many homeowners. You'll save money and gain intimate knowledge of your system. Plus, there's satisfaction in maintaining your own equipment.

The Long-Term Benefits You'll Actually Notice

Regular flushing delivers tangible benefits beyond preventing breakdowns. Energy efficiency improves dramatically—scale-covered heat exchangers work harder to heat water, driving up utility bills. I tracked my gas usage before and after my first flush and saw a 15% reduction in consumption.

Consistent water temperature becomes the norm rather than the exception. No more surprise cold bursts during showers or scalding surges when someone flushes a toilet. Your family stops complaining about shower lottery.

Component lifespan extends significantly. Heat exchangers designed to last 20 years often fail at 7-8 years due to scale damage. Regular flushing helps achieve the manufacturer's intended lifespan. Considering replacement costs approaching $2,000, prevention makes financial sense.

Water flow improves throughout your house. Scale doesn't just accumulate in the heat exchanger—it restricts flow everywhere. Regular flushing maintains the strong water pressure that made you choose tankless in the first place.

Perhaps most importantly, you'll avoid emergency failures. Water heaters, like most appliances, fail at the worst possible times—holiday weekends, during house guests' visits, or amid winter storms. Regular maintenance transforms these emergencies into scheduled events under your control.

My own tankless water heater just celebrated its twelfth birthday, running as efficiently as the day I learned to flush it properly. That emergency service call years ago taught me an expensive lesson, but it's knowledge that's paid for itself many times over. Your tankless water heater represents a significant investment in your home's comfort and efficiency. A few hours of annual maintenance ensures that investment continues paying dividends for decades to come.

Authoritative Sources:

Bradford White Corporation. Tankless Water Heater Maintenance and Service Manual. Bradford White Water Heaters, 2021.

Rinnai America Corporation. Rinnai Tankless Water Heater Installation and Operation Manual. Rinnai America Corporation, 2022.

U.S. Department of Energy. "Tankless or Demand-Type Water Heaters." Energy.gov, Office of Energy Efficiency & Renewable Energy, 2023. www.energy.gov/energysaver/tankless-or-demand-type-water-heaters

Water Quality Association. Hardness in Drinking Water. Water Quality Association Technical Fact Sheet, 2021.

Navien Inc. NPE Series Service Manual. Navien Inc., 2022.

American Society of Home Inspectors. Technical Journal: Tankless Water Heater Inspection and Maintenance. ASHI Reporter, vol. 35, no. 4, 2021, pp. 12-18.