How to Flush a Tankless Water Heater: The Real Story Behind Maintaining Your On-Demand Hot Water System

I'll never forget the first time I watched a professional plumber flush my neighbor's tankless water heater. The guy showed up with what looked like a modified sump pump, a five-gallon bucket, and enough white vinegar to pickle a small cucumber farm. My initial thought? This seems like overkill for something that doesn't even have a tank. Boy, was I wrong.

The truth about tankless water heaters is that they're simultaneously more sophisticated and more vulnerable than their tank-bearing cousins. These compact units push water through a maze of heat exchangers at breakneck speed, and every mineral deposit, every bit of scale buildup, acts like a tiny speed bump in that process. Over time, those speed bumps turn into roadblocks.

Why Your Tankless System Needs This Attention

Let me paint you a picture. Inside your tankless water heater, water flows through narrow passages surrounded by heating elements. In areas with hard water—and that's most of the United States, frankly—calcium and magnesium minerals precipitate out of the water when heated. They cling to these passages like barnacles on a ship's hull.

The kicker? Unlike a traditional tank heater where sediment settles harmlessly at the bottom, in a tankless system, there's nowhere for these minerals to hide. They accumulate right where the action happens, gradually choking off water flow and forcing your heater to work harder. I've seen units that went from providing endless hot water to barely managing a lukewarm trickle, all because nobody told the homeowner about flushing.

The Equipment You Actually Need

Here's where things get interesting. The tankless water heater industry would have you believe you need their proprietary flushing kit, which typically runs $150-200. And sure, those kits work fine. But after years of doing this myself and talking to plumbers who've been at it since these units first hit the American market, I've learned you can accomplish the same thing with:

A small submersible pump (1/6 HP is plenty)
Two washing machine hoses
A five-gallon bucket
Four gallons of white vinegar (the cheap stuff works just as well as the fancy cleaning vinegar)

That's it. Maybe $40-50 if you're buying everything new. The pump is the only specialized item, and you can find them at any hardware store or online. Some folks even use a pond pump they already have lying around.

The Process That Actually Works

First things first—turn off the power to your unit. Gas models need the gas shut off too. This isn't just about safety (though that's paramount); it's about preventing the unit from firing up mid-flush and cooking your vinegar into some ungodly steam.

Now, every tankless heater has service valves. These are your gateway to the system's internals. They're usually hidden behind a panel at the bottom of the unit, color-coded red for hot and blue for cold. If your installer did their job right, there should be isolation valves here too—these let you shut off water to the heater without affecting the rest of your house.

Close those isolation valves. Open the service valves. This is where newbies often get confused because it feels counterintuitive—you're opening valves after closing others. Think of it this way: you're redirecting the flow path from your home's plumbing to your flushing setup.

Connect your hoses. One goes from the pump to the cold water service valve, the other from the hot water service valve back to your bucket. You're creating a closed loop. The vinegar will circulate from bucket to pump to heater and back to bucket, over and over.

Pour your vinegar into the bucket, submerge the pump, and let it run. How long? This is where opinions diverge wildly. Manufacturers typically say 45 minutes. Old-timer plumbers I know swear by 90 minutes minimum. My personal sweet spot? An hour for annual maintenance, 90 minutes if it's been neglected.

What's Really Happening During the Flush

While that pump churns away, the vinegar is doing chemistry, not magic. Acetic acid in the vinegar reacts with calcium carbonate deposits, converting them into calcium acetate, water, and carbon dioxide. The calcium acetate stays dissolved in the vinegar solution, effectively removing it from your heat exchanger surfaces.

You might notice the vinegar getting cloudy or even see small white particles floating around. That's success made visible. I once flushed a unit that hadn't been touched in five years, and the vinegar came out looking like milk. The owner couldn't believe all that gunk had been inside their heater.

The Rinse Nobody Talks About

Here's something most DIY guides gloss over: the rinse cycle is just as important as the flush itself. After your vinegar bath, you need to flush the system with fresh water. Disconnect your pump setup, close the service valves, and open the isolation valves. But before you fire everything back up, open a hot water tap somewhere in your house and let it run for a good five minutes.

Why? Residual vinegar in the system won't hurt your heater, but it will make your morning shower smell like a salad bar. Trust me on this one—I learned the hard way.

When Things Don't Go According to Plan

Sometimes you'll encounter complications. Maybe your unit doesn't have service valves (shame on whoever installed it). Maybe the valves are seized from years of neglect. Or perhaps—and this is more common than you'd think—you start the flush and realize your heat exchanger is already too far gone.

I've seen units where the vinegar flush revealed pinhole leaks that were previously sealed by scale buildup. It's like pulling a bandage off a wound you didn't know was there. In these cases, flushing actually accelerates the inevitable—but better to know now than discover it during your daughter's wedding shower, right?

The Frequency Question

Manufacturers universally recommend annual flushing. But here's my contrarian take: that's a one-size-fits-all recommendation that fits nobody perfectly. If you're on city water that's relatively soft, annual flushing might be overkill. If you're on well water that leaves spots on everything, twice a year might not be enough.

The smart approach? Check your water hardness. You can get test strips for a few bucks. Anything over 7 grains per gallon, and you should lean toward more frequent flushing. Below 4 grains? You could probably stretch it to 18 months between services.

The Professional Alternative

Look, I'm all for DIY maintenance, but I also recognize when professional service makes sense. If you're not comfortable working with water and electricity/gas in close proximity, or if your unit is still under warranty (some manufacturers require professional service records), calling in a pro isn't admitting defeat.

A good plumber will do more than just flush your system. They'll check the flame pattern on gas units, test the flow sensors, inspect the venting, and catch potential problems before they become emergencies. The $200-300 for professional service might sting, but it's pennies compared to a premature heater replacement.

Beyond the Basic Flush

For those who want to go the extra mile, there are additional maintenance steps worth considering. The inlet water filter (yes, your tankless has one) should be cleaned during each flush. It's usually just a small screen, but it can accumulate surprising amounts of debris.

Some folks also swear by descaling solutions specifically designed for tankless heaters. These are typically citric acid-based and arguably more effective than vinegar. They're also more expensive and, in my experience, unnecessary for routine maintenance. Save them for neglected units that need serious intervention.

The Bottom Line on Tankless Maintenance

After years of maintaining these systems, both professionally and on my own units, I've come to appreciate the elegant simplicity of the flushing process. It's preventive maintenance at its finest—a small investment of time and effort that pays dividends in reliability and longevity.

The alternative—waiting until problems develop—inevitably leads to those panicked "no hot water" calls to plumbers, usually at the worst possible times. I've seen too many $4,000 tankless heaters replaced after just five or six years, all because nobody ever flushed them.

Your tankless water heater is a sophisticated piece of equipment that, when properly maintained, can provide decades of efficient service. The flushing process might seem like a hassle, especially compared to the virtually maintenance-free traditional tank heaters. But consider this: you're trading an hour or two of annual maintenance for endless hot water, lower energy bills, and a heater that takes up a fraction of the space.

That's a trade I'll make every time.

Authoritative Sources:

Bradford White Corporation. Tankless Water Heater Installation and Operation Manual. Bradford White Water Heaters, 2021.

Langelier, W.F. "The Analytical Control of Anti-Corrosion Water Treatment." Journal of the American Water Works Association, vol. 28, no. 10, 1936, pp. 1500-1521.

National Association of Home Builders. Residential Water Heater Maintenance Guidelines. NAHB Research Center, 2019.

Rinnai America Corporation. Tankless Water Heater Flushing and Maintenance Procedures. Rinnai Technical Publications, 2022.

U.S. Department of Energy. "Tankless or Demand-Type Water Heaters." Energy.gov, Office of Energy Efficiency & Renewable Energy, 2023.

Water Quality Association. Hardness in Drinking Water: Health Effects and Household Treatment Options. WQA Technical Publications, 2020.