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

I'll never forget the first time I opened up my tankless water heater for cleaning. Standing there with my wrench in hand, I felt like a medieval knight about to battle a dragon made of calcium deposits. Three years of neglect had turned my efficient little heating unit into something that sounded more like a coffee percolator than a water heater.

The thing about tankless water heaters is they're simultaneously brilliant and finicky. Unlike those big tank heaters that sit in your basement like patient water-warming buddies, tankless units are high-strung thoroughbreds. They need attention, and when they don't get it, they let you know – usually when you're covered in shampoo on a cold morning.

Why Your Tankless Heater Actually Needs Cleaning

Most people assume that because there's no tank, there's nothing to maintain. Wrong. Dead wrong. The heat exchanger in your tankless unit – that's the part doing all the heavy lifting – is basically a magnet for mineral deposits. Every gallon of water flowing through carries dissolved minerals, and when that water gets superheated in milliseconds, those minerals decide to throw a party and stick around.

I learned this lesson the expensive way. After ignoring my unit for too long, I ended up with what my plumber colorfully called "limestone stalactites" inside my heat exchanger. The efficiency had dropped so much that my gas bills looked like I was heating a small hotel instead of a three-bedroom house.

The Vinegar Method: Your First Line of Defense

Let me walk you through what actually works. First, you'll need white vinegar – and I mean the cheap stuff from the grocery store, not your fancy cooking vinegar. You'll also need a submersible pump (about $40 at any hardware store), two washing machine hoses, and a five-gallon bucket.

Turn off the power to your unit. This isn't optional – water and electricity are terrible dance partners. Close the water valves, both hot and cold. Now here's where it gets interesting: those service valves your installer hopefully put in? Time to use them.

Connect your hoses to the service ports. One hose goes from your bucket to the cold water service valve, the other from the hot water valve back to your bucket. You're creating a closed loop. Fill your bucket with four gallons of vinegar, drop in your pump, and let it circulate for about 45 minutes to an hour.

The first time I did this, I watched in horrified fascination as the clear vinegar turned into something resembling milk. That was three years of scale breaking loose. Now I do this every six months, and the vinegar stays relatively clear – proof that regular maintenance actually works.

When Vinegar Isn't Enough

Sometimes you need the big guns. If you live in an area with particularly hard water (looking at you, Phoenix and San Antonio), or if you've neglected maintenance for years, vinegar might just laugh at your scale buildup.

Commercial descaling solutions are stronger, but here's the rub – they're also more aggressive on your heat exchanger. I've seen people use CLR or similar products, and while they work, they can also eat away at the copper heat exchanger if you're not careful. If you go this route, follow the dilution instructions religiously and don't let it circulate for longer than recommended.

There's also the nuclear option: calling in a professional. I know, I know – nobody wants to spend $200-300 on something they could theoretically do themselves. But sometimes, especially with older units or severe buildup, a pro with specialized equipment can save your unit from an early grave.

The Forgotten Filter

Here's something that drives me crazy – people obsess over descaling but completely ignore their inlet filter. Every tankless heater has a small screen filter on the cold water inlet. It's usually hidden behind a small cap that unscrews. This little filter catches debris before it can enter your unit, and it gets gunked up faster than you'd think.

I check mine monthly. Takes thirty seconds. Unscrew the cap, pull out the filter, rinse it under the tap, put it back. Yet I've seen filters so clogged they looked like tiny archaeological specimens. One neighbor's filter had enough sediment to start a small beach.

The Combustion Chamber Nobody Talks About

If you have a gas tankless heater, there's another cleaning task everyone seems to forget: the combustion chamber. Over time, dust and debris accumulate on the burner assembly. This isn't just about efficiency – it's about safety.

Once a year, with the unit completely shut down and cool, I carefully vacuum out the combustion chamber area. You need to be gentle here – those components are precise and expensive. A soft brush attachment on a vacuum works well. Some people use compressed air, but I've seen that blow debris deeper into the unit, so I stick with vacuuming.

Water Quality: The Root of All Evil (Or At Least Most Maintenance)

After years of dealing with scale buildup, I finally bit the bullet and installed a whole-house water softener. Game changer. My descaling frequency dropped from every six months to once a year, and even then, there's barely any buildup.

But softeners aren't perfect for tankless heaters either. The sodium in softened water can cause its own issues over time, particularly with the heat exchanger. Some manufacturers actually void warranties if you use softened water. The sweet spot seems to be a water hardness between 7-10 grains per gallon. Too soft, and you risk corrosion. Too hard, and you're descaling constantly.

The Maintenance Schedule That Actually Works

Through trial, error, and one very expensive replacement, here's the schedule I've settled on:

Monthly: Check and clean the inlet filter. Takes less time than making coffee.

Every six months: Full vinegar flush if you have hard water, annually if your water is softer.

Annually: Clean the combustion chamber and air intake. Check the venting for obstructions (bird nests are surprisingly common).

Every two years: Have a professional inspection. They can check things like gas pressure and combustion efficiency that are beyond DIY territory.

Signs You've Waited Too Long

Your tankless heater will tell you when it's unhappy. The error code everyone dreads is usually related to flow – the unit can't push enough water through the scaled-up heat exchanger. But before you get codes, you'll notice other signs.

The "cold water sandwich" gets worse – that's when you get bursts of cold water between hot. The unit starts making new noises: popping, rumbling, or that coffee percolator sound I mentioned. Your hot water takes longer to arrive, or the temperature fluctuates more than usual.

The worst sign? Your energy bills creeping up. A scaled-up heat exchanger works harder to heat the same amount of water. I've seen efficiency drop by 30% or more in neglected units.

The DIY Versus Professional Debate

I'm all for DIY maintenance, but let's be realistic about limitations. Descaling? Absolutely do it yourself. Filter cleaning? No-brainer. But when you start getting into gas pressure adjustments, combustion analysis, or replacing internal components, that's professional territory.

I learned this after attempting to replace a flow sensor myself. Saved $50 on the service call, then spent $400 on repairs after I cross-threaded the fitting and caused a leak. Some victories aren't worth winning.

Final Thoughts from the Trenches

Tankless water heaters are like high-performance cars – amazing when maintained, expensive nightmares when neglected. The cleaning process isn't complicated, but it requires consistency. Set reminders on your phone. Mark it on your calendar. Do whatever it takes to stay on schedule.

The irony is that the very efficiency that makes tankless heaters attractive also makes them vulnerable. They heat water so quickly and efficiently that minerals don't have time to stay in suspension – they just crash out onto your heat exchanger like limestone snow.

But here's the thing: even with all the maintenance, I wouldn't go back to a tank heater. The endless hot water, the energy savings, the space savings – it's all worth it. Just remember that "tankless" doesn't mean "maintenance-free." It means "tank-free," and that's a completely different thing.

Take care of your tankless heater, and it'll take care of you. Ignore it, and you'll be taking very quick, very cold showers while waiting for the repair technician. Trust me on this one – I've been on both sides of that equation, and regular cleaning is definitely the better option.

Authoritative Sources:

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

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

United States 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 Corrosion Control. Water Quality Association Technical Publications, 2020.