As someone who has spent years working with water heating solutions, I can tell you this—your tankless water heater works hard to provide endless hot water, but it won’t stay that way without PROPER MAINTENANCE.
Over time, mineral buildup can clog the system, reduce water pressure, and make your hot showers less satisfying. That’s why FLUSHING and DESCALING it regularly is essential to keeping it running efficiently.
But don’t worry! You don’t need to call a plumber.
In this guide, I’ll walk you through how to flush, descale, and clean your on-demand tankless water heater step by step. With a little time and the right approach, you can keep your system in top shape and enjoy reliable hot water for years to come. Let’s get started!
Flushing, Descaling And Cleaning A Tankless Water Heater: The Best Approach
Over the years, I’ve seen way too many tankless water geysers fail prematurely, simply because people don’t flush them regularly. But cleaning your on-demand heater once a year is the easiest way to keep it running efficiently for years.
When I service water heaters, I never use chemical descalers. Many homeowners drink hot water from their systems, so I stick with food-grade white vinegar—it’s effective, safe, and doesn’t leave harmful residues (material safety data sheet).
If you’ve never flushed your heater before, don’t worry. I’ll walk you through it step by step, just like I do when I train new technicians. I use this method for flushing both gas and electric tankless heaters (tested with Rinnai, Navien and Rheem).
What You’ll Need
Before you start, get these tools and materials ready:
A 5-gallon bucket: This will hold the vinegar and circulate it through the water heater.
Two standard washing machine hoses: These will connect the pump to the tankless heater.
A small submersible utility pump: This pushes the vinegar through the system.
A screwdriver: Needed to open the purge valves.
4 gallons of food-grade white vinegar: The safest and most effective cleaner.
Once you have everything, you’re ready to begin.
Step 1: Power Down the Water Heater
Before touching anything, shut off the power to your heater. I can’t stress this enough—whether it’s gas or electric, you don’t want any surprises.
For electric models: Flip the breaker switch in your electrical panel. (Read this article to know more about electrical connections)
For gas models: Turn off the gas supply valve completely.
I’ve seen cases where people forget this step, and trust me, it’s a mistake you don’t want to make. Here’s a detailed guide about different types of tankless heaters.
Step 2: Shut Off the Water Supply
Next, turn off the water supply so fresh water doesn’t enter the water heater while you’re flushing it.
- Turn OFF the cold water inlet valve: This stops new water from flowing in.
- Turn OFF the hot water valve: This prevents water from leaving the system.
If you’re not sure where these valves are, check your user manual. Every on-demand heater is a little different.
Step 3: Relieve Pressure in the System
Before disconnecting anything, you must relieve any built-up pressure.
- Locate the purge port valves on both the hot and cold water lines.
- Use a screwdriver to slowly open them.
- Let any trapped pressure escape before moving forward.
I learned this the hard way early in my career. One time, I skipped this step and ended up with a face full of hot water when I disconnected the hoses. Don’t make that mistake!
Step 4: Set Up the Flushing System
Now comes the fun part—setting up the flush for your tankless water heater.
- Take one hose and connect it to the cold water valve. Attach the other end to the utility pump sitting inside your bucket. I use my 1/4 HP pump in most cases.
- Take the second hose and connect it to the hot water valve. Place the free end inside the bucket.
- Pour all 4 gallons of white vinegar into the bucket.
Double-check the hose connections. You don’t want leaks when you turn on the pump.
Step 5: Flush the System
Now it’s time to clean out all that scale buildup.
- Turn ON the pump. This will start circulating the vinegar through the instantaneous heater.
- Let it run for at least 45 minutes to an hour.
- Watch for bubbles or discoloration in the vinegar—that’s the scale dissolving.
If your water has a high mineral content, I recommend running the vinegar through for a full hour. Some manufacturers even suggest longer flushing times, so it’s always a good idea to check your manual.
Step 6: Rinse Out the Vinegar
Once the flushing is done, you need to rinse out the system to remove any leftover vinegar.
- Turn OFF the pump and close the purge port valves.
- Disconnect the hoses and discard the used vinegar.
- Turn ON the cold water inlet valve and let fresh water flow through the system for at least 5-10 minutes.
- Open a hot water tap inside your home and let the water run to clear out any remaining vinegar.
I always tell homeowners to do a smell test—if you still detect vinegar in the water, let it run a bit longer.
Step 7: Restore Power & Check for Leaks
The last step is making sure your tankless heater is back up and running.
Turn the gas or electric power back on.
Inspect all connections for leaks.
Run the hot water and watch for any irregularities.
If everything looks good, congratulations—you just flushed your tankless water heater like a pro!
Here is a short video detailing every step in the process-
Why Do Tankless Water Heaters Need Flushing?
A common misconception I hear all the time is that gas and electric tankless heaters don’t need flushing—just because they don’t have a storage tank. That’s completely FALSE.
In my years of working with on-demand water heaters, I’ve seen firsthand how neglecting maintenance can wreck performance, drive up energy bills, and shorten the unit’s lifespan. On the other hand, users who stay on top of descaling enjoy consistently strong water flow and a geyser that lasts for decades.
So, why does flushing matter? Because tankless systems still deal with mineral buildup. If you live in an area with hard water, CALCIUM and MAGNESIUM deposits accumulate inside the heat exchanger and pipes. Cleaning your tank-free water heater regularly prevents these issues and offers those key benefits:
Prevents sediment buildup and keeps efficiency high: When scale accumulates, heat transfer becomes inefficient, forcing the system to overwork. A clean system heats water faster and runs more smoothly.
Lowers energy costs: A clogged heater requires more energy to produce the same amount of hot water. Regular upkeeping keeps energy consumption low, saving you money.
Reduces breakdowns and costly repairs: I’ve lost count of how many times my team has spotted early signs of failure during a routine flush—fixing minor issues before they turn into expensive problems.
Extends lifespan: A properly maintained tankless water heater can last 20+ years, while a neglected one might fail in 10 to 12 years. Descaling protects the heat exchanger and internal components from early wear and tear.
What About Homes With a Water Softener?
Even if you have a water softener, you STILL need to clean your instantaneous water heater—just less frequently. While softened water reduces scale buildup, it doesn’t eliminate it entirely. To keep your system in top condition, I recommend flushing at least every two years to prevent any potential issues.
How Often Should You Descale Your Water Heater?
Regular descaling is crucial for maximizing the performance and longevity of your tankless water heater. But how often is ‘regular’—and what’s the right frequency?
Many people who contact me for on-demand water heater servicing and maintenance are unsure about how often they should flush their systems. Some believe cleaning every 3 to 5 years is sufficient, while others haven’t done it in 15 years! No, I’m not joking. Honestly, I can’t even imagine how risky that must be. They should count themselves lucky their heaters haven’t broken down or caused bigger issues yet!
Let’s clear up the confusion. I generally recommend descaling any tank-free water heater at least once a year, ideally every 12 to 18 months. However, if you live in a region with hard water, more frequent flushing is necessary. Mineral buildup happens faster in such areas, leading to clogging, reduced efficiency, and premature damage.
Here’s a simple guideline based on Total Dissolved Solids (TDS) in water:
| Water TDS | Descaling/ Flushing Frequency |
|---|---|
| 0-60 ppm | 18-24 months |
| 61-120 ppm | 6-12 months |
| 121-180 ppm | 4-6 months |
| 181 ppm and more | 3 months |
To determine your water TDS, you can use a home testing kit or seek professional help. Regular maintenance can save you from costly repairs down the line.
That said, always check your manufacturer’s manual for model-specific cleaning guidelines. And if you notice signs like reduced hot water flow, fluctuating temperatures, or rising energy bills, don’t wait—maintain your heater right away!
Should You Clean Your Tankless Heater Yourself?
You can clean your tankless water heater yourself as it can be a COST-EFFECTIVE option. But before you grab your tools, let’s be real: you need some basic plumbing knowledge and the right equipment. If you’re comfortable with home maintenance tasks and confident in handling them without professional help, you might consider going the DIY route.
The key is to descale the system correctly so it runs efficiently and lasts as long as possible. But if you’ve never tackled home maintenance before and have zero plumbing experience, DIY flushing might not be the best idea. You could miss critical steps, and instead of saving on service costs, you might damage the heater, void the warranty, or even put your safety at risk. That’s a much bigger expense in the long run.
If it’s your first time attempting this, it can feel overwhelming—especially if you’re unfamiliar with how a tankless water heater work. That’s why I always recommend calling in a professional. A skilled technician can get the job done safely, efficiently, and without causing unintended damage.
Plus, professional servicing often includes additional maintenance and inspections, ensuring your heater stays in peak condition. If you decide to go this route, make sure to hire a reliable team that knows what they’re doing. It’s a small price to pay for peace of mind and long-term efficiency.
What Happens If You Don’t Flush Periodically?
I’ve met homeowners who haven’t flushed their water heater in 15 years. When I asked why, they told me, “I thought tankless systems didn’t need it—that’s why I got one in the first place!” My only response? They got lucky. Because skipping regular descaling isn’t just bad maintenance—it’s a ticking time bomb that can lead to serious, expensive problems.
Hard water contains dissolved minerals like calcium and magnesium. When water is heated, these minerals separate and solidify, forming thick layers of scale inside your system. Over time, this buildup chokes your heater’s efficiency and leads to:
Clogged pipes and heat exchanger: Your system has to work harder to push water through, increasing strain.
Inconsistent water temperature: One moment it’s hot, the next it’s lukewarm, then suddenly scalding.
Lower water pressure: Instead of a strong flow, you get weak, sluggish hot water.
Longer wait times for hot water: The on-demand heater struggles to heat properly, leaving you frustrated.
Higher energy consumption: The more scale buildup, the harder your water heater works, driving up utility bills.
Louder operation: Gurgling, rattling, or rumbling sounds are often signs of trapped scale deposits.
If these issues aren’t addressed in time, things only get worse. I’ve personally seen extreme cases where mineral buildup became so severe that it caused overheating, melted internal components, and led to complete system failure. In those cases, there was no easy fix—the only solution was an expensive repair or full replacement.
And here’s something many people don’t realize: Skipping descaling can void your manufacturer’s warranty. That means if your tankless water heater breaks down because of scale buildup, you’ll be paying the entire bill out of pocket.
Regular cleaning isn’t just good maintenance—it’s a NECESSITY. I always recommend flushing at least once a year (or more frequently if you have hard water). A simple, low-cost maintenance routine can:
Keep your heater running at peak efficiency.
Extend its lifespan by years.
Prevent costly repairs and replacements.
Reduce energy bills by keeping performance optimal.
Think of it this way: Would you drive your car for years without changing the oil? Probably not. So why would you risk a breakdown by skipping routine maintenance on your water heater?
Don’t wait until problems start—clean your system before damage sets in.
How Long Does It Take To Flush A Tankless Water Heater?
Flushing a instant water heater typically takes anywhere between 30 minutes to 2 hours, depending on your unit and the manufacturer’s instructions. The actual descaling process—where the cleaning solution circulates through the system—usually takes around 45 minutes to an hour. That said, the total time can vary based on a few key factors:
- Mineral buildup – If your on-demand heater has gone too long without maintenance, the process may take longer to fully dissolve the hardened scale.
- Water hardness – Homes with very hard water (high in calcium and magnesium) often require longer flush cycles to break down deposits.
- Unit size and design – Larger models or those with complex internal structures might take additional time to descale thoroughly.
While a standard flush may take just over an hour, I always suggest setting aside a full 2-hour window to account for unexpected issues—especially if it’s your first time doing it. This ensures you have enough time to properly prepare, wash, and inspect the system without rushing.
A thorough cleaning keeps your tankless water heater running efficiently, reduces energy costs, and extends its lifespan—so it’s well worth the time investment.
Frequently Asked Questions
Both vinegar and commercial descaling solutions are effective for flushing tank-free water heaters. However, I recommend VINEGAR as the go-to option because it is natural, widely available, and budget-friendly. It effectively removes mineral buildup without being toxic or harmful to the environment. Additionally, vinegar is gentle on the heater, reducing the risk of damage. Using about 4 gallons of undiluted, food-grade white vinegar and running the system for at least 45 minutes should break down mineral deposits effectively.
On the other hand, specially formulated descaling solutions may work faster on heavy mineral buildup and be more efficient. However, they are typically more expensive, less readily available, and contain strong chemicals that could potentially damage delicate water heater components if used incorrectly.
Ultimately, the best approach is to follow the manufacturer’s guidelines for cleaning your tankless water heater to ensure you use the most suitable chemical for your specific model.
Yes, you might also consider using citric acid (commonly found in lemon juice) to flush your instantaneous water heater. I have used a 10% citric acid solution in the same way as vinegar, and it worked well. Like vinegar, citric acid is a mild, non-toxic acid, making it a safe option for descaling without the risk of corrosion.
Citric acid is typically sold in powder form and can be purchased online or at home improvement stores.
To flush a tankless water heater, you’ll need a small submersible utility pump that can circulate the descaling solution efficiently without being too powerful, which could potentially damage internal components. For most instantaneous heaters, a 1/4 to 1/6 horsepower submersible pump provides sufficient flow. However, always check the manufacturer’s guidelines, as some models may have specific pump recommendations for optimal performance and safety.
Yes, you can clean a tankless water heater without a pump, but the process will be slower and less effective. Instead of continuous circulation, you’ll need to manually pour the solution into the unit and let it soak for the recommended time. However, using a pump significantly speeds up the process and ensures a more thorough washing by keeping the solution moving through the system.
While CLR (Calcium, Lime, and Rust Remover) is an effective cleaner, most water heater manufacturers do not recommend using it. Its strong chemical formula can potentially damage the heating unit. Instead, they advise using food-grade white vinegar, which is both safe and effective for descaling.
In the USA, the average cost of hiring a professional to flush a on-demand water heater is around $200. However, prices typically range from $160 to $250, depending on factors such as unit size, type, location, and labor costs.
For DIY cleaning, the main expense is the flushing kit and tools, which generally cost between $20 and $50, depending on the brand and quality. Since you’re doing the work yourself, you save on labor costs.
Knowing how to properly flush your tankless water heater isn’t just about maintenance—it’s about keeping your hot water flowing reliably and avoiding those dreaded cold showers, especially in winter.
Regular descaling removes mineral buildup that can clog the system, allowing your heater to operate efficiently year-round. This reduces strain on internal components, lowers energy consumption, and extends the unit’s lifespan—all while keeping your energy bills in check.
While DIY flushing can be an option if you’re confident with the process, a professional service ensures a thorough, risk-free job using the right tools and expertise. Professionals can also inspect your system for potential issues, helping you avoid costly repairs down the line.
Once you find a trusted expert, you’ll have peace of mind knowing your on-demand tankless water heater is in top shape, delivering consistent hot water and peak performance when you need it most!
Quick Recap
Even without a storage tank, mineral buildup can clog your tankless water heater, leading to poor performance, higher energy bills, and early failure. Regular flushing keeps it running efficiently.
If you live in an area with hard water, limescale builds up faster. Adjust your descaling frequency based on water hardness to prevent clogged pipes and weak water pressure.
While flushing your heater yourself can save money, missing steps or using the wrong methods can cause damage. If you’re unsure, hiring a professional can prevent costly mistakes.
Food-grade white vinegar is an effective, safe, and budget-friendly descaler that removes mineral buildup without harming your heater or your drinking water.
Neglecting to clean your heater can void your warranty, increase utility costs, and lead to expensive repairs—or worse, a complete system replacement.
Setting aside just an hour or two once a year for descaling can extend your water heater’s lifespan well beyond 20 years, keeping your hot water reliable and your bills low.
