Do Tankless Water Heaters Need to Be Flushed?

Last Revised: 4/4/25

Are you noticing weaker water pressure or inconsistent water temperatures from your tankless water heater, when you bought it specifically to avoid those problems? Have your energy bills crept back up even though your tankless water heater is supposed to be ultra-efficient? If so, you’re likely dealing with hidden mineral buildup inside your tankless water heater – a common but preventable issue.  

Tankless water heaters are an investment in convenience and efficiency, but like any hardworking appliance, they need regular care to maintain that performance. Flushing your system is the key to preventing scale buildup, protecting heating elements, and ensuring you get the endless hot water you paid for. 

At Mattioni, we’ve been keeping homes comfortable for over 75 years, and we know exactly what it takes to keep your tankless water heater running like new. In this article, we’ll explain why flushing is critical, how often to do it, and whether it’s a DIY job or better left to a pro. 

Let’s start with the basics – why tankless water heaters need to be flushed in the first place. 

Why Do I Need to Flush My Tankless Water Heater? 

Tankless systems heat water only as it’s needed, so when you turn on a hot water tap, cold water flows through a heating element inside the unit. Either a gas burner or electric heating element rapidly heats the water to your desired temperature. This approach to heating water (compared to traditional tank-type water heaters) is highly efficient – but only when heating elements are clean and free from mineral deposits. 

A tankless water heater’s efficiency is directly tied to its heating elements’ ability to make direct contact with the incoming water supply. 

Scale buildup on these components acts as an insulator, making it harder for the heat exchanger to do its job. As a result, your water heater must work harder, using more energy while delivering less hot water. Over time, this can lead to overheating, reduced efficiency, and even damage to the system. In other words, the performance of a tankless water heater can drop off more significantly than a tank-type water heater. 

Flushing removes scale buildup that causes your tankless water heater to work harder and fail sooner.  

Where Does Scale Buildup Come From? 

Scale buildup in your tankless water heater comes from hard minerals naturally present in your water supply – primarily calcium and magnesium. When water is heated, these minerals separate and form hard deposits on the heating components. Over time, this accumulation restricts water flow, reduces heating efficiency, and can cause damage from overheating. 

What Does Scale Buildup Do to a Tankless Water Heater?  

When heated, calcium and magnesium form a layer of scale that clings to the internal components of your water heater. In a tankless system, this buildup happens on the heat exchanger and/or heating elements, reducing its ability to transfer heat efficiently. If left unaddressed, the layer of scale will grow and the performance of your water heater will suffer. You might notice lower water pressure, longer wait times for hot water, or even strange noises like whistling or rumbling – all signs that scale buildup is interfering with performance. 

Pictured: The hot water outlet on a tankless water heater before and after flushing. 

Ensuring Optimal Water Flow 

One of the biggest benefits of a tankless water heater (compared to traditional tank types) is its ability to provide endless hot water. However, if scale buildup begins to clog the narrow passageways inside the heat exchanger, water flow can be restricted. This can lead to fluctuating temperatures, weak water pressure, or even systems shutdowns if the heater detects a blockage. Regular flushing helps prevent this, ensuring that water can flow freely and that your system delivers the consistent performance you expect. 

How Often Should You Flush Your Tankless Water Heater? 

The frequency of flushing depends largely on the hardness of your water. If you have very soft water, you might only need to flush your system every 2-3 years. However, if you live in an area with moderate to hard water, an annual flush is recommended. In areas with very hard water, flushing every 6-12 months is necessary to keep your system running efficiently. 

Test the hardness of your water with a digital hardness meter, an at-home testing kit, or a professional water quality test. 

What If I Have a Water Softener? 

The harder your water is, the faster the scale buildup accumulates. Even if you have a water softener, trace amounts of minerals can still accumulate over time. In other words, a water softener helps reduce scale buildup, which can extend the time between flushes, however it doesn’t eliminate the need for flushing your tankless water heater. 

Will Skipping Flushing Void My Warranty? 

Many tankless water heater manufacturers require regular flushing as part of their warranty terms. If your system fails due to scale buildup and you haven’t performed routine maintenance, you may be responsible for costly repairs. Always check your manufacturer’s guidelines to ensure you’re meeting their requirements. 

DIY vs. Professional Flushing: Do I Need to Call a Pro? 

Flushing a tankless water heater is a nuanced, step-by-step process that must be strictly followed to be safe and effective. It requires plumbing knowledge, the right equipment, and serious attention to detail. If you lack legitimate plumbing experience, it may be a job that is best left to a professional. However, most manufacturers include instructions on how to flush their tankless water heater in the manual, and it’s always possible to learn the skill.  

Before taking on the job on your own, be sure to first understand the basics of how your home’s plumbing system works, what each valve controls, and how your water heater fits into the system. It can be risky taking on the task without assistance, so have a local plumber’s phone number ready in case you need a plumber to quickly come to your home. And if you’re not sure if you’re ready to do it yourself, it’s always best to hire a plumber.  

A trained professional can also inspect the heat exchanger, check for leaks, and ensure your unit is operating efficiently – potentially catching small issues before they become costly repairs. 

The Benefits of Regularly Flushing Your Tankless Water Heater 

At this point you should understand how flushing your tankless water heater regularly helps extend its lifespan, maintain efficiency, and prevent costly repairs. By keeping the heating elements and heat exchanger free from scale, you ensure that your system operates as efficiently as possible, providing reliable hot water whenever you need it. Neglecting this simple maintenance step can lead to higher energy bills, reduced water pressure, and even system failure. 

If you can’t remember the last time your tankless water heater was flushed, Mattioni is here to help. Our team of professional plumbers can flush your system quickly and set you up for continued levels of high efficiency – ensuring you get the most out of your investment. 

To schedule your water heater service or a plumbing inspection, call our friendly team at (610) 400-8510 or book an appointment online. Don’t get stuck with cold showers – schedule your tankless water heater flush today.