Does A Navien Tankless Water Heater Need An Expansion Tank - The

You’ve made a smart investment in a high-efficiency Navien tankless water heater, looking forward to endless hot water and lower energy bills. But now you’re hearing whispers in online forums or from a neighbor about needing something called an “expansion tank.” It’s a common point of confusion that can leave you wondering if your installation is incomplete or even unsafe.

Don’t worry—we’re here to clear the air. As HVACR professionals, we see this question all the time, and the answer is more straightforward than you might think. The truth is, whether you need an expansion tank has less to do with your Navien unit itself and everything to do with your home’s plumbing system.

We promise to give you the straight-up, no-nonsense answer. This comprehensive does a navien tankless water heater need an expansion tank guide will walk you through what thermal expansion is, how to know for sure if your system requires an expansion tank, and why this small component is crucial for protecting your big investment.

Let’s dive in and make sure your system is set up for long-term success.

What is a Thermal Expansion Tank, Anyway?

Before we can answer the main question, let’s quickly cover the basics. Think of an expansion tank as a safety valve or a shock absorber for your plumbing system.

It’s a small metal tank, usually installed on the cold water line leading to your water heater. Inside, a rubber diaphragm separates a chamber of pressurized air from the water in your pipes. Its one and only job is to handle a law of physics called thermal expansion.

When water is heated, it expands. It’s a simple fact. In a traditional tank-style water heater, the extra volume created by this expansion could be absorbed by the air cushion at the top of the tank. But in a modern, sealed plumbing system, that expanded water has nowhere to go.

This is where the expansion tank comes in. As water heats up and its volume increases, it pushes against the diaphragm in the tank, compressing the air on the other side. This safely absorbs the excess pressure, preventing it from building up to dangerous levels in your pipes, fixtures, and your Navien heater itself.

The Critical Difference: Open vs. Closed-Loop Plumbing Systems

Here’s the absolute key to this entire discussion. The need for an expansion tank isn’t determined by the brand of your water heater, but by the type of plumbing system you have.

Years ago, most homes had “open” systems. Water could expand back out toward the municipal water main, relieving pressure naturally. However, modern plumbing codes have changed this for safety and water conservation reasons.

Today, most homes have a “closed-loop” system. This means there is a one-way valve that prevents water from flowing back into the city’s supply. This device is typically one of the following:

Pressure Reducing Valve (PRV): A bell-shaped valve that reduces high municipal water pressure to a safe level for your home. It also acts as a check valve.
Check Valve or Backflow Preventer: A device specifically designed to stop water from flowing backward, preventing potential contamination of the public water supply.

If your home has one of these devices (and it likely does), you have a closed-loop system. When your Navien heats water, the expanded volume gets trapped between your faucets and that one-way valve, causing a rapid and potentially damaging spike in water pressure.

So, Does a Navien Tankless Water Heater Need an Expansion Tank?

Now for the big reveal. We’ve established what an expansion tank does and what a closed-loop system is. This brings us to the definitive answer.

Yes, a Navien tankless water heater absolutely needs an expansion tank if it is installed in a home with a closed-loop plumbing system.

Let’s be crystal clear: the Navien unit itself doesn’t create the need. The closed-loop system creates the need. Any water heater—tank or tankless, from any brand—installed on a closed-loop system requires a thermal expansion tank to operate safely and meet plumbing codes.

Because Navien units are on-demand and don’t store a large volume of hot water, some people mistakenly believe thermal expansion isn’t an issue. However, even the small amount of water inside the unit’s heat exchanger and the hot water in the pipes leaving the unit will expand, and that pressure needs a safe place to go.

How to Quickly Check If You Have a Closed-Loop System

Not sure what kind of system you have? You can perform a quick check. Don’t worry—this is just a visual inspection.

Find where the main water line enters your house. This is often in a basement, crawl space, or utility closet near the front of your home.
Look for a water meter. Near the meter or the main shut-off valve, scan the pipe for a bell-shaped brass device. This is a Pressure Reducing Valve (PRV).
If you see a PRV or any other valve that is clearly labeled as a “check valve” or “backflow preventer,” you have a closed-loop system.

If you find one of these, and you have a Navien tankless water heater, installing an expansion tank is not just a recommendation—it’s a necessity.

The Dangers: Common Problems When Skipping an Expansion Tank

Ignoring the need for an expansion tank can lead to a cascade of issues, turning your high-efficiency upgrade into a source of constant headaches. The uncontrolled pressure spikes from thermal expansion act like a tiny hammer, repeatedly striking every component in your plumbing system.

Here are some of the common problems with does a navien tankless water heater need an expansion tank setups that lack this crucial part:

Constant Dripping from the T&P Valve: The Temperature and Pressure (T&P) relief valve on your Navien is a safety device designed to open and release water if pressure gets too high. If it’s constantly dripping, it’s a red flag that thermal expansion is not being managed.
Premature Failure of the Navien Unit: The delicate internal components of your Navien heater, like the heat exchanger and sensors, are not designed to withstand repeated pressure spikes. This stress can lead to premature failure and costly repairs that are not covered by the warranty.
Damage to Fixtures and Appliances: The high pressure can damage faucet cartridges, toilet fill valves, washing machine solenoids, and ice maker lines throughout your home.
Leaking Pipes: Over time, the constant stress can weaken pipe joints and fittings, eventually causing leaks in your walls or ceilings.

Failing to install a required expansion tank can also void your Navien manufacturer’s warranty, as it’s considered an improper installation.

The Benefits of Doing It Right: Why an Expansion Tank is a Smart Move

Instead of viewing it as another expense, think of an expansion tank as an affordable insurance policy for your entire plumbing system. The benefits of does a navien tankless water heater need an expansion tank being properly installed are significant.

Key Advantages of an Expansion Tank

Protects Your Navien Investment: This is the number one reason. It ensures your high-tech water heater operates within its specified pressure range, dramatically extending its service life.
Safeguards Your Entire Home: It protects every faucet, toilet, and water-using appliance from the damaging effects of high pressure.
Ensures Code Compliance: In virtually all jurisdictions, installing an expansion tank on a closed-loop system is required by the Uniform Plumbing Code (UPC) or International Plumbing Code (IPC).
Promotes a Sustainable and Eco-Friendly System: A well-maintained system prevents water waste from a dripping T&P valve. More importantly, by extending the life of your Navien heater and other appliances, you are reducing manufacturing and landfill waste. A durable system is an eco-friendly does a navien tankless water heater need an expansion tank system.
Gives You Peace of Mind: You can relax knowing your system is safe, protected, and installed according to professional best practices.

Frequently Asked Questions About Navien Expansion Tanks

Where is an expansion tank installed?

The expansion tank should always be installed on the cold water supply line between the main water shut-off and the Navien tankless water heater. It’s often tee’d off this line and mounted securely to a nearby wall or joist.

What size expansion tank do I need?

For most residential tankless applications, a 2.1-gallon thermal expansion tank (like the Amtrol ST-5) is sufficient. However, the correct size depends on your home’s water pressure and the total volume of your hot water piping. A qualified plumber can calculate the exact size needed for your specific setup.

Can I install an expansion tank myself?

While it may seem simple, this is a job best left to a licensed plumber. The expansion tank’s air pressure must be pre-charged to match your home’s water pressure before installation. An incorrect charge can render the tank useless or even create new problems. A professional will have the tools and expertise to do it right.

How do I know if my expansion tank has failed?

A simple check can give you a clue. Gently tap on the tank. The top half (air side) should sound hollow, while the bottom half (water side) should sound solid. If the entire tank sounds solid, the internal bladder has likely failed, and the tank is full of water (waterlogged). Another sign of failure is water sputtering out of the air valve (Schrader valve) when you briefly depress the pin. If it has failed, it needs to be replaced immediately.

Your Final Check and Path Forward

So, let’s bring it all home. The question was never really about the Navien brand, but about the physics of your plumbing.

To summarize, if your home has a pressure reducing valve or a backflow preventer—creating a closed-loop system—then you absolutely need a thermal expansion tank to protect your Navien water heater and your entire plumbing infrastructure. It’s a small, inexpensive device that prevents very large, very expensive problems.

This isn’t just a recommendation; it’s a critical component for safety, longevity, and performance. Following this does a navien tankless water heater need an expansion tank care guide ensures your system remains efficient and reliable for years to come.

Protecting your high-tech Navien heater is simple when you understand the whole system. Stay safe, stay informed, and enjoy that endless hot water!

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David Clark

Founder & Lead HVAC Specialist at HVACRLY

David Clark is the founder of HVACrly and a certified HVAC specialist with 10+ years working on residential and light-commercial systems. He turns complex heating, cooling, ventilation, and refrigeration topics into clear, actionable guides focused on safety and energy efficiency. When he’s not on a service call, he’s testing tools and writing step-by-step tutorials.