Does A Tankless Water Heater Need An Expansion Tank?

Expansion tanks, though not exclusive to tankless water heaters, have long been associated with traditional storage tank water heaters. These devices play a crucial role in managing thermal expansion, a natural occurrence within water heating systems. A tankless water heater, known for its energy efficiency and on-demand hot water supply, has become a popular choice for modern homes.

But as homeowners transition to these compact and efficient systems, questions often arise about the need for an expansion tank. In this comprehensive guide, we’ll explore the role of expansion tanks in tankless water heater systems and address the crucial question: Does a tankless water heater need an expansion tank?

The Basics of Tankless Water Heaters

Tankless water heaters, often referred to as on-demand or instantaneous water heaters, have revolutionized the way we access hot water. Unlike traditional storage tank water heaters, which maintain a reservoir of hot water, tankless units heat water directly as it flows through the unit. 

This results in several benefits, including energy savings and an endless supply of hot water. However, this innovative technology also raises questions about managing thermal expansion.

Understanding Thermal Expansion

Thermal expansion is a natural phenomenon that occurs when water heats up. When the water inside your plumbing system warms, it expands, increasing in volume. In a traditional water heater, with a storage tank, this expansion is accommodated within the tank. However, tankless water heaters do not store hot water, making thermal expansion a topic of concern.

The Role of Expansion Tanks

Expansion tanks are typically associated with traditional water heaters, but their purpose remains vital in tankless systems. When thermal expansion occurs, it creates additional pressure within your plumbing system. This increase in pressure can strain pipes, valves, and the tankless water heater itself. Expansion tanks, equipped with an air-filled chamber, serve as a buffer, absorbing the excess pressure and preventing potential damage.

Factors Influencing the Need for Expansion Tanks

Local Water Pressure

The water pressure in your plumbing system can vary depending on where you live. In some regions, water pressure is naturally high, and this can significantly influence the need for an expansion tank. High water pressure means that water flows through your pipes with more force. When this high-pressure water is heated inside your tankless water heater, it can cause more substantial thermal expansion. 

This makes the installation of an expansion tank even more important. It acts as a pressure relief valve, absorbing the excess pressure created during thermal expansion and preventing potential damage to your plumbing system.

Check Valves

Modern plumbing systems often include a component called a check valve. Check valves are designed to prevent backflow, ensuring that water flows in one direction. While check valves are essential for maintaining the integrity of your plumbing, they can inadvertently create a closed system. In a closed system, water can’t flow backward into the municipal supply when it expands due to heating. 

As a result, the pressure in the system increases, and this is where the expansion tank comes into play. It provides a designated space for the expanding water to go, relieving the pressure and safeguarding your plumbing system from the potential stress caused by thermal expansion.

Tankless Unit Size

The size of your tankless water heater is a key factor when addressing expansion tank considerations. Larger tankless units, designed for higher water heating rates, may experience more significant thermal expansion due to the increased water flow. Therefore, if you have a larger tankless unit, it’s essential to evaluate the potential for temperature sensor failure and the necessity of an expansion tank based on your unique situation.

The increased heating capacity of larger units can generate more thermal expansion, making the tank more relevant for these setups.

The size of your tankless water heater matters. Larger units heat water at a higher rate, potentially leading to more significant thermal expansion.

Installation and Maintenance

Installing an expansion tank with a tankless water heater is a relatively straightforward process. It’s typically connected to the cold water line entering the water heater. Ensuring proper air pressure within the tank is crucial for its effective functioning. Routine maintenance involves checking and adjusting this air pressure to keep the tank in optimal condition.

Advantages and Disadvantages

As with any home improvement decision, there are both advantages and disadvantages to consider when installing an expansion tank with your tankless water heater.


Protects Plumbing System

An expansion tank safeguards your plumbing system from the potentially damaging effects of thermal expansion.

Preserves Appliance Lifespan

By reducing stress on your tankless water heater and plumbing, an expansion tank can extend the lifespan of these components.

Ensures Consistent Wate

Pressure: It helps maintain stable water pressure, preventing fluctuations that can be frustrating during a shower or while using appliances.


Upfront Cost

Expansion tanks come with an initial cost for purchase and installation.

Space Requirements

They require additional space in your home, which may be a consideration in smaller utility areas.


While relatively low, expansion tanks do require periodic maintenance to ensure they function correctly.

here’s a table summarizing the key points and factors discussed in the article:

Factors Influencing the Need for an Expansion Tank
Local Water Pressure
The Presence of Check Valves
Tankless Unit Size
Local Plumbing Regulations
Closed vs. Open Systems
Professional Evaluation

This table provides a quick reference to the factors influencing the need for an expansion tank in tankless water heater systems. It helps homeowners make informed decisions based on their specific circumstances and local regulations.


Is an expansion tank necessary for all tankless water heaters? 

An expansion tank’s necessity depends on factors like water pressure and the presence of check valves. Not all tankless systems require one.

Can I install an expansion tank myself, or should I hire a professional? 

While some DIY enthusiasts may install expansion tanks, it’s often advisable to seek professional installation to ensure it functions optimally.

Are there energy efficiency benefits to using an expansion tank with a tankless water heater? 

While expansion tanks primarily address safety concerns, they can indirectly contribute to energy efficiency by reducing stress on the water heater.

What’s the typical lifespan of an expansion tank?

 Expansion tanks are durable and can last for many years. Regular maintenance is key to ensuring their longevity.

Are there alternatives to expansion tanks for managing thermal expansion in a tankless water heater system? 

Yes, some alternatives, like installing a pressure-reducing valve or a thermostatic mixing valve, can help manage thermal expansion. These options may be more suitable for specific situations.


In conclusion, the necessity of an expansion tank in a tankless water heater system is not a simple ‘yes’ or ‘no.’ Instead, it’s a nuanced decision influenced by various elements, including local water pressure, check valves, unit size, local regulations, and system configuration. In the realm of tankless water heaters, determining the necessity of an expansion tank isn’t a straightforward, one-size-fits-all solution.

These variables interact to define whether or not an expansion tank should be a part of your tankless water heater system. So, it’s essential to assess and weigh these elements to make an informed choice that suits your specific situation.

Understanding these factors is key to making an informed choice that best suits your specific situation, ensuring the efficient and stress-free operation of your tankless water heater. So, while the question may not have a uniform answer, it’s one that can be resolved with careful evaluation and professional guidance.

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