In this article let’s learn in detail about Tankless Water Heater Expansion Tank. We explain what is an expansion tank, what are its functions, where and when it is needed. Many people who use conventional tank water heater know about the thermal expansion tank, as it the major safety component which ensures the safety and longetivity of the water heater. So, when you upgrade to a  tankless water heater a doubt araises “do tankless water heaters need expansion tanks?”. So, read below to know clearly before buying a thermal expansion tank for tankless water heater.

What is an expansion tank in water heaters?

An expansion tank, also known as a pressure vessel, is an essential component in maintaining a stable water pressure in a water heating system. It is a metal container with an elongated or rounded shape that is hermetically sealed to prevent any air or water leakage. The interior of the container is divided into two parts by means of an elastic waterproof membrane. One part is in direct contact with the water in the system, while the other part is filled with air or some type of gas, usually nitrogen.


The expansion tank serves to absorb the increase in water pressure that occurs when the water is heated. As the water is heated, it expands and increases in volume. Without an expansion tank, this increase in volume can cause pressure to build up in the system, which can cause leaks and damage to the pipes and the heater also. The expansion vessel acts as a safety device, absorbing the excess pressure and preventing damage to the system.


The way it works is quite simple, when the water is heated, it expands and presses against the elastic membrane, compressing the air or gas contained in the other part of the container. The air or gas thus absorbs this increase in pressure, ensuring that a stable pressure is always maintained in the system. This stable pressure is crucial for the proper functioning and longevity of the system.

Do tankless water heaters need expansion tanks?

There are two answers YES and Also NO. It depends on which type of tankless water heating system you are using. To understand clearly we will explain few scenarios where expansion tanks are needed and where they are not needed. Before that read the regulation on Thermal expansion control given in 2012 International Plumbing Code (IPC) in the below image.

international plumbing code 2012 on thermal expansion control

Although the above IPC doesnt specify about the tankless water heaters we can understand the concept of it.

According to IPC a thermal expansion tank is not specifically needed for tankless water heaters. But if a tankless heater has a storage tank connected in it to store hot water maybe for recirculations, creates pressure from themal expansion. So, the increased pressure must be stabilized.

Thermal expansion tank for tankless water heaters are available in different sizes like 2 gallon and 5 gallon tanks for domestic water heaters. Larger water heaters require bigger expansion tanks.

The para 607.3.2 is for the storage tank water heaters. So, that doesnt imply for tankless water heaters.

Now lets see few scenarios of tankless water heater systems, so you can understand where and where not thermal extension tanks are needed.

Scenario 1: With no storage tank

In this tankless water heating system, the water is only heated when the hot water outlet is open. When the hot water outlet is open, cold water flows through the heating system, gets heated, and exits through the outlet. The thermal expansion of the water occurs, but as the system is open, the pressure is released directly through the hot water outlet.

tankless water heater without expansion tank opens system

When the hot water outlet is closed, this system becomes closed. But as the water flow stopped the heater stops heating the water, thus no pressure is formed. Therefore thermal expansion tank is not needed in this system.

Scenario 2: With storage tank, Closed systems, Recirculations

This is a closed type as the water service line has a backflow preventer (such as a dual check valve at the water inlet and outlet). Some tankless water heater systems require storage tanks for recirculation of hot water to decrease the delay in providing hot water at any outlet tap or when to provide large amounts of hot water due to the high demand.

tankless water heater with expansion tank and storage tank recirculation diagram

Just an example to show the circuit.

In this system, initally hot water is sent to storage tanks and if there is no demand the hot water stays for long time in tanks and gets cooled down, these water again sent to the heater and gets heated with the help of pump. Heated water get back to the tanks. This process is called recirculation, if there is no demand in hot water and outlet taps are closed, pressure is created due to thermal expansion. So, expansion tank is needed in this system.

Note: The pressure relief valve on a water heater cannot be used as the device to control the pressure caused by thermal expansion. The Uniform Plumbing Code requires thermal expansion tanks if there is a check valve, pressure regulator, or backflow preventer installed in the main supply line, regardless of the type of water heater.

With Buffer tanks:

Few tankless water heater models are build with integral buffer tanks to increase the efficiency by absorbing the excessive heat and release them when needed. These tanks will be small but they can create high pressures due to thermal expansion. Thus an expansion tank must be incorporated in the system to prevent it from busting due to high pressures.

Location to Fix:

Expansion tanks are advised to fix on the cold water line near to the heater, but you can place on cold pipe anywhere but without any values in between heater and expansion tank.


Expansion tank is not required for tankless water heaters. But if it has storage tank expansion tank is required.

If you have any questions or concerns, don’t hesitate to leave a comment below! Our team is here to assist you and provide all the information you need. Your feedback is important to us, so please feel free to share your thoughts and we’ll respond as soon as possible.

Categorized in: