Tankless Water Heater Expansion Tank: A Must-Read Guide Before You Buy

Learn what is an expansion tank and Do we need thermal expansion tank for tankless water heater. If yes, what are the reasons and if No, what are the reasons.

In this article let’s learn in detail about Tankless Water Heater Expansion Tank. In February 2024 when joseph went to install a tankless water heater in Colorado replacing with the traditional tank heater. So, while installing it the home owner got a doubt about the expansion tank and asked joseph, does a tankless water heater need an expansion tank?. Without any delay Joseph explained her what is an expansion tank, what are its functions, where and when it is needed and finally is it required in tankless setup.

Many people who use conventional tank water heater know about the thermal expansion tank, as it the major safety component. So, when you upgrade to a tankless unit multiple doubts arises. So, read below conversation between Joseph and Home owner to know clearly before buying a thermal expansion tank for a tankless water heater.

Homeowner: Hey Joseph, what is this cylindrical thing on this old water heater?

Joseph: hmm, this is a thermal expansion tank, it is used to control the pressure inside the hot water tank when the water expands while heating.

Homeowner: Do we need it on our new tankless unit?

Joseph: No, we don’t need it in our tankless setup, this tankless unit doesn’t has any storage tank and it heats water only when there is a demand of hot water. As this is an open system the pressure generated with thermal expansion goes out through the hot water faucet itself.

Homeowner: Ok, I got it. but as you said ours is an open system, so what is a closed system and do we need an expansion tank in closed system?

Joseph: When the hot water faucets are farther from the heater, as soon as we turn on the faucet water flows which turn on our tankless heater and heats water. But, it takes few seconds for the hot water to reach the faucet till then you get cold water. To avoid this and get instant hot water buffer tanks with recirculation systems are used where a small tank is placed near the faucets and hot water recirculates regularly. This type of system is called a closed system and as it has a buffer storage tank in it, we need to install an expansion tank in the system.

What is an expansion tank in water heaters?

An expansion tank is an essential component in maintaining a stable water pressure in a water heating system. It is a metal container with an elongated or cylindrical shape that is airtight 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 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. 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 also to the heater.

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 doesn’t 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 for recirculation’s, creates pressure from thermal expansion. So, the increased pressure must be stabilized with it.

Expansion tank for tankless water heaters are available in different sizes like 2 gallon and 5 gallon tanks you can choose them according to the heater size and capacity. Larger water heaters require bigger expansion tanks.

The para 607.3.2 is for the storage tank water heaters. So, that doesn’t points to 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: Open system with no storage tank

In this scenario, the water is heated only when the hot water faucet is open. When the hot water faucet is open, cold water flows through the heater gets heated, and exits through the faucet. 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 faucet is closed, this system becomes closed. But as the water flow stopped the heater stops heating the water, thus no pressure is formed. Therefore expansion tank is not required in this system.

Scenario 2: Closed system with storage tank, Recirculation’s

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 if they are at longer distance from the heater 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, initially 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.

Location to Fix:

Expansion tanks are advised to fix on the cold water line near to the heater, you can place on cold water line 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 on expansion tanks or about water heaters don’t hesitate to ask joseph by leaving a comment below! Joseph is here to assist you and provide all the information you need. Your feedback is important to us, so please feel free to share if this information helped.

TanklessHelp Staff

TanklessHelp Staff has an experienced writers and researchers in the field of water heating technology. With a background in HVAC and plumbing, Tanklesshelp staff has in-depth knowledge in many types of waterheaters in America and Canada. They have a passion for educating homeowners and businesses on solving easy to hard repairs and guide them on how to maintain them to save money.

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