If you’ve been using a tank-type water heater at your home, you might have come across terms like water heater expansion tank or thermal expansion tank. But what does a water heater expansion tank mean?

It is basically a safety device designed to defend the plumbing in your house from thermal expansion. Generally, if you own a tankless water heater, the possibility of pressure damage from thermal expansion is not much of a concern. However, for those who own a tank-type water heating system, it is essential to know that your plumbing system might be at risk.

With the help of thermal expansion, once 50 gallons of cold water is heated, it can convert into at least 52 gallons. However, the additional two gallons will no longer fit in your regular water tank. That’s where a water heater expansion tank comes to use.

To figure out whether you require an expansion tank on your heating system, you first need to determine the type of water heater you own, what exactly thermal expansion tank means, and how does the system work. In this article, we are going to answer all these questions to help you understand the basics of the thermal expansion tank and figure out if you actually need it for your water heater.

Table of Contents

What Type of Water Heater Do You Have?

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Whether you need an expansion tank for your water heater or not entirely depends on this question. If you own a tankless water heater at your home, you do not have to worry about thermal expansion. It is because tankless units do not have a primary tank for expansion.

However, if you’re using a conventional tank-type water heating system, you will certainly need to consider if your water heater requires an expansion tank.

How Does a Thermal Expansion Tank Work?

Basically, a thermal expansion tank includes a pressurized air bladder that expands and contracts to absorb additional water.

As the water in your heating system begins to heat, it expands and increases the pressure inside the tank as well as the plumbing system. Instead of allowing more pressure to build, the excess water passes to the expansion tank.

Now, whenever you open the faucet at your home, the hot water stored in the thermal expansion tank is released into your primary system. A water heater expansion tank does not contain water on a regular basis, rather it stores the excess water that overflows from the main tank.

Why Do You Need a Thermal Expansion Tank?

If you’re using a traditional tank-type water heater, not having a thermal expansion tank might be risky. Even though most people do not consider having an expansion tank, we recommend you to invest in one. An expansion tank will protect and increase the lifespan of your appliances, plumbing, and even your water heater.

Believe it or not, overflowing of hot water may cause the pressure inside the tank to rise to a dangerous level, which may even result in the breakage of components. Similarly, overflowing hot water may potentially leak out through the T&P Valve, which can cause extensive water damage.

Even if the components of the system do not break or cause any leakage, not having an expansion tank can still be risky. If your water tank is often filled to capacity, the increased strain may cause the spare parts to wear out sooner than expected and shorten the lifespan of the water heater.

What Size Expansion Tank Do I Need?

Even if you have decided to have an expansion tank, you may end up in a situation where you are unsure regarding the right size of the expansion tank for your water heater. To figure out the right size of the expansion tank for your home, you need to consider two significant factors — water heater capacity and household water pressure.

Water Heater Capacity – To find out the maximum capacity of your water heating system, all you have to do is check the factory label attached to the unit.

Household Water Pressure – To find out the water pressure within the plumbing system of your home, you will require a gauge. All you have to do is attach the gauge to a faucet or hose and turn the water on. For a reliable result, it is recommended to check the water pressure of a 24-hour period. The water pressure is measured in pounds per square inch (psi).

Keep in mind, each expansion tank manufacturer will have a different chart and calculator that will help you indicate the right size and model of the thermal expansion tank for you. There is actually no universal approach to determine the right size of the expansion tank, so you may have to do a bit of research or contact a plumber for the right suggestion.

Basic Guidelines for Selecting the Thermal Expansion Tank Size

If you’re still unsure about what size of expansion tank you should consider for your residential water heater, this chart will provide you a rough idea on how to choose the size of an expansion tank. Please note that this chart is based on the assumption that the temperature of the unit is set to 150-degrees.

Water Heater Capacity (Gallon)Household Water Pressure (psi)Expansion Tank Size  (Gallon)
40 to 6040 to 502
40 to 6060 to 803.2
8050 to 603.2

If the capacity and static supply pressure readings of your water heater do not fall in this chart, your best bet is to contact a professional who can come up with the necessary calculations and help to install the expansion tank.

What if the Expansion Tank is of the Wrong Size?

It is crucial that you purchase the right sized thermal expansion tank by making all the necessary calculations for your water heater. However, if you’re in doubt regarding the size of the expansion tank, it is best to go for a larger tank rather than a smaller one.

A water heater expansion tank that is a bit too large for your system will still be able to handle the excess water. However, if you buy an expansion tank that is too small for your system, it can actually trigger the pressure relief valve as well as temperature. Not only that, but a tank that’s too small also may not be able to store enough water, resulting in overflowing.

How to Install an Expansion Tank?

Installing an expansion tank is sort of a technical task. Therefore, it is better to contact a professional to do the job for you. However, if you enjoy DIY projects and have a few mechanical skills, you should be able to install the expansion tank without hiring a professional.

Installing an Expansion Tank

  • Firstly, determine the water pressure of your household using a pressure gauge. If the water pressure of your house is above 80 psi, make sure to install a pressure reducing valve.
  • Then, examine the air pressure on the expansion tank using a tire gauge.
  • Once you get the result, adjust the expansion tank’s air pressure to match the maximum water pressure using a hand pump. Remember, you should not use an air compressor for this.
  • Now, install the expansion tank in the cold water line.
  • Once the installation is completed, open the faucet and let it open until you notice a steady stream of water. This process is mandatory as it will help eliminate trapped air inside the tank if any.

What is the Actual Life Expectancy of an Expansion Tank?

The life expectancy of an expansion tank depends upon various factors, so it is quite unpredictable. It may depend on various factors like the quality of the tank, proper inflation of the tank, and water quality, to name a few.

While there are cases where an expansion tank has failed in as quickly as two years, most of them are built to last for eight to ten years.

Additionally, the expansion tanks are usually backed by one to five years warranty, depending upon the manufacturers.

Can the Expansion Tank Leak?

Yes, of course! Like any other part of the plumbing system, the water heater expansion tank can leak as well if not maintained properly.

Normally, the expansion tank can face leakages due to the wearing out of the rubber bladder. This causes the tank to stop functioning as it looks for ways to mitigate thermal expansion.

Another reason for expansion tank leakage might be due to the corrosion in the connection between the tank and water piping.

The first thing you should do to ensure you have no leakage issues in the expansion tank is to get it installed by a licensed plumber. A skilled plumber will make sure that the system is connected and positioned properly to minimize the risk of leakage.

Make sure to purchase a high-quality expansion tank that offers a maximum period of warranty to boost the longevity of the system. Also, it is suggested to check the expansion tank at least once a year to get the optimum performance.

What Should I Do If I Have Multiple Water Heaters?

If you have multiple water heaters connected to a single plumbing system, you need to make sure you have a thermal expansion solution that is sufficient for your system. For instance, if you have a 50-gallon water heater, you should at least have a 2-gallon expansion tank.

If you have more than one water heater at your home, you should either have separate small expansion tanks for each of them, or you can also invest in a single tank that is large enough to handle all the units.

Final Say

If you have a traditional storage-tank water heater, it is advised that you invest in an expansion tank. I hope this article has helped you understand the basics regarding the water heater expansion tank and what size of the tank would be suitable for your system. For your convenience, we recommend you to contact a plumbing expert to make sure you have the right size of the tank.

(Last Updated on June 17, 2020 by Sadrish Dabadi)

Nina Howell is a Rewenable Energy researcher and consultant based out of Houston, Texas Area. She earned her Master's Degree in Energy and Earth Resources from Austin Jackson School of Geosciences in 2010, and a Bachelor's Degree in Environmental Science from State University of New York College of Environmental Science and Forestry in 2008. Nina has been working in the energy sector since 2011. She worked as an Energy Supply Analyst from 2011 to 2017 in Bounce Energy and then as a Research and Energy Consultant at GE Renewable Energy from March 2017 to February 2020 . Nina is a mom of 2 beautiful children who are joy to her life. She strongly believes in eco-friendly living and is vocal about renewable energy, environmental issues, water crisis, and sustainable living.