What Is a Water Heater Thermal Expansion Tank and Is It Necessary?

Posted by
John Woodard on February 17, 2023

A water heater, like many things, is only noticed when it is not working. In 2007 and 2008, videos of water heater explosions went viral, and many viewers became concerned about the safety of their home water heater. Around the same time, building codes were updated to accommodate the excess pressure inside these systems. One of these updates involves the use of expansion tanks with water heaters to relieve pressure caused by thermal expansion. Below you can find information about what water heater expansion tanks are, whether they are necessary, their pros and cons, and how to test for a faulty expansion tank.

What is a water heater expansion tank?

A water heater expansion tank, also known as a thermal expansion tank, is a small overflow tank that compensates for the thermal expansion of water inside a water heater. As the temperature of water increases, the water expands. Without relief, the pressure inside the water heater will increase, causing damage to valves, plumbing, or the water heater itself. Prior to the requirement of check valves, excess pressure would force water back into the municipal water lines. Regulations on backflow have necessitated the use of check valves or other similar devices to prevent contamination of the public water supply, but they have also placed greater stress on water heaters. Thermal expansion tanks solve this problem by accommodating overflow water that results from this added pressure.

Learn more: What is a backflow prevention device?

Do I need a thermal expansion tank for my water heater?

Expansion tanks are required by code in all new water heater installations in closed systems. A closed water system uses check valves, backflow prevention devices, pressure-reducing valves, or other devices on the supply line to prevent water from being sucked back into the municipal water supply. In 1992, the EPA required municipal water suppliers to protect their lines from backflow contamination. This means homes must incorporate check valves to prevent water from backing up into the supply lines. As a result, these homes are closed-loop systems, so all new water heater installations must incorporate a thermal expansion tank alongside the water heater.

If your current water heater is not equipped with an expansion tank, the addition of a water heater expansion tank can benefit your system. High pressure in a water heater can lead to problems such as leaks, ruptures, and corrosion within your system, all of which can be expensive to fix. While a water heater expansion tank is an added cost, it can save you money in the long run by preventing these problems.

What size water heater expansion tank do I need?

The size of your water heater expansion tank should be based on the capacity of your tank and your home’s water pressure. The capacity of your water heater will be printed on the tank’s exterior, and your home’s water pressure can be measured with a pressure gauge. An expansion tank that is too large for the system will not adversely impact its performance. If you are unsure if one size of expansion tank will be large enough for your system, you can size up without fear of negative effects.

To ensure your expansion tank is large enough for your system, stick to the following sizes for your expansion tank.

Water heater capacity up to 60 gallons

  • Water pressure < 60 psi – 2-gallon expansion tank
  • Water pressure 60 psi or greater – 3.2-gallon expansion tank

Water heater capacity > 60 gallons

  • Water pressure < 50 psi – 2-gallon expansion tank
  • Water pressure 50-60 psi – 3.2-gallon expansion tank
  • Water pressure > 60 psi – 4.4-gallon expansion tank

How much do water heater expansion tanks cost?

The overall cost of a water expansion tank installation, including the cost of the tank, averages a little over $200. The ease of access to your system affects the labor cost of installation, as well as the size of the expansion tank being installed. An expansion tank can be installed without a professional, but you risk additional costs if you install the tank incorrectly.

How often do you need to replace a water heater expansion tank?

A thermal expansion tank will need to be replaced once every five to ten years. The best way to elongate the life of an expansion tank is to ensure the tank’s pressure matches the incoming water pressure. This preserves the life of the bladder and prevents leak issues with valves. Regardless of how well a tank is maintained, it will need to be replaced at around the ten-year mark.

Advantages of water heater expansion tanks

Water heater expansion tanks are so beneficial to your home’s plumbing system that they are now required by code. Thermal expansion tanks provide the following advantages to your water heater system.

  • Prevent leaks caused by pressure buildup
  • Provide a second layer of security alongside the water heater’s pressure relief valve
  • Save money caused by water damage or pipe bursts
  • Inexpensive to purchase and install
  • Take up little space

Disadvantages of water heater expansion tanks

If you do not have a water heater expansion tank in your home because the system was installed before they were required, you may wonder if an expansion tank should be installed in your current system. While water heater expansion tanks are extremely advantageous, they do possess a couple of disadvantages.

  • Many components are susceptible to failure
  • Small installation errors can cause problems within the system

With proper installation and calibration, a water heater expansion tank is a reliable safety feature of any water heater. Even if an expansion tank fails, failure within the tank is less expensive and problematic than failure within a home’s plumbing. Consequently, the benefits of thermal expansion tanks outweigh their negatives.

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How to test a water heater expansion tank

Water heater expansion tanks can fail easily. As a result, it is a good idea to test if your tank is working if you believe something is off in your system. You can perform simple tests that indicate high water pressure in a system, or you can test the pressure inside the tank itself.

Tap the outside of the tank

Using a metal object to tap the exterior of the expansion tank can give you a clue that something is wrong, but it should not be used as a final determiner. In a properly functioning expansion tank, water will fill the bottom of the tank, and air will fill the top. Lightly tap the top and bottom of the tank with a metal object. The sounds these sides give off should be distinctly different from each other. If the sounds are similar, the tank may not be filling as it should.

Observe water pressure

If you do not use hot water for an extended period, observe the water pressure coming out of the next fixture where use hot water. If the water surges out faster than normal and calms down shortly after, there is pressure buildup within your system, and the expansion tank may be at fault. Many other factors, such as faulty water pressure regulators, can cause pressure fluctuations within a system. Consequently, this method is unreliable in determining if an expansion tank is faulty. However, it is a warning sign that should prompt further investigation into the problem.

Learn more: What is a water pressure regulator?

Check the air valve

All thermal expansion tanks contain an air valve called the Schrader valve that looks similar to the air valve on a tire. Bleed a small amount of air out of the Schrader valve. If air comes out, the tank is operating correctly. However, if water leaks out of the valve, the tank’s bladder has ruptured. A ruptured bladder cannot be repaired, so the tank must be replaced. At this stage, it is recommended to check the tank’s pressure to ensure that the bladder or other tank components are not at risk of damage.

Check the tank’s pressure

When a water heater expansion tank is installed, its pressure must be set to match the incoming water pressure. Expansion tanks are pre-charged to about 40 PSI, but the pressure must be adjusted based on the home’s water pressure. If the pressure within the tank is too high or too low, it can cause the tank to fail. Should this happen, the rest of your plumbing system risks being damaged by high water pressure. You should check the pressure on your expansion tank at least once each year. Throughout the life of the tank, the pressure inside should remain consistent with the pressure it was initially charged with upon installation.

To check a thermal expansion tank’s pressure, you can use the same pressure gauge that you use on your car’s tires. Follow these steps to discover the pressure within the tank.

  1. Remove the cap protecting the valve at the top of the tank. This valve is known as the Schrader valve.
  2. Use a tire pressure gauge on the Schrader valve the same way you would use it on a tire.
  3. Ensure the pressure inside the tank matches your home’s water pressure. This number should match what the tank was initially charged with. If you do not know your home’s water pressure, you can learn how to use a pressure gauge to check it yourself.

Now that you know your expansion tank’s pressure, you may need to lower or raise the pressure inside. To lower pressure within the tank, allow small amounts of air to escape, checking the pressure inside the tank after each interval. Once the pressure matches the desired PSI, you are finished.

To raise the pressure inside a water heater expansion tank, you will need a small tire hand pump. Ensure that you do not use a motorized pump, as it may increase the pressure too rapidly, causing the bladder to rupture. Pump small amounts of air into the tank at a time. Measure frequently to ensure the pressure does not rise above the desired PSI and stop immediately once the appropriate pressure is reached.



If you have any additional questions, please do not hesitate to contact us.

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