Keep Your Plumbing Efficient and Safe: Learn How to Replace a Water Heater Expansion Tank

If you have a water heater, you may already know about the importance of having an expansion tank installed. However, over time, your expansion tank may start to malfunction or fail altogether, requiring a replacement.

While it may seem hard, replacing a water heater expansion tank is a relatively straightforward process that can be done with basic plumbing tools.

Expansion tank (photo: amazon)

If you are looking to learn how to replace a water heater expansion tank and avoid costly damage, our expert guide is here to help. In this guide, we’ll walk you through the steps to replace your expansion tank, provide essential tips, and help you keep your plumbing in top shape.

Factors to Consider When Buying and Replacing a Water Heater Expansion Tank

What Is an Expansion Tank?

An expansion tank is a safety device that is installed in closed-loop water heating systems to help regulate the pressure and prevent damage to the plumbing system.

When water is heated, it expands and causes pressure to build up in the system. If you don’t replace a water heater expansion tank, the increased pressure from heated water can cause damage to pipes, valves, and other components, leading to leaks or even a ruptured water heater.

By installing a new expansion tank, the excess water pressure is absorbed, reducing the risk of damage and prolonging the lifespan of the water heating system.

Additionally, it can help improve the efficiency of the system and lower energy costs. In short, an expansion tank is an important safety device that can save you money in the long run by preventing costly repairs and replacements.

How Does an Expansion Tank Work?

An expansion tank works by providing a space for the water to expand into as it heats up. The tank is typically installed near the water heater and is connected to the plumbing system with a T-shaped pipe. The top portion of the T-pipe connects to the water heater, while the bottom portion connects to the cold water supply line.

When the water in the water heater heats up, it expands and increases the pressure in the plumbing system. The excess water is then pushed into the expansion tank, which has a flexible membrane inside that expands to accommodate the increased volume of water. This process helps regulate the pressure in the system and prevents damage to the plumbing, such as leaks or burst pipes.

As the water cools down, the pressure in the system decreases, and the membrane in the expansion tank contracts back to its original size. This allows the water to flow back into the system, ready to be heated again the next time it’s needed. By providing a safe and effective way to regulate pressure, an expansion tank helps protect your water heater and plumbing system, keeping your home safe and comfortable.

Expansion Tank Types

If you need to replace a water heater expansion tank, there are three types to choose from:

  • Compression type
  • Bladder type
  • Diaphragm (bag) type

The compression type, also known as plain steel, is the oldest type, and it does not include the bladder. This is just an empty tank with the air cushion above the water, which must be controlled for the proper operation.

The bladder type uses the bladder that keeps the water separated from the precharged air cushion. As the water volume increases in the storage tank, it is directed into the expansion tank, and as it contracts, the fluid goes back into the heating system. The disadvantage of the bladder type is the potential corrosion of the tank, especially when exposed to potable water.

The diaphragm-type is designed as the container, divided into two sections, separated by a non-replaceable rubber diaphragm, which looks like a bag. The system water is contained inside the diaphragm and connected to the plumbing system, while the other side includes air under pressure. The advantage here is that the risk of corrosion is avoided as the metal tank is not directly exposed to the water.

During thermal expansion and volume increase, water enters the expansion tank, moving the diaphragm and compressing the air out to create space for additional water. Any air that enters with the water is then removed from the system into the atmosphere. With these types of expansion tanks, you can prevent the harmful effects of thermal expansion on your water heating system and ensure that it operates efficiently for years to come.

How to Size and Select the Right Expansion Tank for Your Water Heater

Depending on the heater’s capacity, the right size of the potable water expansion vessel should be used so it can safely deal with the thermal expansion.

When shopping for a new expansion vessel, it is important to determine the appropriate size for your heating system. This can be determined by considering the capacity of the storage tank and the pressure of the plumbing system. If you are replacing an old expansion tank, it is recommended to obtain the same size as the previous one.

There is an excellent selection of the expansion tanks for water heaters on, including the popular Amtrol, Watts, and Falcon.

For the proper sizing, you need two pieces of information; the capacity of the storage tank and the pressure of the plumbing system. Then go to the manufacturer site and select the device that meets these two requirements.

You can also use this sizing calculator.

Even if you install a device with a larger capacity, it won’t hurt – it is better to oversize than undersize.

When Should I Replace My Water Heater Expansion Tank?

Expansion tanks for water heaters should be checked periodically to ensure proper functioning. Common signs that it’s time to replace a water heater expansion tank include:

  1. Water leakage: Leakage from the expansion tank can be a sign that it’s damaged or no longer functioning correctly. If you notice water dripping from the tank, it’s time to replace it.
  2. Age: Like most components of a water heating system, expansion tanks have a lifespan. Most expansion tanks last between five and ten years, so if your tank is reaching its maximum lifespan, it’s wise to replace it.
  3. Low water pressure: A malfunctioning expansion tank can lead to decreased water pressure in your plumbing system. If you notice a drop in water pressure, check the tank to ensure it’s functioning correctly.
  4. Inconsistent water temperature: An expansion tank that’s no longer working properly can cause the water temperature to fluctuate or become inconsistent.
  5. Increased water bills: A malfunctioning expansion tank can cause your water bills to increase. If you notice a sudden increase in your water bills, it’s worth checking the expansion tank to ensure it’s functioning correctly.

By knowing when to replace a water heater expansion tank, you can ensure that your water heating system is operating correctly and efficiently.

How to Replace a Water Heater Expansion Tank

Replace a water heater expansion tank following the steps below:

  • Turn off the power to the water heater: Before you start replacing the expansion tank, turn off the power supply to the water heater. If you have an electric heater, switch off the power at the circuit breaker. If you have a gas heater, turn off the gas supply.
  • Turn off the water supply: Turn off the water supply to the water heater by closing the valve on the cold water supply pipe.
  • Drain the water heater: You need to drain some water from the tank to relieve the pressure and make it easier to replace the expansion tank. Connect a garden hose to the drain valve at the bottom of the heater and run the hose to a nearby drain or outside.
  • Remove the old expansion tank: Use a pipe wrench or adjustable wrench to disconnect the old expansion tank from the water heater. If it’s a bladder type tank, carefully depressurize it first by opening the valve on top.
  • Install the new expansion tank: Install the new expansion tank by threading the nipple fitting into the opening on top of the tank. Make sure the tank is oriented in the correct position and that the Teflon tape or joint compound is applied to the threads.
  • Reconnect the water supply: Reconnect the cold water supply line to the water heater, using Teflon tape or joint compound on the threads.
  • Turn the water supply back on: Open the cold water supply valve and turn on a hot water faucet to allow air to escape from the system.
  • Check for leaks: Check all the connections and joints for leaks. If there are any leaks, tighten the connections or replace the faulty parts.
  • Turn the power back on: If you have an electric heater, turn on the power at the circuit breaker. If you have a gas heater, turn on the gas supply and relight the pilot light.
  • Test the expansion tank: Turn on the water heater and let it heat up. Check the pressure on the tank’s gauge to make sure it’s within the manufacturer’s recommended range. You can also test the pressure relief valve to make sure it’s working properly.


Replacing a water heater expansion tank is a simple process that can be done with basic plumbing tools. It’s important to choose the right size expansion tank for your water heater and to follow the manufacturer’s instructions for installation. Properly installing and maintaining an expansion tank can help prolong the life of your water heating system and prevent damage from extreme pressure. If you are not comfortable with installing an expansion tank yourself, it’s recommended to hire a professional plumber to ensure that the work is done correctly and safely.


Are Expansion Tanks Required?

Expansion tanks are required by plumbing codes in most regions for closed-loop water heating systems. The codes mandate the use of expansion tanks to protect the plumbing system and prevent water damage due to thermal expansion. The requirements for expansion tanks vary depending on the local plumbing codes and the type of water heating system installed.

How Do I Know If My Water Heater Expansion Tank Needs to Be Replaced?

You may need to replace your water heater expansion tank if you notice that the pressure relief valve on the water heater is spurting water, or there is extreme pressure on the gauge, or the expansion vessel is full of hot water. Additionally, if you notice any leaks or corrosion on the tank, it may need to be replaced.

How Long Does It Take to Replace a Water Heater Expansion Tank?

Replacing a water heater expansion tank is a straightforward plumbing work that typically takes about 30 minutes to complete. However, the time it takes to complete the installation may vary depending on the type of pipes in the system and the specific installation requirements.

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