What is a thermal expansion pack?

A thermal expansion tank can prevent your water heater from early failure or serious damage.

Not everyone has one of these connected to their system, but it sure isn’t a bad idea. Whether you need one or not could depend on what type of water heater you have.


A thermal expansion tank looks a lot like the propane tank you hook up to your barbeque grill. It’s a smaller tank that is connected to the top of your water heater system. There is a threaded pipe connection on the bottom and Schrader valve (like you see on some bicycle tires) on the top.

How Do Thermal Expansion Tanks Work?

The water in your main tank expands at it heats up. This is referred to as thermal expansion. This extra pressure can generally be absorbed by surrounding water supply system, and it becomes negligible. But over time, this pressure can also become trapped in your home’s plumbing system and put stress on your fixtures, supply lines, and appliances.

A thermal expansion tank is meant to absorb some of this excess pressure. A portion (about half) of the tank is filled with water from your home’s system. The other half contains compressed air. The middle of the tank has a butyl rubber bladder.

When the water in your water heater gets hot and expands, it will push against that rubber bladder and compress the air on the other side. This reduces your water heater’s internal pressure to safer levels and protects your tank and the rest of your home’s systems from wear, tear, and damage.

Do I Need a Thermal Expansion Tank?

Many people don’t actively think about what sort of water heater is in their home, but you’ll need this information to figure out if you should get a thermal expansion tank. For example, if you have a tankless water heater, you probably know this, and you don’t need one of these extra systems.

However, a traditional tank-style water heater could benefit from a thermal expansion tank, particularly ones that are on a “closed” plumbing system. With this type of system, water can’t return to the city water lines once it enters your home. This means that the pressure has no place to escape.

In fact, some manufacturers will even void your warranty if you are on a closed system and don’t have a thermal expansion tank installed. In some cities across the country, this is also a requirement.

Not sure if you have a closed or open system? If your home has a backflow prevention device installed on its main water line, which prevents water from flowing back to the water supply, you have a closed plumbing system.

If you need a thermal expansion tank install in your home, or have questions about this system, contact us. Fast Water Heater will inspect your system, answer your questions, and provide you with a quote for the work.

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