Thermal Expansion Tanks Explained

Is a Thermal Expansion Tank required for water heaters?

Also possibly known as a Water Heater Expansion Tank, a Thermal Expansion Tank is an essential unit to pair with a water heater in a closed-loop system. These tanks are required for closed HVAC systems.

When water is heated, it causes molecular agitation - basically, the molecules get excited, pressure builds, and more room is needed. Think about what happens in a pot of boiling water. You need a pot larger than the original volume of water in order to be sure that the water stays contained.

What happens without a Thermal Expansion Tank?

The purpose of the thermal expansion tank is to relieve the pressure built when the water expands during heating. Without the extra space supplied for the water expansion, water pressure can build up and cause many problems. One of the most obvious of these problems is the sudden bursting of a pipe, usually caused by ice building up in pipes in the winter.

A lesser-known issue is an undetected, slow leak which causes the deterioration of pipes and fixtures associated with the hot water system throughout the building. This includes everything from showers and faucets to your washing machine and dishwasher. These essential home structures can only handle a certain level of pressure. If a leak goes undetected, the structure can wind up with damages such as a rotting wood frame and growing into toxic mold growth.

The expansion tank is installed on the line going into the water heater and is pressurized to match the hot water system’s water pressure. There are two primary categories of thermal expansion tanks for your HVAC system - Compression Type and Bladder Type. The primary difference between the two is that the Bladder Tank contains a membrane isolating the air from the water, and the Compression Tank (or Plain Steel Tank) does not.

Plain Steel (Compression) Tanks

Compression-type tanks are typically referred to as an air management system or air management tank and are the original design of expansion tank. Another term for this type of tank is the Plain Steel Tank, which is how we list our WPS line of Compression-type tanks at American Wheatley.

This tank must be placed above the air collector/separator (read more about Air Separators here). This tank absorbs the pressure by allowing water from the boiler to compress air inside this expansion tank. Essentially, the amount of air will remain the same. But as the volume of water builds with the increase in temperature, this type of tank will compress the air to a smaller volume. This stabilizes the pressure by allowing the water volume to expand necessarily during heating.

Plain Steel Expansion Tank
Expansion Bladder Tank

Bladder Expansion Tanks

Bladder Expansion Tanks act similar to the Plain Steel Tank, except they contain a membrane that separates the water from the air being compressed. This prevents air absorption into the water system, eliminates any potential water-logging issues, and prevents tank corrosion. Because the membrane contains the water separately, this type of tank is ideal for potable water use.


We outlined our BDT Bladder Expansion Tank earlier this year in our blog, ‘What is a Multi-Purpose Bladder Expansion Tank?’ Here, we go more into detail about how the BDT or “Multi-Purpose” Bladder Tank works.


We also carry three other Bladder Tank Series. If you’re interested to know more, please check the “Catalog Sheets” Tab on our Bladder Tanks Product page.

Because of the importance of these tanks in your HVAC system, it is important to have a quality unit. An ASME constructed tank is one way to ensure that your tank is safe and reliable. Read more about ASME standards on their site.


This information is just one of many facets of a typical closed HVAC system. If you’re looking for more information or need some guidance, don’t hesitate to reach out to us.