Air Management and Pressurization Part 5: Is It A Compression Tank or an Expansion Tank?

By Chad Edmondson

 As we bring our first series about air management in a hydronic system to a close, we’re going to revisit tanks – not what they do (we’ve already covered that) but what we call them.  Specifically, we want to address whether or not there is a difference between what some industry professionals call an “expansion tank” and what others call a “compression tank.” 

Short answer:  It depends on whom you ask.


Long answer:  Yes, there is a difference.  But practically speaking about today’s usage, the words are frequently used interchangeably.  So it is very important to clarify the physical attributes of the tank in question.  

 This all begs a little bit of a history lesson.  Remember in Part 1 when we said that many years ago most hydronic systems were design as open systems and the water was allowed to expand in a tank that was open to the atmosphere?  Technically, those were expansion tanks – not the closed vessels (plain steel or bladder/diaphragm tanks) we use today. 

Compression tanks are what we use today, and what we’ve been discussing throughout this series on Air Management – even though we too sometimes refer to them as expansion tanks.  Compression tanks are pneumatic cushioning devices that absorb the fluctuating pressures result from temperature changes in the water.  Yes, water is allowed to “expand” into these tanks, but they are not open to the atmosphere like the original “expansion tanks” were.  Rather they contain an air cushion that “compresses” as the water expands.  A compression tank can be a plain steel tank where the air cushion and water actually touch or a bladder/diaphragm tank where there system water and air cushion are separated by a physical barrier.

 It’s easy to see how the term “expansion tank” got carried over as a way of referring to the tanks used in more modern closed systems.  After all, they do give the water room to expand – but technically they were and always will be designed to be compression tanks that allow for the expansion of water in a closed hydronic system.