What is an Expansion Tank?

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  • Written By: C. Daw
  • Edited By: O. Wallace
  • Last Modified Date: 06 October 2019
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An expansion tank is a small tank that is generally used to contain excess water pressure within domestic and closed water heating systems. Thermal expansion is created during the heating of water, which can lead to problems if the system does not have a way to store some of the excess amounts of water and steam. The tank itself is designed to have two separate sections, each side separated by a rubber diaphragm, which allows the tank to push steam from one side to the other without having to manually expel it. Domestic applications of this device are also equipped to conform to basic drinking water regulations, so these tanks allow a heating system to vent pressure properly without causing a decrease in every day usability.

The basic purpose of the expansion tank is to ensure that the plumbing lines stay full with water, and that the thermal expansion does not burst pipes or heating units. As the temperature rises, the water within the pipes expand when the expansion tank takes over. It allows the excess volume to flow into it. After the temperature decreases the water will contract, leaving open spaces within the pipes. Once again, the expansion tank solves this problem by expelling the extra amounts of volume that it has stored, allowing the plumbing lines to stay full of fluid at all times. The less air that plumbing lines have, the better the entire system works.


Expansion tanks come in various shapes and sizes because each specific system will call for a specially designed unit. High temperature plumbing systems will require a different tank than cold systems will. The more water that is pushed through the lines, the bigger the tank will have to be in order to accommodate the excess amounts. In most basic systems, the temperature within the tank can not go over a certain limit, and the storing capacity of the tank can not be more than the amount of extra water expected, along with the air that is already present within the unit. Basically, the tank has to be large enough to accept all of the excess water and steam, while allowing plenty of room at the top for the air that is present within it.

All domestic and closed heating systems have to be installed with a tool to release pressure within the lines, as well as to allow extra water to flow freely back and forth, which is what the expansion tank is designed for. The plumbing lines of today have far less air within the lines, air which used to allow spaces for expansion, so this tank was designed specifically with that in mind. To make this tank even more valuable within a heating system, an air regulator is also used on top of it, which allows the tank to not only do what it was made for, but to release the excess pressure and air from the system without having to have constant monitoring and maintenance. The expansion tank is a way to move water back and forth through the plumbing system as it expands and contracts, allowing the pipes to always remain full of water.


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