Expansion tanks are designed to adjust to a changing hot water system supply. These kinds of tanks are usually smaller than the typical hot water heater, and hold around 2 gallons. Many local authorities now recommend that these tanks be fitted whenever new houses are built.
Advantages to Expansion Tanks
Expansion tanks are designed to relieve the high water pressure of your hot water system. This pressure can sometimes build up so much that the hot water tank can be explosive. Expansion tanks help to remove some of this pressure, thereby preventing a serious build-up. In order to relieve the pressure on your hot water tank, you need to carefully position the expansion tank so that it is not too far away from the pipes leading to the hot water system.
Disadvantages to Expansion Tanks
Most of the disadvantages to these tanks are caused by poor installation. Expansion tanks are sometimes used to replace the pressure relief valve on your hot water system tank. This is not their purpose, and without this valve, pressure will keep building in the tank until it fails. Another problem is that they are often not positioned high enough above the hot water system, meaning that water cannot properly drain off.