Some renewable energy heat sources require large thermal storage tanks. Examples include systems’ biomass boilers and solar thermal collectors. These “wild” heat sources can operate over a wide range of conditions and often at temperatures significantly higher than required by modern low-temperature hydronic distribution systems. Managing these heat sources within a system is very different from just turning a conventional heat source on and off.
When storage volume requirements reach a few hundred gallons, some designers look to unpressurized thermal storage tanks because they are less expensive on a dollar per gallon basis, and easier to install compared to pressure-rated steel tanks. In some cases, an unpressurized thermal storage tank, which ships in a “knock-down” configuration, is the only practical option when the tank will be located in an area that’s only accessible through a narrow door.