Designed to offer a solution for solar thermal and ground-source applications with an emphasis on hydronic space heating, the multifunctional Thermal-Stor™ stratified solar thermal storage tank operates as a storage tank and hydronic buffer tank while occupying the space of a single unit. It uses the natural buoyancy of heated water to efficiently layer the temperatures in the tank, maximizing the heat transfer capability of the renewable energy source and hydronic space-heating system. Energy from a solar thermal system or ground-source heat pump is transferred to the storage vessel through a spherical heat exchanger. As renewable energy is added, the stored water stratifies within the tank. Equipped with connections on the side of the tank, Thermal-Stor allows for a space-heating system to be easily integrated with the renewable-energy system. The hot water in the top of the tank permits the space-heating system to access the hottest possible water for distribution to the heat emitters. The cooler water in the bottom of the tank allows for increased collector efficiency and greater opportunity for solar thermal heat generation. A back-up hydronic heat source, such as a Knight® boiler, ensures that the storage vessel is consistently kept at a sufficient temperature to meet hydronic heating demands. As demand for space heating is initialized, Thermal-Stor acts as a hydraulic separator, decoupling the system demands from the boiler output. This allows for extended boiler runtime, eliminating short cycling of the boiler and constant temperature delivery to the heating system. The storage tank is available in nine models in a range of 125-900 gallon capacities.