Most hydronic heating systems that have renewable energy heat sources, such as solar thermal collectors, an air-to-water or geothermal heat pump or a biomass boiler also have an auxiliary heat source. In most cases, that heat source is a boiler, especially when an existing system is being retrofitted with a renewable heat source.
Many of these systems also have one or more thermal storage tank that absorbs excess heat when it’s available from the renewable heat source, and park it for later use. The thermal storage tanks used in systems with solar thermal collectors and biomass boilers often contain several hundred of gallons of water.