Many hydronic systems include thermal storage tanks — especially systems with renewable energy heat sources. When the heat input is from an array of solar thermal collectors or a biomass boiler, it’s possible for the water temperature in those tanks to reach temperatures of 180°-200° F. Thermal storage tanks supplied by electric boilers operating on off-peak electricity can also reach these high temperatures by the end of the charging cycle.
A 500-gallon tank filled with 190° water contains a lot of heat. However, the usefulness of that heat depends on the supply temperature required by the heat emitters in the system. The lower the tank temperature can be while still meeting the heating load of the system, the more useful heat it delivers.