This invention provides a thermal energy system comprising a heat exchanger for transferring thermal energy between a source and a load, the heat exchanger having a primary side associated with the source, and a secondary side for conducting a fluid associated with the load, wherein the secondary side of the heat exchanger is passively back-flushed upon consumption of a portion of the fluid. Passive back-flushing prevents fouling of the heat exchanger due to sediments, scale,and mineral deposits which may be present in the circulating fluid.