What characteristic do solar thermal collectors, hydronic heat pumps and thermal storage tanks supplied by biomass boilers have in common? Answer: They all perform best when coupled to distribution systems that operate at low supply water temperatures.
But that’s not what’s waiting for them in most North American homes. Instead, it’s common to encounter space heating distribution systems designed in the days of lower-cost fuels, and when boilers burning those fuels were by far the most common heat source. Heat emitters were sized around water temperatures of 180° F or higher. The design rationale was simple, higher water temperatures meant small heat emitters and lower installation cost. Boilers of that era were not designed to condense flue gases, so efficiencies in the range of perhaps 85-87% represent the practical “ceiling” of thermal performance. In short, there was little if any incentive to design distribution systems around lower water temperatures.