Last month we began discussing direct-to-load hydronic heat pump systems. They combine a variable speed compressor with variable speed circulator to modulate both heat output and flow to match the needs of a distribution system without need of a buffer tank or hydraulic separator.
Both air-to-water and water-to-water heat pumps are often combined with a boiler to serve as a supplemental and backup heat source. In a direct-to-load system the boiler can be tied into the system using a pair of closely spaced tees as shown in figure 1.
