Results of an experimental study of a full-scale passive solar hot water heating system utilizing a boiling collector are presented. The self-pumping system alternates between two modes of operation. During the run cycle, vapor pressure drives the evaporated refrigerant downward from the collector to the condenser. Once a preset quantity of refrigerant is condensed, vapor pressure is again used to force the return of the condensate to the collector during the pump cycle. In order to assess the thermal penalty of self-pumping operation, the system is also operated with a mechanical pump. Daily operating characteristics and seasonal performance are discussed. Performance of the self-pumping system is strongly influenced by the duration of the pump cycle. The average thermal loss attributed to self-pumping during one heating season in Fort Collins, Colo. is approximately nine percent of the solar energy incident on the collector. Refinement of the system design, including individual components, is required to reduce losses and improve performance.

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