Field Measurement of the Interactions Between Heat Pumps and Attic Duct Systems in Residential Buildings
Research efforts to improve residential heat-pump performance have tended to focus on laboratory and theoretical studies of the machine itself, with some limited field research having been focused on in-situ performance and installation issues. One issue that has received surprisingly little attention is the interaction between the heat pump and the duct system to which it is connected. This paper presents the results of a field study that addresses this interaction. Field performance measurements before and after sealing and insulating the duct systems were made on three heat pumps. From the pre-retrofit data it was found that reductions in heat-pump capacity due to low outdoor temperatures and/or coil frosting are accompanied by lower duct-system energy delivery efficiencies. The conduction loss reductions, and thus the delivery temperature improvements, due to adding duct insulation were found to vary widely depending on the length of the particular duct section, the thermal mass of that duct section, and the cycling characteristics of the heat-pump. In addition, it was found that the use of strip-heat back-up decreased after the retrofits, and that heat-pump cycling increased dramatically after the retrofits, which respectively increase and decrease savings due to the retrofits. Finally, normalized energy use for the three systems which were operated consistently pre- and post-retrofit showed an average reduction of 19% after retrofit, which corresponds to a change in overall distribution-system efficiency of 24%.