Air source heat pumps (HPs) are a highly efficient space heating technology that have been used for decades; however, there is a scarcity of published field data characterizing their performance. This research utilizes data collected from pre-commercial residential cold climate HPs developed for the U.S. Department of Energy’s Residential Cold Climate Heat Pump Challenge. This research investigates defrost mode specifically, which is necessary for optimal heating functionality when frost forms on the outdoor unit. This paper presents findings from 22 residential sites concerning the characteristics of defrost operation, its impact to performance, and a categorization of various strategies employed by the units. The results suggest that defrost mode becomes more prevalent at higher humidities and at outdoor air temperatures from 10 °F to 30 °F (-12.2 °C to -1.1 °C). None of the strategies for defrost were observed to have significant impacts to average room temperature throughout the homes (median impact all less than 1 °F 0.6 °C). However, the units that ran the indoor supply fan during defrost (except the ones that simultaneously supplied 10 kW or more of auxiliary heat) were associated with low supply air temperatures during defrost mode (median values less than 60 °F 15.5 °C), which could have comfort impacts during winter to occupants especially near supply vents. It was observed that many of the units were initiating defrost cycles at times where there did not appear to be a need for it, either due to a fixed interval feature or control settings that did not consider frost formation or impact to performance. Although these units are pre-commercial and may undergo modifications before entering the market, the findings underscore the importance of developing a deeper understanding of the energy and comfort impacts of defrost mode strategies for improving HP performance.
Keene et al. (Mon,) studied this question.