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During the mid-1970s, following the first oil embargo, heat pumps became of great interest to northern U. S. electric power companies, particularly for those experiencing large peak demands during the heating season, as shortages of natural gas and oil led to increased usage of direct electric heating. Air-source heat pumps (ASHP), given their wider market presence relative to other heat pump types, almost universal applicability, and inherent efficiency and capacity issues at cold outdoor temperatures were of primary interest as an electrical heating system alternative at that time. ASHPs based on the simple vapor compression cycle suffer both heating capacity (output) and efficiency (coefficient of performance or COP) degradation as the outdoor ambient temperature drops. At the same time, building heat demand is increasing so ASHPs require a supplemental heating source – usually direct electric resistance heating elements to bridge the gap between the building heat demand and the ASHP heating output. This feature causes lower seasonal performance and limits peak electric demand reduction potential leading to limited acceptance of ASHPs in areas that experience large numbers of hours at cold temperatures (loosely defined as ≤-7°C for purposes of Annex 41). A primary criterion for ASHPs to achieve good seasonal performance in cold areas is achieving high heating output at low ambient temperatures so as to minimize reliance on supplemental heat sources and maximize the overall system heating seasonal performance factor (HSPF or SPFh).

IEA HPP Annex 41 Cold Climate Heat Pumps: Task 1 Report Literature and Technology Review United States - ORNL/TM-2013/472