Measured Performance of a Low Temperature Air Source Heat Pump

Research output: NRELSubcontract Report


A 4-ton Low Temperature Heat Pump (LTHP) manufactured by Hallowell International was installed in a residence near New Haven, Connecticut and monitored over two winters of operation. After attending to some significant service issues, the heat pump operated as designed. This report should be considered a review of the dual compressor 'boosted heat pump' technology. The Low Temperature Heat Pumpsystem operates with four increasing levels of capacity (heat output) as the outdoor temperature drops. The system was shown to select capacity correctly, supplying the appropriate amount of heat to the house across the full range of outdoor temperatures. The system's Coefficient of Performance (Seasonal COP, or SCOP) over two entire winters was calculated, based on measured data, to be 3.29over the first winter and 2.68 over the second winter. A second seasonal efficiency calculation by a different method yielded a SCOP of 2.78 for the first winter and 2.83 for the second winter. This second seasonal efficiency calculation was determined by comparing measured heat pump energy use to the in situ energy use with resistance heat alone. This method is the ratio of the slopes of thedaily energy use load lines.
Original languageAmerican English
Number of pages46
StatePublished - 2013

Bibliographical note

Work performed by Consortium for Advanced Residential Buildings (CARB), Norwalk, Connecticut

NREL Publication Number

  • NREL/SR-5500-56393

Other Report Number

  • DOE/GO-102013-3788


  • Building America
  • cold climates
  • COP
  • defrost
  • efficiency
  • low temperature air-source heat pump
  • residential
  • residential buildings
  • space conditioning


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