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The EnergyPlus.IDD file currently defines the minimum compressor operation temperature as 5C . (This file lives in the same directory as the EnergyPlus executable). If I were you, the first thing I'd try is modifying this file to a lower value that is more realistic for your particular heat pumps. I think this minimum limit was established before cold climate heat pumps. I have encountered the same issue with air source heat pumps.

  WaterHeater:HeatPump,
   ...
   N4 , \field Minimum Inlet Air Temperature for Compressor Operation
   \type real
   \units C
   \minimum 5
   \default 10
   \note Heat pump compressor will not operate when the inlet air dry-bulb temperature
   \note is below this value.