VRF system shows very low COP in heating mode

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I suggest you review the VRF performance curve(s) for EIR. You might input a 3.5 COP, however, as outdoor temperatures fall, the EIR also falls due to the cold weather. It is not uncommon for heat pumps to perform poorly with low COPs in cold weather.

Plot the curve results versus outdoor wet-bulb temperature (or dry-bulb depending on the input field Heating Performance Curve Outdoor Temperature Type). Also include a range of indoor dry-bulb temperatures to see the impact of all possible operation conditions within your simulation/geography. See Figure 7 in http://www.fsec.ucf.edu/en/publicatio...

AirConditioner:VariableRefrigerantFlow,
 VRFHeatEIRFT,  !- Heating Energy Input Ratio Modifier Function of Low Temperature Curve Name
 VRFHeatEIRFTBoundary,    !- Heating Energy Input Ratio Boundary Curve Name
 VRFHeatEIRFTHi,  !- Heating Energy Input Ratio Modifier Function of High Temperature Curve Name
 WetBulbTemperature,      !- Heating Performance Curve Outdoor Temperature Type

Curve:Biquadratic,
  VRFHeatEIRFT,            !- Name
  0.87465501,              !- Coefficient1 Constant
  -0.01319754,             !- Coefficient2 x
  0.00110307,              !- Coefficient3 x**2
  -0.0133118,              !- Coefficient4 y
  0.00089017,              !- Coefficient5 y**2
  -0.00012766,             !- Coefficient6 x*y
  15,                      !- Minimum Value of x
  27,                      !- Maximum Value of x
  -20,                     !- Minimum Value of y
  12,                      !- Maximum Value of y