negative coil bypass factor calculated

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My case might help someone. I got the error "negative coil bypass factor calculated" when I hardsized Gross Rated Total Cooling Capacity but forgot to hardsize Rated Air Flow Rate. The cooling capacity and the air flow rate of DX coils should be both autosized or both hardsized.

   ** Warning ** Sizing: Coil:Cooling:DX:VariableRefrigerantFlow "33F_PLANT_ROOM_5 VRF COOLING COIL": Rated air volume flow rate per watt of rated total cooling capacity is out of range.
   **   ~~~   ** Min Rated Vol Flow Per Watt=[4.027E-005], Rated Vol Flow Per Watt=[1.648E-005], Max Rated Vol Flow Per Watt=[6.041E-005]. See Input Output Reference Manual for valid range.
   ** Severe  ** Coil:Cooling:DX:VariableRefrigerantFlow "33F_PLANT_ROOM_5 VRF COOLING COIL" -- negative coil bypass factor calculated.
   **   ~~~   **  Environment=WUHAN ANN CLG 1% CONDNS WB=>MDB, at Simulation time=07/21 24:00 - 24:10
   **  Fatal  ** Coil:Cooling:DX:VariableRefrigerantFlow "33F_PLANT_ROOM_5 VRF COOLING COIL" Errors found in calculating coil bypass factors
   ...Summary of Errors that led to program termination:
   ..... Reference severe error count=105
   ..... Last severe error=Coil:Cooling:DX:VariableRefrigerantFlow "33F_PLANT_ROOM_5 VRF COOLING COIL" -- negative coil bypass factor calculated.
   ************* Warning:  Node connection errors not checked - most system input has not been read (see previous warning).
   ************* Fatal error -- final processing.  Program exited before simulations began.  See previous error messages.

The first sentance of the warning is the clue. It says the standard ratings calculation is not at the AHRI test condition due to curve out of bound. The standard ratings calculation looks at each of the performance curve objects min/max limit (Minimum/Maximum Value of x or y). If these limits exceed the values required for the calculations, this warning is printed. So this warning directly relates to the performance curves. Look at each curves limits and compare to the values shown below.

Comparing your limits for x and y minimum and maximum, e.g., 12.78 and 23.89 for indoor wet-bulb and 18 and 46.1 for outdoor temperature, I don't see anything that is ouside the range of these StandardRatings test limits.

IndoorCoilInletAirWetBulbTempRated(19.44);   // 19.44C (67F)  Tests A2, B2, B1, and F1
OutdoorCoilInletAirDryBulbTempRated(35.0);   // 35.00C (95F)  Tests A2, B2, B1, and F1
OutdoorCoilInletAirDryBulbTempTestA2(35.0);  // 35.00C (95F)  Test A2 (high speed)
OutdoorCoilInletAirDryBulbTempTestB2(27.78); // 27.78C (82F)  Test B2 (high speed)
OutdoorCoilInletAirDryBulbTempTestB1(27.78); // 27.78C (82F)  Test B1 (Low speed)
OutdoorCoilInletAirDryBulbTempTestF1(19.44); // 19.44C (67F)  Test F1 (Low speed)

Curve:Biquadratic,
  DXCoolCapFT,            !- Name
  0.942587793,             !- Coefficient1 Constant
  0.009543347,             !- Coefficient2 x
  0.000683770,             !- Coefficient3 x**2
  -0.011042676,            !- Coefficient4 y
  0.000005249,             !- Coefficient5 y**2
  -0.000009720,            !- Coefficient6 x*y
  12.77778,                !- Minimum Value of x  <---
  23.88889,                !- Maximum Value of x  <---
  18.0,                    !- Minimum Value of y  <---
  46.11111,                !- Maximum Value of y  <---
  ,                        !- Minimum Curve Output
  ,                        !- Maximum Curve Output
  Temperature,             !- Input Unit Type for X
  Temperature,             !- Input Unit Type for Y
  Dimensionless;           !- Output Unit Type