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negative coil bypass factor calculated

asked 2020-09-04 16:53:39 -0500

Wooyoung's avatar

updated 2022-07-14 13:51:23 -0500


I have an err file saying the following.

 ** Warning ** The Standard Ratings is calculated for Coil:Cooling:DX:SingleSpeed = SPLIT MNW COOLING COIL but not at the AHRI test condition due to curve out of bound.
   **   ~~~   **  Review the Standard Ratings calculations in the Engineering Reference for this coil type. Also, use Output:Diagnostics, DisplayExtraWarnings for further guidance.
   **   ~~~   ** CheckCurveLimitsForStandardRatings: The max and/or min limits specified in the corresponding curve objects
   **   ~~~   **  do not include the AHRI test conditions required to calculate one or more of the Standard Rating values.
   **   ~~~   ** Coil:Cooling:DX:SingleSpeed=SPLIT MNW COOLING COIL:  Integrated Energy Efficiency Ratio (IEER) calculated is not at the AHRI test condition.
   **   ~~~   **  Check limits in Total Cooling Capacity Function of Temperature Curve, Curve Type = Curve:Biquadratic, Curve Name = SPLIT DX COIL CAP-FT
   **   ~~~   **  Check limits in EIR Function of Temperature Curve, Curve Type = Curve:Biquadratic, Curve Name = SPLIT DX COIL EIR-FT
   ** Severe  ** Coil:Cooling:DX:SingleSpeed "SPLIT MNW COOLING COIL" -- negative coil bypass factor calculated.
   **   ~~~   **  During Warmup, Environment=NEW.YORK-JOHN.F.KENNEDY.INTL.AP_NY_USA ANN HTG 99.6% CONDNS DB, at Simulation time=01/21 00:00 - 00:10
   **  Fatal  ** Coil:Cooling:DX:SingleSpeed "SPLIT MNW COOLING COIL" Errors found in calculating coil bypass factors

I think this is related to weather data (as when I feed another NewYork weather data, although I get warnings, the simulation never fail) and the minimum/maximum X and Y values (according to this answer (

Any clue for this issue would be highly appreciated. Thanks.

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Any clue for soving this severe error?

Keigo's avatar Keigo  ( 2022-07-13 23:06:25 -0500 )edit

2 Answers

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answered 2023-05-18 00:51:07 -0500

Keigo's avatar

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.
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I had run into this issue as well, hard sizing seemed to fix the problem for me

TGE2023@km's avatar TGE2023@km  ( 2024-02-20 12:19:07 -0500 )edit

The warning shows Rated Vol Flow Per Watt=[1.648E-005] which is very low. You might try increasing the supply air temperature in cooling mode in Sizing:Zone to see if that helps with this warning (e.g., from 12 C to 16 C, this should increase the zone design cooling air flow rate).

rraustad's avatar rraustad  ( 2024-02-20 12:27:19 -0500 )edit

answered 2022-07-14 07:24:06 -0500

updated 2023-05-19 12:21:08 -0500

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)

  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
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Asked: 2020-09-04 16:53:39 -0500

Seen: 13,112 times

Last updated: May 19 '23