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# Error when using PTAC

I'm trying to use the PTAC system to make redundancy to a chilled water coil in a IT Room.

To run the simulation with PTAC, when I change only Rated Total Cooling Capacity for the actual equipment, the below error occurs indicating that Rated air volume flow rate per watt of rated total cooling capacity is out of range.

I've tried several changes alternatives, but every time I change the autosized condition of any field, the same error occurs.

I do not think the error message is correct, because I fix the air flow to the indicated range and the error persists.

Is this a bug or am I making a mistake?

   ************* Beginning Plant Sizing Calculations
** Severe  ** Sizing: Coil:Cooling:DX:SingleSpeed "0A_SL DE EQUIP_CYCLING ELEC PTAC 1 SPD DX CLG 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=[1.368E-005], Max Rated Vol Flow Per Watt=[6.041E-005]. See Input Output Reference Manual for valid range.
** Severe  ** For object = Coil:Cooling:DX:SingleSpeed, name = "0A_SL DE EQUIP_CYCLING ELEC PTAC 1 SPD DX CLG COIL"
**   ~~~   ** Calculated outlet air relative humidity greater than 1. The combination of
**   ~~~   ** rated air volume flow rate, total cooling capacity and sensible heat ratio yields coil exiting
**   ~~~   ** air conditions above the saturation curve. Possible fixes are to reduce the rated total cooling
**   ~~~   ** capacity, increase the rated air volume flow rate, or reduce the rated sensible heat ratio for this coil.
**   ~~~   ** If autosizing, it is recommended that all three of these values be autosized.
**   ~~~   ** ...Inputs used for calculating cooling coil bypass factor.
**   ~~~   ** ...Inlet Air Temperature     = 26.67 C
**   ~~~   ** ...Outlet Air Temperature    = -15.94 C
**   ~~~   ** ...Inlet Air Humidity Ratio  = 1.125000E-002 kgWater/kgDryAir
**   ~~~   ** ...Outlet Air Humidity Ratio = 3.159972E-003 kgWater/kgDryAir
**   ~~~   ** ...Total Cooling Capacity used in calculation = 5280.00 W
**   ~~~   ** ...Air Mass Flow Rate used in calculation     = 8.288767E-002 kg/s
**   ~~~   ** ...Air Volume Flow Rate used in calculation   = 7.227805E-002 m3/s
**   ~~~   ** ...Air Volume Flow Rate per Watt of Rated Cooling Capacity is also out of bounds at = 1.3689025E-005 m3/s/W
**   ~~~   **  During Warmup, Environment=SANTAREM AEROPORTO ANN CLG 1% CONDNS DB=>MWB, at Simulation time=10/21 00:00 - 00:15
**  Fatal  ** Check and revise the input data for this coil before rerunning the simulation.

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The important thing to note from the error message is that it:

**   ~~~   ** The combination of rated air volume flow rate, total cooling capacity and sensible  heat ratio yields coil exiting air conditions above the saturation curve.
**   ~~~   ** Possible fixes are to reduce the rated total cooling capacity, increase the rated   air volume flow rate, or reduce the rated sensible heat ratio for this coil.
**   ~~~   ** If autosizing, it is recommended that all three of these values be autosized.


EnergyPlus requires capacity, flow rate, and SHR to complete sizing calculations. When you set one of these fields to 'Autosize' you essentially ask EnergyPlus to calculate the appropriate sizing condition for that input given the other fixed inputs that you assign a value to. The values you are entering cause EnergyPlus to calculate a value for air flow per capacity to be below the acceptable minimum of 300 cfm/ton.

Try to run a simulation with all three of these input fields set to 'Autosize'. You can then look at the sizing results in the Component Sizing Summary and Equipment Summary output tables and compare them to your own sizing data (if you have that). If you want to hard-set values for the coil's input fields, the sizing results would be a good starting point. You can look at questions related to coil sizing HERE, HERE, and HERE for more info.

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