Why does the sizing subroutine design on only the first design day?

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I am sizing radiant floor heating system, and have 4 different summer design days, followed by 3 winter design days in the input files.

Design days is selected in my Sizing Zone object for both heating and cooling and the floor objects are set as shown below. However, the sizing routine seems to pickup the only the first summer design day, and calculates a much greater load than when I run a copy with only the winter design days. (see image of different outputs below

I thought the system would run all of the design days and size for the largest heating load, and I cannot really understand why it picked the summer day and had design loads so much larger than when only the winter design days were available.

I want to keep the summer days in to either size summertime ac, or run the floors in a cooling mode, as my basement floors can cool water for the upper floors in late spring and summer and provide at least part cooling duty... but I have to understand why this heating autosize is behaving as it is.... Any ideas?

ZoneHVAC:LowTemperatureRadiant:VariableFlow,                                           
    K Radiant Floor,         !- Name                                                   
    Always On Discrete hvac_library,  !- Availability Schedule Name                    
    Kit Bkfst,               !- Zone Name                                              
    Low Temp Radiant Var Flow 1Floors,  !- Surface Name or Radiant Surface Group Name  
    0.013,                   !- Hydronic Tubing Inside Diameter {m}                    
    Autosize,                !- Hydronic Tubing Length {m}                             
    MeanAirTemperature,      !- Temperature Control Type                               
    HeatingDesignCapacity,   !- Heating Design Capacity Method                         
    Autosize,                !- Heating Design Capacity {W}                            
    ,                        !- Heating Design Capacity Per Floor Area {W/m2}          
    ,                        !- Fraction of Autosized Heating Design Capacity          
    Autosize,                !- Maximum Hot Water Flow {m3/s}                          
    K in,                    !- Heating Water Inlet Node Name                          
    K out,                   !- Heating Water Outlet Node Name                         
    2,                       !- Heating Control Throttling Range {deltaC}              
    VVRH Air Temp Kitchen,   !- Heating Control Temperature Schedule Name              
    CoolingDesignCapacity,   !- Cooling Design Capacity Method                         
    0,                       !- Cooling Design Capacity {W}                            
    ,                        !- Cooling Design Capacity Per Floor Area {W/m2}          
    ,                        !- Fraction of Autosized Cooling Design Capacity          
    0,                       !- Maximum Cold Water Flow {m3/s}                         
    ,                        !- Cooling Water Inlet Node Name                          
    ,                        !- Cooling Water Outlet Node Name                         
    0.5,                     !- Cooling Control Throttling Range {deltaC}              
    ,                        !- Cooling Control Temperature Schedule Name              
    Off,                     !- Condensation Control Type                              
    1,                       !- Condensation Control Dewpoint Offset {C}               
    CalculateFromCircuitLength,  !- Number of Circuits                                 
    106.7;                   !- Circuit Length {m}