DX coil sizing and part-load flow component

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1) If you draw a straight line between a DX coil's entering air conditions and leaving air conditions, then let that line continue to the saturation curve, the intersecting temperature is called the Apparatus Dew Point. This represents the temperature at which air would be cooled to IF the DX coil was 100% effective (reference HERE). The coil bypass factor describes how close the actual leaving conditions are to the coil's apparatus dew point. However, if the straight line between the DX coil's entering air conditions and leaving air conditions doesn't intersect the saturation curve, there is no apparatus dew point and the coil bypass factor can't be calculated.

From the Engineering Reference for Rated SHR during DX Coil Sizing in EnergyPlus, the rated inlet air condition used to size the coil is ALWAYS T_DB = 26.7C and T_WB = 19.4C. Since that can't change, and since you are auto-sizing rated SHR, then EnergyPlus is calculating SHR based upon your supply air conditions. Supply air conditions are set inSizing:Zone and Sizing:System objects depending upon if your DX coil is part of zone HVAC equipment (PTAC, Window AC, etc.) or a central air system. Try adjusting your design outlet air state in these objects to be colder and/or more humid.

2) Frost coil conditions often happen when a DX coil is operating with cold inlet air or low air flows (can read more about freezing coil causes HERE). Since you're operating at night and forcing reduced air flow, you have both impacts together. There are a few things I would check:

• I'm guessing that you're trying to use only the first stage of your 2-stage DX coil for night dehumidification. Either your first stage capacity is too low, and the second stage is also operating during night dehumidification, or your first stage capacity is too high and causing coil freezing.
• There is an input field "Outdoor Air DX Cooling Coil Leaving Minimum Air Temperature" at the end of the CoilSystem:DXCooling object. Make sure this value isn't too low (E+ default value is 2.0 C).