EnergyPlus: how to work with the DDY file

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You need to copy and paste the Site:location and SizingPeriod:DesignDay objects for your design conditions into the IDF file. It is important to use the Site:location and SizingPeriod:DesignDay that is consistent with the EPW weather file selected.

In IDF Editor, use the menu item FILE.. OPEN.. or else the OPEN icon on the toolbar. Then select in the bottom right corner of the dialog box just above the Open and Cancel buttons the dropdown list and select Design Day (*.DDY). Now navigate to the directory that contains the DDY files usually \EnergyPlusV8-2-0\WeatherData and select the file that you want. The DDY file usually contains a number of SizingPeriod:Design objects that correspond to different design conditions such as heating 99% or heating 99.6%. Pick a heating and a cooling SizingPeriod:Design that is appropriate for your project. Now copy the SizingPeriod:Design objects for the design days that you want from the DDY file you just opened into your file and don't forget to also copy the Site:location objects. Make sure you delete any old Site:location and SizingPeriod:DesignDay objects from your file. This can also be done in a text editor.