Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
The ddy files that come with the EnergyPlus epw files are based on ASHRAE handbook data so the 0.4% etc. values are based on 20-30 years of weather data (and not necessarily the same period of record as the TMY3, IWEC, etc. weather file because these sources have been updated on different cycles, check the comments in the ddy file to find the vintage of the design data). Doing the same percentage calculations from a single year weather file, whether it's a single actual year or a composite year (like TMY3) will not necessarily give the same result. The ideal solution would be for the weather data vendor to generate these values from a multi-year data set. If that is not available and you want to base the sizing on the weather file, you could use SizingPeriod:WeatherFileDays. But remember if you do this, you will have solar present during the winter, unlike the standard winter design day objects which zero out the solar.
