[EnergyPlus_Support] theory behind SizingPeriod:DesignDay vs. SizingPeriod:WeatherFileDays

["Patricia" <patricia@xxxxxxxxxxxxxxxx>]

Ok, this may also be a dumb question but I'll ask anyway.

If one had a weather file, would it be necessary to have design days? I thought that design days were a few days in the year where you specify certain weather parameters. Then you let the engine extrapolate values for those parameters for the rest of the year based on some internal algorithm.

However, if you have weather files, don't they give this data for the entire year? Why would or should you also use DesignDays? I'm reading pg 137  (of 1052) in the EngineeringReference.pdf file (version 4). It states: 

"The input object 'SizingPeriod:DesignDay' describes design days (meant to mimic ASHRAE design conditions but in a whole day profile) using certain characteristics for the day and then EnergyPlus supplies the remaining portions to complete outdoor conditions needed for program execution. SizingPeriod:DesignDay are perhaps the best objects for sizing equipment as the ASHRAE specified design conditions can be input AND weather files may or may not have the conditions necessary to size equipment properly."

It sounds like the weather files aren't enough and have to be supplemented with Design Days? ('best for sizing equipment')

So, do I always need to add both Design Days and weather data? 

Furthermore, I hook my weather file to the EP-Launch screen. why do I need to also add data into the IDF-Editor.

I apologize for all of these questions. (Please ignore them if they bother you.) I'm sure that these answers are obvious but I am just not quite understanding the big picture yet.

Sincerest thanks,
Patricia



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