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

["Crawley, Drury" <Drury.Crawley@xxxxxxxxxx>]

The weather files (EPWs) that we use is 'typical' of the patterns -- not necessarily the long-term measurements.  Yes, you could use the weather files to do the sizing... but from experience it is better to use ''design conditions' that are representative of a 15-20 period of record.  This contains more of the normal extremes that you may not see in the 'typical' weather files.  If you're interested, I can send (separately, not to the general list) references and papers that show how typical vs. actual weather files influence weather data.

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From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Patricia
Sent: Thursday, October 22, 2009 11:36 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] theory behind SizingPeriod:DesignDay vs. SizingPeriod:WeatherFileDays

 

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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