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Setting up SizingPeriod:DesignDay objects for purchased weather data

asked 2014-12-18 20:27:56 -0500

scottb gravatar image

updated 2015-09-29 07:38:16 -0500

How do others set up Design Day objects (0.4%, 1%, 2% and 99%, 99.6% variants) for purchased weather data sources (WeatherAnalytics, etc)? Should I be nagging WA for more information?

I have been using the dated DOE weather sources (slight tendency to oversize in my climate zone). I know ASHRAE handbooks provide information for some climates. I assume best practice would be to extract this directly from the weather file. If not, how do others do it?

Curious if anyone is willing to share a script to automate this process.


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@scottb Since you used EnergyPlus syntax in the question title, I've added a tag for energyplus. If your question is broader than that, please edit.

MJWitte gravatar imageMJWitte ( 2014-12-19 07:38:15 -0500 )edit

The EnergyPlus WeatherConverter program contains the complete Climatic Tables from the ASHRAE HOF (go to PreProcess\WeatherConverter and look for the ASHRAE*.csv files), so you can write a script to search by WMO and then print out the data in the form required by the DDY. Alternatively, you should be able to use the WeatherConverter to "convert" a dummy file with the same WMO# and get the *.DDYs :-) :-). I don't know whether ASHRAE has any copyright issues about such use of their data, though.

Joe Huang gravatar imageJoe Huang ( 2014-12-19 12:58:32 -0500 )edit

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answered 2014-12-19 07:36:42 -0500

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.

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answered 2014-12-19 12:45:32 -0500

Joe Huang gravatar image

updated 2014-12-19 13:14:35 -0500

I'm going to echo what Mike has written above about the basis for the DDY files, but expand on that to say what I think should be done for locations that are absent from the ASHRAE HOF, which is to calculate them from the raw data used to create the EPW files. In my opinion that is actually preferable and more methodologically consistent than using the DDYs from the ASHRAE HOF, since the periods of records as well as station location may not be the same. For example, if I'm using a typical year weather file created from the last 12 years, why should I use a DDY file that's from the last 30 years? For that reason, when I created the CZ2010 weather files for the California Energy Commission, I also calculated and provided the DDY files to go with them.

(warning: following note refers to commercial weather data products) This is also done for the ASHRAE IWEC2 weather files, i.e., they all come with DDY files calculated from the same long-term record, as well as any weather file from White Box Technologies. (note: Joe Huang is the president of White Box Technologies, Inc.)

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Asked: 2014-12-18 20:27:56 -0500

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Last updated: Dec 20 '14