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Importance of DDY and STAT weather files

asked 2018-09-16 08:15:37 -0500

mokhtar gravatar image

updated 2018-09-16 11:16:11 -0500


I generated my own EPW files with the help of "Elements" software. OpenStudio runs well with the files. However, I wanted to use the "Change Building Location" measure. Hence, I used the "Weather.exe" application of E+ to generate the needed DDY and the STAT files. Yet, the files are not complete when generated and this gives an error when running OpenStudio with this measure.

I got the DDY and STAT data for a nearby weather station, rename it, and ran the program and it worked. However, I am not sure about the accuracy of the results.

My question is: how important is the accuracy of these input files? It seems E+ mainly uses EPW files for the hourly calculations. The DDY seems to be useful for the sizing of the equipment. I assume one or two degrees will not make that big of a difference. I am not sure what the STAT file is used for. Hence, not sure about its value for the accuracy of the results.


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answered 2018-09-16 22:53:18 -0500

updated 2018-09-16 22:54:57 -0500

It definitely matters, but by how much is the question. If you are interested in design day peak load calculations, which is used for equipment sizing, it will matter. Mechanical engineers size based on specific sets of design weather data, often obtained from ASHRAE, and based on dry-bulb and mean-coincident wetbulb temperatures which correspond to 99.6% annual cumulative frequency for heating, and 0.4%annual cumulative frequency for cooling. This is the information that is in the DDY (design day) file.

You can find the ASHRAE design day data here:

If you are just interested in energy, it can still affect results. Peak design day conditions do not necessarily correlate to the peak conditions found in an annual weather file. They can be higher or lower. This will affect the equipment size, and in turn affects the energy use of that equipment in your simulation.

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Thank you very much

mokhtar gravatar imagemokhtar ( 2018-09-20 01:57:26 -0500 )edit

answered 2018-09-16 13:09:04 -0500

Avi gravatar image

I believe you are right about DDY data especially when you make sure that your sizing factors are larger than 1.

The STAT file is a Statistics Report of the annual weather data. You can use the Preprocessor WeatherConverter found in your E+ installation folder under the PreProcess folder to convert EPW data to STAT as suggested in this post. Try reading first the AuxiliaryPrograms pdf found in the documentation folder.

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Thank you very much

mokhtar gravatar imagemokhtar ( 2018-09-20 01:57:28 -0500 )edit

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Asked: 2018-09-16 08:15:37 -0500

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Last updated: Sep 16