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OpenStudio weather file design days

asked 2016-05-02 17:49:08 -0600

Matt Koch's avatar

updated 2020-01-20 13:35:15 -0600

The uploaded file below shows the OpenStudio Site tab, and at the bottom of it a number of design days. I suppose I only need one (the worst case) design cooling day and one (the worst case) design heating day, yet there are three of each for this (San Antonio International Airport) weather file.

Is there a recommendation as to which of the many to keep - can I delete the others?

The cooling design days are all 0.4%, but then there are DB=>MWB, DP=>MDB, Condns Enth=>MDB and Condns WB=> MDB. The heating design days are all 0.4% as well, but then there are Condns DB, Condns WS=>MCDB and Condns DP=>MCDB.


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Matt, I took the liberty of editing your post to include the picture directly in there and removing the greetings as per our convention.

Great to have you on board though, and tanks for the encouraging words :)

Julien Marrec's avatar Julien Marrec  ( 2016-05-03 00:57:57 -0600 )edit

Also, considering this is actually not an OpenStudio specific question, I'd tend to rename it to something like "Which design days do I need for sizing purposes". What do you think?

Julien Marrec's avatar Julien Marrec  ( 2016-05-03 02:45:14 -0600 )edit

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answered 2016-05-03 00:54:59 -0600

Short story, the "most classic" [1] is to use the .4% DB=>MWB for coolng and .4% Condns DB for heating which corresponds to the hottest and coldest months respectively (with highest/coldest dry bulb temperature).

Obviously the other types of design days have value too, depending on the equipment/application you have. Cooling design days based on wetbulb are useful for sizing cooling towers for example. Below I've tried to summarize which design days are useful for which application.


  • DB=>MWB: Chillers and air conditionners
  • Condns WB=> MDB: Cooling Towers and other evaporative coolers
  • DP=>MDB: These values are especially useful for humidity control applications, such as desiccant cooling and dehumidification, cooling-based dehumidification, and fresh-air ventilation systems
  • Condns Enth=>MDB: this is used for calculating cooling loads caused by infiltration and/or ventilation into buildings. Enthalpy represents the total heat content of air (the sum of its sensible and latent energies)


  • Condns DB: General heating applications
  • Condns DP=>MCDB: to size equipment for humidification applications.
  • Condns WS=>MCDB: Wind design data provide information for estimating peak loads accounting for infiltration


All above information is based on Chapter 14 of the 2009 ASHRAE Handbook of Fundamental.

There are newer sources in the ASHRAE HoF 2013, and the resulting EnergyPlus .ddy here

[1] From ASHRAE HoF 2009:

Annual Heating and Humidification Design Conditions.

The month with the lowest mean dry-bulb temperature is used, for example, to determine the time of year where the maximum heating load occurs.

The 99.6 and 99.0% design conditions are often used in sizing heating equipment.

Annual Cooling, Dehumidification, and Enthalpy Design Conditions.

[...] The 0.4, 1.0, and 2.0% dry-bulb temperatures and mean coincident wet-bulb temperatures often represent conditions on hot, mostly sunny days. These are often used in sizing cooling equipment such as chillers or air-conditioning units.

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Only [very minor] comment to add to the great info above would be for a performance rating model, Appendix G dictates to used the 99.6% heating and the 1% dry-bulb and 1% wet-bulb for cooling. (this is true for '07, '10, '13 versions).

dradair's avatar dradair  ( 2016-05-04 09:46:12 -0600 )edit

Thank you kindly for all your answers, collectively. I suppose I should have had the wherewithal to look this up in ASHRAE Fundamentals myself. I do understand about Appendix G, but the weather file I have seems to only provide 0.4%, not 1%?

Matt Koch's avatar Matt Koch  ( 2016-05-04 10:13:56 -0600 )edit

The weather file downloaded? I cant personally speak to that per se. But, the 2013 HoF tables have the 99.6%, 99%, 0.4%, 1%, and 2% values; so it should be verifiable. I am not as familiar with how OS does it, but I'd bet these can be manually added to the model file. I know its possible in E+ and eQuest.

dradair's avatar dradair  ( 2016-05-04 10:40:45 -0600 )edit

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Asked: 2016-05-02 17:49:08 -0600

Seen: 2,344 times

Last updated: May 03 '16