> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/user-guide/workflows/design-day.md).
# Design Day
### Set up a Design Day Model
A Design Day in HVAC pertains to using extreme climatic conditions to size and effectively design heating, ventilation, and air conditioning systems. They are crucial in HVAC for several reasons. They represent extreme weather conditions, ensuring systems are sized to handle peak loads effectively. By focusing on the hottest and coldest days, design days provide a reliable basis for dimensioning heating and cooling systems, preventing underperformance during temperature extremes and in real-world conditions.
Design Days are established automatically through EPW (EnergyPlus Weather) files, encompassing location-specific weather data. A DesignDay object is employed to create these days, specifying parameters such as dry-bulb temperature, humidity, solar radiation, etc. Alternatively, you can create your Design Day to produce multiple Design Days based on project-specific parameters.
More information can be found on Design Day Weather Overview and EnergyPlus Sizing Calculations.
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