- I have heard mixed messages about design day objects- for instance I have been told that all design day objects should be ran to produce accurate sizing by Linda & Drew, but told by Paulo the same as you say of only two needed. For using two I wonder what is the best way to choose the correct design days, and for some time thought it made more since to use Weather file:extreme summer week/Weather file:extreme winter week so that I could run the same building at many locations during parametric runs and so I did not have to make arbitrary choices of Design Days. Lately I have just been running files with all Design Days to be on the safe side. I guess ASHRAE standards would be a good place for me to research for the best day type to choose/use. (your advise on this was appreciated)