Design day schedules can be useful depending on what you need equest to simulate. In equest, design days and the associated design-day-schedules are used to size equipment. For an early design you may not know the capacities of your system and equest will auto size items. These design days and schedules are used for this sizing.

You might want to do this to account for over sizing. As an example, say a client is requesting all equipment be able to handle 5 w/sf of equipment load. The building may never reach 5w/sf however, so we make a daily fractional schedule that peaks around 60% or so. If we want equest to properly size an Air handling unit, make a fractional schedule for the design day only that peaks at 100%.

This is important in designs that may operate differently at part load. A vav with reheat system will operate much worse if grossly oversized than on that is right sized, or auto sized by equest.

Make sure to set the actual design day condition as well. The default equest picks is not always correct for the location. 90.1 has some design day conditions, ashrae fundamentals has them all.

Hope this helps!

Neil Bulger