Thanks for the quick reply, but to be honest I'm still not
convinced. If a
design day is intended to be worst case scenario (as opposed to realistic
weather simulation) then why wouldn't summer design days also have a 0
daily
temperature range?
Winter days are not just one temperature -- there is no solar (so there
is no possibility that winter sun will help warm the space) They
are designated as "Winter Design Day" in our definitions that
are supplied. Usually there won't be people, etc. Yes, Winter
is usually when heating happens and equipment or people or sun into the
space will help.
Summer days have range due to different diurnal climates and summer more
often is about humidity. Here it is "Summer Design Day"
and the people, equipment etc are supplying extra load to the
space.
The Design Days are intended to be used for "sizing" equipment
giving one an idea of potential extremes of their buildings.
Read about Loads Calculations in the ASHRAE Handbook of Fundamentals
(Chap 14-19 in 2009 HOF) for more information. That will give you
some background on philosophy.