I'll always defer to the AC experts above, but I'll tell you on the sly that I've been a fireman for 21 yrs and we (not me, these other 'uns) run the AC when it's substantially lower than 60 outside. I've come to work many times in the winter with station temp really cold -- colder than outside. The city buys cheap units and I'll be danged if we don't run them through the gauntlet. They seem to last year after year.

Bruce