In many buildings, there are isolated spaces with a continuous cooling load even when the buildings are unoccupied. These can be server rooms, communication closets, main distribution frame rooms, laboratory equipment rooms, etc., and they are often scattered throughout a building.
If these spaces are cooled by the same central air handlers that serve the occupied spaces, the 24/7 cooling demands will require the air handlers to operate 24/7. With the ability to program time-of-day schedules with unoccupied set back temperatures and airflows for individual zone terminal units, it is now possible to reduce the after-hours demands on the central equipment. However, the true 24/7 cooling loads typically represent a very small percentage of the central system load. As such, it is necessary to circulate more air than is strictly required to cool the 24/7 spaces because the air handling equipment can only turn-down a certain amount before becoming unstable or unable to deliver cooling air to the spaces requiring it.