Large air-handling units of a constant-volume system provide both heated and cooled air to a zone. Each unit has both heating and cooling coils that are alternatively active during the winter and summer. Variable air volume systems usually have cooling coils in the unit itself and reheat coils at the VAV box located near each space served. These small remote coils are helpful for dehumidification during the cooling season and for tempering the cooled air delivered to interior spaces.
Units that are constant-volume, and some variable-volume air handlers, do not require heating in the summer. (This is especially true of single-story systems with a roof load, making all zones effectively exterior zones.) Improperly adjusted controls or faulty valves cannot completely disable the heating coils, so the unit simultaneously heats and cools the air in a needless waste of energy. The safe thing to do is to physically valve off the hot-water or steam supply to the air-handler coils in the summer, to be sure no energy is lost to reheat. Or, if the VAV reheat uses electric resistance heaters, turning the breakers off will disable them.
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Excerpt from Energy Conservation Projects.