Most buildings have more than one thermal zone, and each zone can have significantly different gains and losses over a given time period. A thermal zone is a space within a building that generally has its own thermostat. Consider for instance, an office tower with rooms facing north, east, south, and west. During the morning, the east facing rooms may be receiving a great deal of solar gain, while the rest of the building is receiving only diffuse sky radiation. Heat gains would be much greater for the east facing rooms, which may require cooling, while the rest of the building may require heating. Similarly, interior spaces with no exterior walls, roofs, or floors have only internal gains from people, lighting, and equipment. These spaces may require cooling almost all of the time. Mechanical systems in most large buildings are designed to serve the needs of all the thermal zones in the building and must be capable of simultaneously heating one zone while cooling another.
The thermal zones between the hallway on the left and the gymnasium above are completely different. The hallway may require continuous heating and cooling during operating hours, whereas the gymnasium may require heating and cooling only during certain hours. The gymnasium may also require a larger cooling load during the summer time to compensate for larger occupant loads.
Next: Thermal Movements