Office high-rise buildings have many zones by their nature. Often individual tenant’s inhabit entire floors or even a part of the floor. Each of these tenant spaces must be designed separately and as a unique zone in the building. Within each tenant space, you also face the design program of private offices, conference rooms, open office areas, IDF computer/network rooms, elevator lobbies, and corridors. All of these rooms have different heating and cooling needs just as the tenant space as a whole has unique needs from adjacent tenants.
Often a medium-pressure system is put into place for the base-building’s HVAC system. Large roof top units, chillers, cooling towers, etc are put into place. On each floor of a typical office tower, you have a core consisting of elevators, stairs, restrooms, and storage. It is here that you also find the medium pressure loop. This is a large duct that brings primary air to the floor usually at 55F for further conditioning. This large duct loop can be tapped into as needed as long as you are staying in the base-building’s system limits.
Each tenant on the floor taps into this medium pressure loop and ties into a VAV or FPB box to further condition the air. A Variale Air Volume box will use the thermostat input from the user to module a damper and let different volumes of air into the space. A Fan Powered Box will bring in a given amount of primary air that the fan can handle. Often on each of these (but mostly on the FPB’s), we see reheat coils. Water is fed to and taken from these coils to then feed the base-building’s exhaust to a heat sink (outdoor air). These boxes feed diffusers which let the air go into the space. Often the return air is not ducted and goes straight into the above-ceiling plenum. In a FPB situation, this return air can reenter the box and mix with primary air. Other systems may include a base-building boiler which serves water-based radiators or induction units on each floor (at the perimeter). All inputs come from the zone’s thermostat. A zone may be a series of rooms or one open office area. There will be one box per zone.