Exterior zones are affected by varying weather conditions wind, temperature, and sun and, depending on the geographic area and season, may require both heating and cooling at different times. While the engineer has many options in choosing a system, the system must respond to these variations. The considerable flexibility to meet such variations enables the greatest advantages from VAV systems to be realized. The need for separate perimeter zone heating is determined by
• Severity of the heating load (i.e., geographic location).
• Nature and orientation of the building envelope.
• Effects of downdraft at windows and the radiant effect of the cold glass surface (i.e., type of glass, area, height, and U-factor).
• Type of occupancy (i.e., sedentary versus transient).
• Operating costs (e.g., in buildings such as offices and schools that are unoccupied for considerable periods, fan operating cost can be reduced by heating with perimeter radiation during unoccupied periods rather than operating the main supply fans or local unit fans.)
Separate perimeter heating can operate with any all-air system. However, its greatest application has been in conjunction with VAV systems for cooling-only service. Care in design minimizes simultaneous heating and cooling. The section on Variable Air Volume has further details.
