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area minimizes service needs and reduces potential harm to occu-
pants, furnishings, and processes.
• These systems offer the greatest potential for use of outside air
for economizer cooling instead of mechanical refrigeration for
cooling.
• Seasonal changeover is simple and adapts readily to automatic
control.
• A wide choice of zoning, flexibility, and humidity control under
all operating conditions is possible, with the availability of simul-
taneous heating and cooling even during off-season periods.
• Air-to-air and other heat recovery may be readily incorporated.
• They permit good design flexibility for optimum air distribution,
draft control, and adaptability to varying local requirements.
• The systems are well suited to applications requiring unusual
exhaust or makeup air quantities (negative or positive pressuriza-
tion, etc.).
• All-air systems adapt well to winter humidification.
• By increasing the air change rate and using high-quality controls,
it is possible for these systems to maintain the closest operating
condition of ±0.15 K dry bulb and ±0.5% rh. Today, some sys-
tems can maintain essentially constant space conditions. All-air systems have the following disadvantages:
• They require additional duct clearance, which reduces usable
floor space and increases the height of the building.
• Depending on layout, larger floor plans are necessary to allow
enough space for the vertical shafts required for air distribution.
• Ensuring accessible terminal devices requires close cooperation
between architectural, mechanical, and structural designers.
• Air balancing, particularly on large systems, can be more
difficult.
• Perimeter heating is not always available to provide temporary
heat during construction.
Heating and Cooling Calculations
Basic calculations for airflow, temperatures, relative humidity,
loads, and psychrometrics are covered in Chapters 6 and 29 of the
ASHRAE Handbook—Fundamentals. The designer should under-
stand the operation of the various components of a system, their
relationship to the psychrometric chart, and their interaction under
various operating conditions and system configurations. The HVAC
designer should work closely with the architect to optimize the
building envelope design. Close cooperation of all parties during
design can result in reduced building loads, which often allows the
use of smaller mechanical systems.
Zoning—Exterior
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 sys-
tem must respond to these variations. The considerable flexibility to
meet such variations enables the greatest advantages from VAV sys-
tems 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 unoccu-
pied periods rather than operating the main supply fans or local
unit fans.)
Separate perimeter heating can operate with any all-air system.