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However, its greatest application has been in conjunction with VAV
systems for cooling-only service. Care in design minimizes simul-
taneous heating and cooling. The section on Variable Air Volume
has further details.
Zoning—Interior
Interior spaces have relatively constant conditions because they
are isolated from external influences. Cooling loads in interior
zones may vary with changes in the operation of equipment and
appliances in the space and changes in occupancy, but usually inte-
rior spaces require cooling throughout the year. A VAV system has
limited energy advantages for interior spaces, but it does provide
simple temperature control. Interior spaces with a roof exposure,
however, may require similar treatment to perimeter spaces requir-
ing heat.
Space Heating
Although steam is an acceptable medium for central system pre-
heat or reheat coils, low-temperature hot water provides a simple
and more uniform means of perimeter and general space heating.
Inpidual automatic control of each terminal provides the ideal space comfort. A control system that varies the water temperature
inversely with the change in outdoor temperature provides water
temperatures that produce acceptable results in most applications.
To produce the best results, the most satisfactory ratio can be set
after the installation is completed and actual operating conditions
are ascertained.
Multiple perimeter spaces on one exposure served by a central
system may be heated by supplying warm air from the central sys-
tem. Areas that have heat gain from lights and occupants and no heat
loss require cooling in winter, as well as in summer. In some sys-
tems, very little heating of the return and outdoor air is required
when the space is occupied. Local codes dictate the amount of out-
side air required (see ASHRAE Standard 62 for recommended opti-
mum outside air ventilation). For example, with return air at 24°C
and outside air at 18°C, the temperature of a 25% outdoor/75%
return air mixture would be 13°C, which is close to the temperature
of the air supplied to cool such a space in summer. In this instance,
a preheat coil installed in the minimum outdoor airstream to warm
the outdoor air can produce overheating, unless it is sized so that it
does not heat the air above 2 to 5°C. Assuming good mixing, a pre-
heat coil located in the mixed airstream, prevents this problem. The
outdoor air damper should be kept closed until room temperatures
are reached during warm-up. A return air thermostat can terminate
the warm-up period.
When a central air-handling unit supplies both perimeter and
interior spaces, the supply air must be cool to handle the interior
zones. Additional control is needed to heat the perimeter spaces
properly. Reheating the air is the simplest solution, but it is not
acceptable by most energy codes. An acceptable solution is to vary
the volume of air to the perimeter and combine it with a terminal
heating coil or a separate perimeter heating system, either base-
board, overhead air heating, or a fan-powered terminal unit with
supplemental heat. The perimeter heating should be inpidually
controlled and integrated with the cooling control. Resetting the
supply water temperature downward when less heat is required gen-
erally improves temperature control. For further information, refer
to Chapter 12 in this volume and Chapter 45 of the ASHRAE Hand-
book—Applications.
Temperature Versus Air Quantity
Designers have considerable flexibility in selecting the supply
air temperature and corresponding air quantity within the limita-
tions of the procedures for determining heating and cooling loads.