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ASHRAE Standard 55 also addresses the effect of these variables
on comfort. In establishing the supply air temperature, the initial
cost of lower airflow and low air temperature (smaller fan and duct
systems) must be calculated against the potential problems of dis-
tribution, condensation, air movement, and the presence of in-
creased odors and gaseous or particulate contaminants. Terminal
devices that use low-temperature air can reduce the air distribution
cost. These devices mix room and primary air to maintain reason-
able air movement in the occupied space. Because the amount of
outside air needed is the same for any system, the percentage in low-
temperature systems is high, requiring special care in design to
avoid freezing of preheat or cooling coils. Also, the low-tempera-
ture air supply reduces humidity in the space. Lower humidity dur-
ing cooling cycles costs more in energy because the equipment runs
longer. Also, if the humidity is too low, it may cause respiratory
problems.
Other Considerations
All-air systems operate by maintaining a temperature differential
between the supply air and the space. Any load that affects this dif-
ferential and the associated airflow must be considered. Among
these loads are the following:
• All fans (supply, return, and supplemental) add heat. The effect of
these gains can be considerable, particularly in process work. If
Energy
the fan motor is in the airstream, the inefficiencies in the motor
must also be counted as heat gain to the air. If the fan is placed
after the cooling coil (draw-through,) the total pressure (static
plus velocity pressure) must be calculated. If the fan is placed
before the coil (blow-through), only the velocity pressure needs
to be considered, and the amount of supply air can be reduced.
The heat gain in medium-pressure systems is about 1 K per kilo-
pascal static pressure.
• The supply duct may gain or lose heat from the surroundings.
Most energy codes require that the supply duct be insulated,
which is usually good practice regardless of code requirements.
• Attempting to control humidity in a space can affect the quantity
of air and become the controlling factor in the selection of supply
airflow rate. VAV systems provide only limited humidity control,
so if humidity is critical, extra care must be taken in design.
First, Operating, and Maintenance Costs
As with all systems, the initial cost, or first cost, of an air-han-
dling system varies widely depending on location, condition of the
local economy, and preference of the contractor—even for identical
systems. For example, a dual-duct system is more expensive
because it requires essentially twice the amount of material for ducts
as that of a comparable single-duct system. Systems requiring
extensive use of terminal units are also comparatively expensive.
The operating cost depends on the system selected, the skill of the
designer in selecting and correctly sizing components, the effi-
ciency of the duct design, and the effect of the building design and
type on the operation. All-air systems have the greatest potential to
minimize operating cost.
Because an all-air system separates the air-handling equipment
from the occupied space, maintenance on major components in a
central location is more economical. Also, central air-handling
equipment requires less maintenance than other comparable equip-
ment. The many terminal units used in an all-air system do, how-
ever, require periodic maintenance. Because these units (including
reheat coils) are usually installed throughout a facility, maintenance
costs for these devices must be considered.