In each terminal unit, this primary air is discharged
in such a way that it draws in a much larger volume of
secondary air from the room, which is filtered and passed
through a coil for additional heating or cooling (Figure
10.11). The use of primary air as the motive force eliminates
the need for a fan in the induction unit. The cooling
coil is often deliberately kept at a temperature
greater than the dew point temperature of the room air
which passes through it, eliminating the need for a condensate
drain. Although the standard air-water induction
system is a cooling-only system, room terminals can
employ reheat coils to heat perimeter zones.
Despite the high pressures and velocities required
for the primary air distribution, distribution energy is
minimized by the relatively small volume of primary air.
But the energy saved in primary air distribution is more
than offset by the energy consumed in the indirect control
and distribution of cooling water, making air-water
induction systems among the least energy efficient.
Air-water induction units tend to be noisy and the
system provides negligible control of humidity. The applicability
of these systems is limited to buildings with
widely varying cooling or heating loads where humidity
control is not necessary, such as office buildings. Concerns
about indoor air quality limits their use as well.