since expansion valves provide better superheat control over a
wide range of operating conditions. Split systems may include
the following configurations:
a) Direct expansion coil and supply fan combined with
b) Direct expansion coil, supply fan, and compressor
combined with a remote condenser, cooling tower, or ground-loop
These systems generally have lower first cost than
central systems but may have higher life cycle costs. If part
load operation is anticipated for a majority of equipment
operating life, consider multiple unitary equipment for superior
operating efficiencies and added reliability. Refer to ASHRAE
Handbook, Equipment for size and selection criteria.
Room Air Conditioning Units. These units are
self-contained units serving only one space. These units are
typically referred to as window or through-the-wall type air
conditioners. Rooms served by these units should have a separate
HVAC unit to provide ventilation air for a group of rooms. Use
them when they are life cycle cost effective, and in accordance
with MIL-HDBK-1190. Refer to ASHRAE Equipment Handbook.
Built-up Systems. These systems consist of individual
components assembled at the building site. Generally, use them
when a large volume of air is handled. These systems may be used
as remote air handling systems with a central cooling plant.
They are generally more efficient and better constructed than
unitary air handling units. Determine the number of air handling
units by an economic division of the load, considering: (a) the
value of space occupied by equipment; (b) the extent of ductwork
and piping; (c) the multiplicity of control, maintenance, and
operating points; and (d) energy conservation factors.
Heating Systems. Heating sources can be either steam,
hot water, natural gas, oil, electricity, or a renewable
resource. Select these sources based on life cycle cost.
Heating systems may be combined with ventilating systems when
at windows in office spaces.
Individual Heating Plants. Locate individual heating
plants in the building they serve or in a separate, adjoining