efficient lighting and electronics. Incandescent lighting generates large
amounts of heat while lighting an area. Modern energy-efficient fluorescent
lighting provides the same and often better light quality with much lower heat
output. Computer systems and servers also generate significant heat, and can
be upgraded to more energy-efficient versions. Dark colors on the outside of
a building increase the absorption of solar heat, increasing the thermal load
of an HVAC system.
Reducing cooling loads can be accomplished by installing better insulation
and more efficient windows and sealing air leaks. Interior spaces in a building
often need cooling during times when the outdoor air temperature and humidity
are sufficiently low to provide cooling without running refrigeration equipment.
Economizers use controls and supply and return air dampers to provide “free”
cooling by circulating outdoor air into a building when conditions allow.
Equipment Sizing
Correctly sizing HVAC equipment is extremely important. HVAC systems are
often designed to handle the maximum heating and cooling loads possible for
an area, keeping a building cool on the hottest days and warm on the coldest
days. This leads to the HVAC system being oversized to handle just a few days
out of the year. Oversizing HVAC equipment wastes energy and can also be
uncomfortable for inhabitants by overheating or overcooling a space. Simi-
larly, undersizing can lead to discomfort and underventilation, which can lead
to high carbon levels and poor health for inhabitants.
Sizing is such an important step that it needs to be well thought out before a
system is even designed. Buildings with floor areas less than 20,000 square feet
typically use factory-built, air-cooled “unitary” (packaged) equipment; build-
ings from 20,000 to 100,000 square feet typically use multiple large packaged
units; and buildings larger than 100,000 square feet and multibuilding cam-
puses generally use water-cooled systems specifically built for that location.
Despite these “rules of thumb” the correct sizing of an HVAC system’s
cooling capacity should not be based purely on the size of a building. The
envelope load (windows, walls, and roof), the internal thermal loads (lights,
people, and equipment), and the ventilation load must also be considered.
ASHRAE provides proven methods for calculating heating and cooling loads.
To improve efficiency an HVAC system must be considered as a whole sys-
tem rather than a number of parts or components. For example, in a chilled-
water system the chiller is the main component and the largest energy con-
sumer. However, simply selecting a high-efficiency chiller does not guarantee
a high performance system. Auxiliary equipment, such as fans and blowers,
Heating, Ventilating, and Air Conditioning Systems 41