can also have substantial effects on efficiency. According to the U.S. Depart-
ment of Energy, fans, and pumps account for 64% of the electricity consumed
by a commercial HVAC system.
Inefficient auxiliary equipment and a poorly designed, sized and placed
system can severely reduce overall system efficiency. For example, exhausts
and intakes must have maximum airflow without restrictions, condensing units
should be spaced far enough apart so as to prevent the recirculation of heat,
and outdoor units should be located close to indoor units.
HVAC Sequence of Operation
HVAC systems operate based on a predetermined sequence of operation. The
sequence of operation is critical to the efficiency and effectiveness of the
HVAC system. It is the sequence of operation that will determine how system
components interact. As an example, an HVAC system is typically triggered
when the thermostat senses that a space’s air temperature differs from the ther-
mostat’s set temperature. When that happens, contacts in the thermostat are
closed and control voltage is supplied to the control board terminals, which
causes the blower in the AHU to start.
The AHU control board then supplies control voltage to the condenser, caus-
ing a device known as a contactor to close its contacts. This supplies power to the
compressor, which then raises the pressure and temperature of the refrigerant
gas in the system, forcing it through the condenser coils. In the condenser the
refrigerant is cooled and condenses from a gas to a liquid and is then pumped
indoors to the evaporator through a metering device that regulates the flow to
the evaporator coil, causing the refrigerant pressure to decrease. Air is then cir-
culated over the evaporator coil by the fan in the AHU, transferring heat from
the air to the refrigerant. This heat raises the temperature of the refrigerant above
its saturation point, changing it back into a gas. The refrigerant gas is then
pumped back to the condenser where the process repeats itself until the thermo-
stat senses that the temperature is at the desired level, opening the contacts and
shutting off the control voltage supplied to the furnace or the air conditioner.
Maintenance
The placement of certain components also affects how often a system will
undergo maintenance. Maintenance of an HVAC system is critical to maintain its
efficiency and performance and adequate space to perform such maintenance is
crucial. When equipment is crammed into a space that makes it difficult for a
42 Smart Building Systems for Architects, Owners, and Builders