technician to service, it is less likely that the equipment will receive regular
maintenance. Routine maintenance such as replacing filters, and less routine
maintenance, such as replacing compressors, fans, belts, shafts, bearings, and
coils is essential to maximize a system’s life-cycle efficiency.
Displacement Ventilation
The concept of “displacement ventilation” can drastically improve HVAC sys-
tem efficiency and ventilation quality. Traditional ventilation mixes a turbulent
stream of fresh air with waste air that has been exhaled by occupants of a space,
creating a constant mix of medium-quality air. Displacement ventilation uses a
slow-moving stream of fresh air from the floor to displace the waste air, which
is forced to the ceiling and then out of the room through exhaust panels.
This creates two levels of air in a room, with cool fresh air in the occupied
lower part of the room, and warm waste air in the unoccupied upper part of
the room. This also creates natural convection, because as the cool air rises it
cools the occupants and then the heat taken from them is expelled from the
room. Displacement ventilation can eliminate the need for large HVAC equip-
ment, not only reducing energy costs but initial and maintenance costs as well.
HVAC Controls
HVAC systems must control variable conditions of the system and its com-
ponents. These conditions include liquid and gas pressure, temperature,
humidity, the flow rate of liquids and gases and the speed and on/off state
of mechanical equipment.
A number of instruments and terminal devices available in the field are used
to gather data on the system and assist in controlling it. System controllers use
input and data from sensor devices to make decisions about the system, and
then, based on the input information, control actuator devices.
Sensors and transmitters include thermostats, liquid differential-pressure
transmitters for pumps and chillers, differential pressure sensors for fluids and
airflow, static pressure sensors, air-pressure sensors, and humidity sensors.
An example of an actuator or operator is an actuator for a damper that is
mounted to the damper shaft and triggers the start of the damper operation.
That operation could be a temperature sensor detecting a high temperature
and sending a signal to the controller, which results in the controller sending
a signal to an actuator to engage a motor that opens or closes a damper or vent.
These devices may communicate to each other or to the controller with ana-
log or digital signals. Analog inputs to a controller can be a continuously
Heating, Ventilating, and Air Conditioning Systems 43