Air Terminal Units
Air terminal units (ATUs) address specific HVAC thermal loads or zones.
Thermal loads in a space can consist of exterior loads (outside air temperatures
increasing or decreasing) and interior loads (people, lighting, computers, and
other sources). A thermal zone is a space or group of adjoining spaces in a
building that have similar thermal loads.
Defining thermal zones in a building reduces the amount of HVAC subsys-
tems needed because a single subsystem can usually handle an entire thermal
zone. ATUs compensate for these thermal loads and zones by varying the
air temperature, varying the air volume, or doing both. While constant air vol-
ume (CAV) systems provide air at a variable temperature and constant flow
rate, variable air volume (VAV) systems provide air at a constant temperature
and regulate the room temperature by changing the flow rate of the air into the
room. VAVs can be pressure independent, where the flow is maintained con-
stant regardless of the inlet pressure, or they can be pressure dependent, where
the flow rate of the VAV is dependent on the inlet pressure and, typically, the
position of its damper or speed of a fan.
VAVs are more energy efficient than CAVs. VAVs use less air volume
resulting in less energy used for heating, cooling, and operation. Most residen-
tial HVAC systems are CAVs, while most new commercial buildings have
VAV systems.
Efficiency
Air conditioner efficiency is essentially a ratio of the cooling capacity of the
unit versus its required power input and is usually measured as follows: the
energy efficiency ratio (EER), seasonal energy efficiency ratio (SEER), or coef-
ficient of performance (COP). EER is a measure of an air conditioner’s effi-
ciency at its maximum air conditioning load, and is calculated by dividing
the unit’s cooling or heating capacity in BTUs per hour by watts of electrical
input. EER is based on a constant 95F outdoor temperature. A better and
more widely used measure of efficiency is SEER.
Although it uses the same units as EER, SEER provides a more accurate
measure of a unit’s efficiency because it is taken as an average of various
operating conditions and seasonal temperatures. It is designed to give the most
accurate representation of actual operating conditions for the air conditioner.
Because it is based on seasonal external temperatures SEER ratings depend on
geographical location and thereby differ by area.
Heating, Ventilating, and Air Conditioning Systems 39