Form PA102.20-N1
107
Installation & Assembly Manual │
PACE
SECTION 3 - HANDLING, STORAGE, AND INSTALLATION
Issue Date: 07/03/2018
3
VAV SYSTEMS
Overhead VAV systems are the preferred method of
air distribution because they offer greater energy effi-
ciency and better control of building diversity than con-
stant volume systems (CVS). Unlike a CVS, in which
the leaving air temperature is adjusted to satisfy the
cooling load, the air temperature in a VAV system re-
mains constant, and the air volume is varied to meet
the cooling requirements.
A VAV system has four components:
1. AHU with airflow control (i.e. Variable Frequency
Drives (VFDs)),
2. VAV boxes,
3. Zone thermostats, and
4. Duct static pressure sensors.
These components must work together to provide good
temperature control, and a comfortable environment.
The zone thermostats control the VAV boxes. As the
zone temperature increases, the VAV boxes open to
allow greater airflow into the space, As the zone tem-
perature decreases, the VAV boxes close to decrease
the airflow to the space.
As the VAV boxes open and close, the static pressure
in the ductwork changes. When a box opens, the duct
static pressure decreases, and when a box closes, the
duct static pressure increases. The duct static pres-
sure sensor controls the AHU supply fan. Since an in-
crease in duct pressure relates to a decrease in the re-
quired zone airflow, the supply fan volume decreases.
Conversely, a lower duct static pressure indicates a
need for increased zone airflow; the supply fan volume
increases. A change in supply air volume is accom-
plished using a VFD or similar device.
In the AHU, a decrease in airflow through the DX coil
will result in a corresponding decrease in the suction
gas pressure while an increase in airflow will result
in an increase in the suction gas pressure. Since the
system is designed to maintain a constant suction gas
pressure, the compressors will be turned on or off,
as needed, to meet the increase or decrease in load
demand. The system should be designed to operate
smoothly, avoiding transients that could upset system
balance and cause liquid flood back.
Problems can arise if the airflow de-
creases more quickly than the com-
pressor control can respond to the load
change. Therefore, airflow should never
change at a rate faster than 3% per min-
ute on VAV systems.
This limitation will promote stable control of the system
and minimize fluctuations in zone temperature. Under
any circumstances, a minimum of 350 FPM face ve-
locity across the coil must be maintained for DX split
systems.
