Specifications

JOHNSON CONTROLS
2-60
FORM 102.20-N1 (1109)
Installation
There are four basic components in a VAV system -
an air-handling unit with airow control (i.e. variable-
speed drives), VAV boxes, zone thermostats and duct
static pressure sensors. All of 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 airow into the space
and as the zone temperature decreases, the VAV boxes
close to decrease the airow to the space.
As the VAV boxes in the system 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 pressure sensor controls the air handling unit
supply fan. Since an increase in duct pressure relates
to a decrease in the zone airow required, the supply
fan volume decreases in response. Conversely, a lower
duct static pressure indicates a need for increased zone
airow; therefore, the supply fan volume increases
in response. The change in supply air volume is
accomplished using a Variable Frequency Drive or
similar device.
In the air-handling unit a decrease in airow through
the DX coil will result in a corresponding decrease in
the suction gas pressure while an increase in airow
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 staged
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 ood back.
Problems can arise if the airow 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 minute on VAV systems.
This limitation will promote stable control of the
system and minimize uctuations in zone temperature.
Under any circumstances, a minimum of 350 FPM
face velocity across the coil must be maintained for
DX split systems.