67
JOHNSON CONTROLS
FORM 100.50-EG12 (918)
Single Zone VAV (SZVAV) Mode
Units configured for SZVAV operation shall contain a supply fan VFD. The unit shall switch
between cooling mode, heating mode, and standby mode based on zone temperature.
In cooling mode, the supply fan speed shall be varied based on zone temperature. If the
zone temperature gets warmer, the supply fan speed shall increase. Conversely, if the
zone temperature gets cooler, the supply fan speed shall decrease. In heating mode, the
supply fan shall run at full speed. When the zone temperature is satisfied, the unit is nei-
ther in cooling mode nor heating mode, and the supply fan shall run at minimum speed.
Control of cooling and heating stages shall operate as described in the following section.
Unit Mode Determination (Hardwired or Communicated) – The unit compares
the analog wired zone temp or communicated zone temp input to the occupied zone
cooling, occupied zone heating, unoccupied zone cooling, or unoccupied zone heating
setpoints to determine the sub-mode of operation. Figure 5 on page 67 shows what
the unit mode would be based on the difference between the zone temperature and the
zone temperature setpoints.
The only difference between hardwired and communicated is the method the unit control-
ler uses to determine the zone temperature. In the hardwired mode the input is an analog
input to the control. In the communicated mode the input is a serial input from a bas con-
trol system.
LD19888
FIGURE 5 - OPERATIONAL MODE: SINGLE ZONE VAV (SZVAV)
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