CEMP-E
TI 810-11
30 November 1998
Figure 4-17a. Control system schematic for VAV HVAC system without return fan.
Figure 4-17b. Control system ladder diagram for VAV HVAC system without return fan.
Figure 4-17c. Control system equipment for VAV HVAC system without return fan.
Figure 4-17d. Control panel interior door layout for VAV HVAC system without return fan.
Figure 4-17e. Control panel back panel layout for VAV HVAC system without return fan.
Figure 4-17f. Control panel terminal block layout for VAV HVAC system without return fan.
Figure 4-17g. DDC control system schematic for VAV HVAC system without return fan.
Figure 4-17h. DDC control system ladder diagram for VAV HVAC system without return fan.
Figure 4-17i. DDC control system equipment for VAV HVAC system without return fan.
Figure 4-17j. DDC control system I/O table and data terminal strip layout for VAV HVAC system without
return fan.
b. General sequence of operation.
(1) Supply fan off. When the fan is off, the cooling coil valve and the outside air, relief air, and
supply fan inlet vane dampers are closed. The return air damper is open.
(2) Supply fan operating. When the supply fan is on, the cooling coil valve and the control
dampers are operated as required by the system's mode of operation. The control dampers are either
positioned for full recirculation of air, positioned to introduce minimum outside air, or modulated to
maintain the mixed air temperature. The cooling coil valve is either closed, or modulated to maintain
unit discharge temperature.
(3) Control of supply fan. Unless the fan is stopped as the result of a safety shutdown, it is on or
off as required by the control system's mode of operation. When the fan is on, the inlet vanes are
modulated to maintain the required supply air static pressure.
(4) Safety shutdown of the fan. The control system shuts down the fan if there is a low
temperature condition, if smoke is detected, or if a high static pressure condition is detected.
(5) Low temperature detection. On a fall in temperature to its setpoint, a freezestat stops the
supply fan. To restart the fan, the freezestat and the control panel must be manually reset.
(6) Smoke detection. Duct smoke detectors stop the supply fan whenever either detects the
presence of smoke. To restart the fan, the smoke detectors and the control panel must be manually
reset.
(7) High static pressure detection. On a rise in static pressure above the setpoint of a high limit
static pressure switch downstream of the supply fan, the fan stops. To restart the fan, the control panel
must be manually reset.
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