30 November 1998
(1) Supply and return fans off. When the fans are off, the cooling coil valve and the outside air,
relief air, supply fan inlet vane, and return fan inlet vane dampers are closed. The return air damper is
(2) Supply and return fans operating. When the fans are 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 mixed air temperature. The cooling coil valve is either closed, or modulated to maintain unit
(3) Control of supply and return fans. Unless the fans are stopped as the result of a safety
shutdown, they are on or off as required by the control system's mode of operation. When the fans are
on, the inlet vanes are modulated to maintain the required supply air static pressure and return air flow.
(4) Safety shutdown of the fans. The control system shuts down the fans 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 and return fans. To restart the fans, the freezestat and the control panel must be manually reset.
(6) Smoke detection. Duct smoke detectors stop the supply and return fans whenever either
detects the presence of smoke. To restart the fans, the smoke detectors and the control panel must be
(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 fans stop. To restart the fans, the control panel
must be manually reset.
(8) Filter condition. Filter conditions are monitored by a pressure gauge and a differential
pressure switch. When the pressure drop across the filter exceeds the switch setpoint, the switch turns
on a pilot light.
(9) Minimum Outside Air Flow Control. A minimum outside air flow controller accepts a signal
from a outside air flow station transmitter. When the fans are on, with the control system in the occupied
mode, and with the ventilation delay mode off, the outside air flow controller modulates the minimum
outside air damper to maintain the control setpoint.
(10) Economizer control. The economizer controller closes its PV contacts when the return air
temperature indicates that the building requires cooling rather than heating. The economizer controller
closes its DEV contacts when the outside air temperature is sufficiently below the return air temperature
to be effective for cooling. When both these contacts close, the dampers are modulated by the mixed air
(11) Mixed air temperature control. A mixed air temperature controller, with its temperature
transmitter in the mixed air, modulates the economizer outside air damper, relief air damper, and the
return air damper to maintain a mixed air temperature of 13 degrees C (55 degrees F). As the
economizer outside air and relief air dampers open, the return air damper closes.
(12) Cooling coil temperature control. During the ventilation delay and occupied modes, a
cooling coil temperature controller, with its temperature transmitter in the unit discharge air, modulates