30 November 1998
(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 fan is 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
the cooling coil valve to maintain its setpoint temperature. During the unoccupied mode, the cooling coil
valve is not controlled and remains closed.
(13) Space temperature control. Each controlled space is equipped with a variable air volume
(VAV) box, which is controlled by a microprocessor based VAV box controller. The controller receives
temperature signals from a temperature sensing element in the space served and from a flow sensor
upstream of the VAV box. On a fall in space temperature, the controller modulates the damper toward
minimum position, to maintain the cooling mode setpoint. After the minimum position is reached, the
controller is inactive while the space temperature falls through a temperature deadband. On a further fall
in temperature below the temperature deadband, the heating coil valve modulates to maintain the
(14) Unoccupied mode of operation. Throughout the unoccupied mode, the outside air and
relief air dampers and the cooling coil valve remain closed, and the return air damper remains open.
The supply fan is cycled by the system's night thermostat to maintain its low limit space temperature
(15) Ventilation delay mode of operation. During the ventilation delay mode, the dampers
remain as they were throughout the unoccupied mode, and the supply fan runs continuously. Until the
ventilation delay mode ends, return air is recirculated, to bring the building to comfort conditions using a
minimum of energy.
(16) Occupied mode of operation. The supply fan runs continuously, the minimum outside air
damper is modulated to maintain minimum outside air flow, and the economizer outside air and relief air
dampers are either closed or are under mixed air temperature control as previously described.
c. Detailed sequence of operation.