30 November 1998
(2) Supply air and return air fans operating. When the fans are on, the control dampers and the
cooling coil valve are operated as required by the system's mode of operation. Zone heating coil valves
and dampers are under the control of their zone thermostats. 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 cold deck temperature.
(3) Control of supply air and return air 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.
(4) Safety shutdown of the fans. The control system shuts down the fans if there is a low
temperature condition, or if smoke is detected.
(5) Low temperature detection. On a fall in temperature to its setpoint, a low temperature
protection thermostat stops the supply air and return air fans. To restart the fans, the thermostat and the
control panel must be manually reset.
(6) Smoke detection. Duct smoke detectors stop the supply air and return air fans whenever
either detects the presence of smoke. To restart the fans, the smoke detectors and control panel must
be manually reset.
(7) Filter condition. Filter condition is monitored by a pressure gauge and a differential pressure
switch. On a rise in pressure drop across the filter to the switch setpoint, the switch turns on a pilot light.
(8) Economizer control. When the control system modes of operation no longer require the
outside air, return air, and relief air dampers to be in their full recirculation positions, the dampers are
positioned to admit outside air for ventilation. The dampers then remain at minimum position until the
economizer controller closes both its PV and DEV contacts. 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 mixed air temperature control.
(9) Mixed air temperature control. A mixed air temperature controller with its temperature
transmitter in the mixed air modulates the outside air and relief air dampers between minimum position
and fully open, to maintain a mixed air temperature of 13 degrees C (55 degrees F). As the outside air
and relief air dampers open, the return air damper closes.
(10) Cold deck temperature control. A cold deck temperature controller with its temperature
transmitter in the cold deck discharge modulates the cooling coil valve to maintain its setpoint
temperature. During the unoccupied mode, the cooling coil valve is not controlled and remains closed.
(11) Space temperature control. On a rise in temperature, each zone thermostat first modulates
its zone reheat coil valve toward closed. On a further rise in temperature after a space temperature
deadband, each zone thermostat modulates its cold deck damper toward open and simultaneously
modulates its bypass damper toward closed.
(12) 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 and return fans are cycled by the system's night thermostat to maintain its low limit space