30 November 1998
(4) Safety shutdown of the fan. The control system shuts down the fan 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 fan. To restart the fan, the thermostat 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 control panel must be manually reset.
(7) Filter condition. Filter condition is monitored by a pressure gauge and a differential pressure
switch. When the rise in pressure drop across the filter reaches the switch setpoint, the switch turns on a
(8) Economizer control. When the system does not need full return air recirculation, the
dampers are set 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 as part of the space temperature
(9) Space temperature control. On a rise in space temperature, the heating coil valve is
modulated toward closed. On a further rise in temperature, the outside air and relief air dampers are
modulated from minimum position toward fully open while the return air damper is modulated toward
fully closed. On a still further rise in space temperature, the cooling coil valve is modulated toward open.
The reverse occurs on a fall in temperature.
(10) 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
(11) 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 circulated, to bring the space to comfort conditions using a
minimum of energy. Heating coil and cooling coil valves are under space temperature control.
(12) Occupied mode of operation. The supply fan runs continuously, and the outside air and
relief air dampers are at a minimum position or are under space temperature control as previously
c. Detailed sequence of operation.
(1) Timeclock CLK-XX-01 has two independent sets of contacts, which between them determine
the mode of system operation. Five minutes before the scheduled beginning of the occupied mode, the
ventilation delay contacts close, energizing relay R-XX-04 and lighting pilot light PL-XX-02. The normally
closed contacts of relay R-XX-04 open, preventing relay R-XX-03 from being energized. The normally