30 November 1998
(3) Control of the 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.
(4) Safety shutdown of the fans. The control system shuts down the fans if there is a low
temperature condition or 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 in the supply and return air 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. On a rise in pressure drop across the filter to the switch setpoint, the switch turns on a pilot light.
(8) Unoccupied mode of operation. Throughout the unoccupied mode, the outside air and relief
air dampers 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 setpoint.
(9) 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, the HVAC system circulates return air to bring the building to comfort conditions, using
a minimum of energy.
(10) Occupied mode of operation. The supply fan runs continuously. The heating coil valve
and control dampers are modulated, with a deadband between them, by the space temperature
c. Detailed sequence of operation.
(1) Time clock CLK-XX-01 has two independent sets of contacts, which between them determine
the mode under which the system operates. Five minutes before the scheduled beginning of the
occupied mode, the ventilation delay contacts close, energizing relay R-XX-03 and turning on pilot light
PL-XX-02. The normally closed contacts of relay R-XX-03 open, to prevent relay R-XX-02 from being
energized. The normally open contacts of relay R-XX-02 prevent signals from reaching current-to-
pneumatic transducer IP-XX-01. The dampers remain in their normal positions, with outside air and relief
air dampers closed and return air damper open.
(2) When the occupied contacts of time clock CLK-XX-01 close, relay R-XX-01 is energized and
pilot light PL-XX-01 is turned on. Contacts of relay R-XX-01 energize the supply fan. The auxiliary
contacts of the supply fan starter energize relay R-XX-04.
(3) When the ventilation delay contacts of time clock CLK-XX-01 open to end the ventilation
delay mode, relay R-XX-03 is de-energized and pilot light PL-XX-02 is turned off. With the now closed
but normally open contacts of energized relays R-XX-01 and R-XX-04, the normally closed contacts of
relay R-XX-03 energize relay R-XX-02. The normally open contacts of relay R-XX-02 connect the output
signal of high signal selector TY-XX-01 to current-to-pneumatic transducer IP-XX-01. The pneumatic
output of transducer IP-XX-01 places the dampers at the minimum position set on minimum position