30 November 1998
(2) Supply fan operating. When the fan is on, the pump is on, and the outside air damper, heat
exchanger valve, preheat coil valve, cooling coil valve, reheat coil valve and humidifier valve are
operated as required by the system's operational modes. The outside air damper is either open or
closed. The preheat coil and reheat coil valves are under the control of their respective controllers. The
cooling coil valve is either closed or modulated to maintain the space temperature and/or humidity.
(3) Control of 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 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 they detect 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) Preheat coil control. The preheat coil valve is modulated to maintain a constant preheat coil
(9) Space temperature control. The reheat coil valve and cooling coil valve are modulated in
sequence to maintain a constant space temperature.
(10) Space humidity control. The humidifier valve and cooling coil valve are modulated in
sequence to maintain a constant relative humidity in the space. A high limit humidity control overrides
the space control of the humidifier valve when necessary, to prevent the relative humidity in the supply
duct from exceeding its high limit setpoint.
(11) Unoccupied mode of operation. Throughout the unoccupied mode, the outside air damper
and cooling coil valve remain closed. The supply fan is cycled by the night thermostat to maintain its low
limit space temperature setpoint.
(12) Ventilation delay mode of operation. Throughout the ventilation delay mode, the outside air
damper remains closed. The reheat coil and cooling coil valves are under the control of the room
temperature controller. Return air is circulated, to bring the building to comfort conditions, using a
minimum of energy.
(13) Occupied mode of operation. The supply fan runs continuously with the outside air
dampers open. The space temperature and humidity are controlled as previously described.
c. Detailed sequence of operation.
(1) Timeclock 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