30 November 1998
under the control of the hot deck temperature controller. Zone dampers are under the control of their
(2) Supply and return fans operating. When the fans are on, 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 the
cold deck discharge temperature. The heating coil valve is modulated to maintain the hot deck
discharge temperature. Zone dampers are under the control of their zone thermostats.
(3) Control of supply fan and return fan. 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 and return fans. To restart the fans, the thermostat and the
control panel must be manually reset.
(6) Smoke detection. Duct smoke detectors stop the supply and return fans whenever either
detects the presence of smoke. To restart the fans, the smoke detectors and control panel must be
(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 mode of operation no longer requires the
outside air, return air, and relief air dampers to be in their full recirculation operating 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 outside 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 temperature controller.
(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 ventilation delay and occupied modes.
(11) Hot deck temperature control. A hot deck temperature controller with its temperature
transmitter in the heating coil discharge modulates the heating coil valve to maintain its temperature
setpoint. Outside air temperature controller TC-XX-04 with its temperature transmitter in the outside air
lowers the setpoint of the hot deck controller as outside air temperature rises. On a fall in outside air
temperature, the reverse occurs.