30 November 1998
(2) Supply and return fans operating. When the fans are on, the control dampers and the
cooling coil and heating coil valves are operated as required by the system's mode of operation. 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 duct discharge temperature setpoint. The heating coil valve is modulated
to maintain the hot duct discharge temperature. Dual-duct boxes are under the control of their
(3) Control of supply and return 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 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. When the pressure drop across the filter reaches the switch setpoint, the switch turns on a pilot
(8) Economizer control. When the control system's mode of operation no longer requires the
outside air, return air, and relief air dampers to be in their full recirculating 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 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 damper 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 duct temperature control. A cold duct temperature controller with its temperature
transmitter in the cold duct discharge modulates the cooling coil valve to maintain its setpoint
temperature, during the ventilation delay and occupied modes.
(11) Hot duct temperature control. A hot duct temperature controller with its temperature
transmitter in the heating coil discharge modulates the heating coil valve to maintain its temperature
setpoint. Another controller with a temperature transmitter in the outside air lowers the setpoint of the hot
duct controller as outside air temperature rises. On a fall in outside air temperature the reverse occurs.