30 November 1998
Figure 4-22g. DDC control system schematic for single zone HVAC system with DX coil.
Figure 4-22h. DDC control system ladder diagram for single zone HVAC system with DX coil.
Figure 4-22i. DDC control system equipment for single zone HVAC system with DX coil.
Figure 4-22j. DDC control system I/O table and data terminal strip layout for single zone HVAC system
with DX coil.
b. General sequence of operation.
(1) Supply fan off. When the fan is off, the outside air and relief air dampers are closed and the
return air damper is open. The heating coil valve is under control.
(2) Supply fan operating. When the fan is on, the control dampers are either positioned for full
recirculation of air, positioned to introduce minimum outside air, or modulated to maintain space
temperature. The cooling is either de-energized or cycled in stages to maintain space temperature.
(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) 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 amount of ventilation air then remains at minimum
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 control.
(9) Space temperature control. On a rise in space temperature, the heating coil valve is
modulated toward closed. On a further space temperature rise, economizer control permitting, the
outside air and relief air dampers are modulated between minimum position toward fully open, and
simultaneously the return air damper is modulated toward fully closed to maintain space temperature.
On a still further rise in temperature, the stages of cooling are cycled to maintain the space temperature.