Direct Digital Control (DDC)
Sequence of Operation
Microprocessor Controller
This unit is
equipped with a
DDC controller
that is factory
programmed,
mounted and
tested. The
controller
has an LCD
screen for easy
monitoring of unit operation and has a set of push
buttons for changing set points. Factory-installed
sensors are already mounted and wired, but there
are several optional sensors and control devices
that must be field-installed. Refer to the unit-specific
wiring diagram and the manual supplied with the DDC
controller.
Unit Start Command:
• Factory mounted and wired Outdoor Air actuator is
powered
• Supply fan starts
• Heating and cooling operation
Unit Stop Command (or De-energized):
• Supply fan and tempering options are de-energized.
• Outdoor Air damper actuators are de-energized and
damper will close.
Remote On / Off
Unit DDC may have an input allowing the unit to be
started or stopped by others.
Occupied / Unoccupied Modes:
These modes are based on a 7-day time clock that is
internal to the DDC. The schedule must be set by the
end user. When the user initiates an override input,
the DDC would switch from unoccupied to occupied
mode. The DDC will return to the scheduled occupied
/ unoccupied mode after the override time has expired
(60 minutes, adjustable). If the internal time clock is
disabled, a remote contact or a BMS can control the
occupied / unoccupied mode.
•
Occupied Mode
1. Supply Fan ON
2. Heating
3. Cooling
•
Unoccupied Mode (Unit Off)
Default setting when there is no room
temperature sensor.
Cooling Sequence
The DDC controller will power the compressed
refrigerant system to maintain the supply temperature
set point. The mechanical cooling will be locked out
when the outside air is less than 55° - 2°F hysteresis,
adjustable.
Dehumidification Sequence
The cooling is controlled to maintain the cooling-
coil set point. The dehumidification sequence will be
locked out when the OA is less than 10°F above the
cold-coil set point. The mechanical cooling will be
locked out when the outside air is less than 55° - 2°F
hysteresis, adjustable.
Reheat Sequence
While the unit is in dehumidification mode, the outdoor
air can be reheated for space neutral applications.
Heating Sequence
The heating is controlled to maintain the supply
temperature set point. The heating will be locked
out when the outside air is greater than 70° + 2°F
hysteresis, adjustable.
This unit may be optionally equipped with either an
indirect gas-fired furnace or an SCR controlled electric
heater.
•
Indirect Gas-Fired Furnace
The DDC will energize and modulate the heating
capacity of the furnace to maintain the Supply Air
temperature set point.
•
Electric Heater
The DDC will energize and modulate the heating
capacity of the electric heater to maintain the
Supply Air temperature set point.
Supply Set Point Reset Function
Either a room temperature sensor or the outdoor
air reset function (if there is no room temperature
sensor wired to the DDC) will determine the supply
temperature of the unit.
•
Outdoor Air Reset Function
Without a room temperature sensor available,
the DDC will default to discharge temperature
control, based on outdoor air temperature. The
DDC will monitor the OA temperature and reset
the supply temperature set point based upon
the outdoor air reset function.
•
Optional Room Temperature Sensor
With a room temperature sensor, the DDC
will adjust the supply temperature set point
up / down accordingly to satisfy the desired
room temperature. Cooling and heating are
determined by a difference in temperature of
the room temperature sensor compared to the
desired room temperature set point (adjustable).
Typical DDC Controller
16
Model MPX Make-Up Air Unit
Model XMPX Make-Up Air Unit
