Minimum Position - EngA C-TRAC3 Installation, Operation And Maintenance Manual

A
C-TRAC3
If input terminal E is powered (24Vac), the modulating output signal will drop to the minimum outside air
position setting. An optional enthalpy control is normally connected to this terminal.
Unoccupied mode will disable the mixing dampers to 100% return air.

MINIMUM POSITION

The normal setting for the minimum amount of outside air is 15%, although there are many situations
where this number could vary substantially. Refer to the unit function sheet to determine the required
amount.
The mixed air minimum position setpoint is typically adjusted at the C-TRAC3 using the face mounted
minimum position potentiometer; however a variety of methods could be employed. Input terminal MP
may be used to set the minimum position from a remote 0-10Vdc signal (10Vdc = 100% outside air). The
minimum position can also be programmed into the C-TRAC3 through the RS232 communication port from
a computer or directly controlled from an EMS signal. If the minimum position potentiometer is not used
it should be set to zero (turned fully CCW) as it remains in the hardware control circuit.
The mixed air minimum position may overridden from another device, such as a building static pressure
sensor or CO sensor. Refer to the unit function and wiring diagram to clarify. If used, this is normally a
"flow through" device that would input to the MP terminal on the C-TRAC3 first. The C-TRAC3 would then
output the appropriate signal to the damper actuator(s), rather than the minimum position signal directly
feeding into the damper actuator.
Variable Frequency Drives
If the equipment is employing variable frequency drives to control the fan speed (and program options are
selected), a feedback signal of actual speed (10Vdc = 60Hz) is connected to the C-TRAC3. This is to ensure
the airflow is adequate to correctly operate and safeguard the equipment.
As the total air volume drops when the VFD speed decreases, the amount of minimum outside air also
decreases. To compensate for this, the C-TRAC3 will automatically increase the minimum outside air
signal to the damper actuator(s) to allow for more outside air.
Under low air volume conditions the C-TRAC3 will limit the maximum amount of heating and cooling
output for improved temperature control and to safeguard the equipment. This is referred to as 'load
shedding'. This feature has been removed in versions 3.6 and later.
Refer to the variable air volume section of this manual for additional information.
Mixed Air Low Limit Override
To avoid low limit failure, if the discharge air temperature approaches the low limit setpoint (typically
40°F) the C-TRAC3 will try and keep the unit operational by decreasing the minimum outside air position
down to ½ of its setpoint.
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