Fan Control; Sensor 2 (Fan Status Or Discharge Air Temp); Economizer - KMC Controls FlexStat Application Manual

Fan Control

The following is the base sequence for fan operation.
The occupied and the unoccupied modes have separate configurations
for fan control. Each scheduled mode can be configured to operate the
fan in either continuous or automatic mode.
If the configuration for a particular occupancy mode is set to "On," there
will be a continuous call for fan during that scheduled state. If the fan
is set to "Auto," the fan will cycle only on a call for heating or cooling.
Because PI control is used in heating and cooling, values may be slightly
skewed, but generally a call for heating turns on at active heating set-
point (minus deadband) and off at active heating setpoint. Generally, a
call for cooling turns on at active cooling setpoint (plus deadband) and
off at active cooling setpoint.
There are certain application-specific variations of how the fan control
functions that are detailed in the specific application sections below.
The FlexStat has two options concerning fan status:
• If the Sensor 2 option is set to either "not used" or "discharge air
temp," the unit is configured to not have fan status. The fan mo-
tor is then assumed to always respond when commanded to run.
• If the unit is configured to have fan status, and if the fan is com-
manded to run and the FlexStat does not see a response on the
fan status input within 5 seconds, a fan alarm is generated. Upon
fan alarm, all outputs other than the fan are de-energized.
There is a fan status icon on the user interface that indicates the fan run
command status. This is controllable through Control Basic.
If a low limit alarm is present, all fan outputs are de-energized.
BAC-1xx63/1xxx63 Series (6 Relays and 3 Analog Outputs) Section

Sensor 2 (Fan Status or Discharge Air Temp)

An option, Sensor 2, determines the use of analog input #2. If sensor 2 is
configured for fan status, the FlexStat performs the fan control sequence
as described above. If sensor 2 is configured for discharge air temp, the
FlexStat configures analog input #2 for a 10K type III thermistor. This
selection is intended for monitoring discharge air temperature of the
HVAC unit.
Note: This selection is for monitoring only—there is no HVAC
control associated when selecting discharge air temp.
Additionally, a trend log is set up for discharge air temp with a
10 minute interval. Lastly, sensor 2 may be configured for "not
used."

Economizer

Certain applications can be configured for an economizer option. If the
economizer option is enabled, the economizer mode is determined by
comparing outside air temperature (OAT) to the economizer enable
temperature. The economizer mode is enabled if the OAT is 1.0° F (0.56°
C) below the economizer enable temperature and disabled if the OAT is
1.0° F (0.56° C) above the econ enable temp.
During occupied mode, if the economizer mode is enabled, the outside
air damper will open to the greater of the minimum damper position
setting (adjustable, for ventilation) or the mixed air temperature PI loop.
The mixed air temp loop will modulate from 0 to 100% as the mixed air
temp rises above the mixed air temp setpoint.
The PI loop "P" component is calculated and reset based on an OAT
reset schedule. "P" increases as OAT gets colder, adding loop stability
at colder OA temperatures. When the OAT is at or above 3° F (1.67° C)
below MAT Min Temp (53° F or 11.7° C), the Econ Loop (LOOP4) "P"
factor is set to OAD Warm Temp (15° F or 8.3° C). The Econ Loop "P"
factor is set to OAD Cold Temp (30° F or 16.7° C) when the OAT is at or
below twice the difference between the above calculated temperature (3°
F below MAT Min Temp) and MAT Low Limit (45° F or 7.2° C). The "P"
factor resets linearly in between the above values. The following table il-
lustrates various "P" factor values using the default Fahrenheit settings:
34 Application Guide L