28
Fig. 19 — Minimum Damper Position Curves
FREE COOLING
The economizer will be enabled for cooling (OK TO FREE
COOL? = Yes) if the supply air temperature sensor reading is
valid, there are no applied lockouts, and economizer is opera-
tional. Economizer Operational (ECON OPERATIONAL?)
indicates if an economizer is installed (ECON INSTALLED?)
and feedback indicates it is operational. The three economizer
lockouts that determine if free cooling should be used to help
with cooling are: Dry Bulb Lockout (DRY BULB LOCKOUT),
Enthalpy Lockout (ENTHALPY LOCKOUT), and Unoccu-
pied Free Cooling Lockout (UFC LOCKOUT?). Any one of
these lockouts will disable economizer free cooling. See below
for how each lockout occurs.
When the economizer is available for free cooling (OK TO
FREE COOL? = Yes) and the compression is not on, the damper
will start opening from the damper’s minimum Position in Effect
(EFFECTIVE MIN POS) based on the supply air temperature
(SUPPLY AIR TEMP) to provide free cooling. A low cooling
demand (DEMAND = LOW COOL) will utilize the Low Free
Cooling SAT Setpoint (LOW COOL SAT SP) as the Free Cool-
ing Setpoint (FREECOOL SAT SP) to control the economizer.
A medium or high cooling demand (DEMAND = MED COOL
or HIGH COOL) will utilize the High Free Cooling SAT Set-
point (HIGH COOL SAT SP) as the Free Cooling Setpoint
(FREECOOL SAT SP) to control the economizer.
During free cooling, the fan will start at the dedicated free
cooling speed (FREE COOL IDF SPD). After the economizer
(ECON CMD POSITION) reaches 100% (or Max) for 5 min-
utes, the fan will be changed to the High Cool Speed (HIGH
COOL IDF SPD). When a high cooling demand (DEMAND =
HIGH COOL) is active, the control will use the High Cool
Speed (HIGH COOL IDF SPD). The compressor will be al-
lowed for use after the fan and economizer are 100% (or Max)
for 5 minutes. Once compression is turned on, the economizer
and fan will remain at 100% until the call for cooling is re-
moved or until the unit is no longer allowed to free cool (OK
TO FREE COOL = No).
Dry Bulb Lockout
Dry Bulb Lockout (DRY BULB LOCKOUT) occurs when any
of the following are true:
• The Outdoor Air Temperature (OUTDOOR AIR TEMP)
is invalid.
• When Differential Dry Bulb Control is disabled (DIFF
DRY BULB CTL = Disable) and the Outdoor Air Tem-
perature (OUTDOOR AIR TEMP) is greater than the con-
figured Free Cooling Maximum Temperature (FREE
COOL
MAX OAT) or less than the configured Free Cool-
ing Minimum Temperature (FREE COOL MIN OAT).
• When Differential Dry Bulb Control is enabled (DIFF
DRY BULB CTL = Enable) and the return air temperature
(RETURN AIR TEMP) plus the Differential Dry Bulb
deadband (DIFF DB DEADBAND) is lower than the out-
door air temperature (OUTDOOR AIR TEMP).
Enthalpy Lockout
The control uses the Outdoor Air Temperature (OUTDOOR
AIR TEMP), Outdoor Relative Humidity (OARH LEVEL),
and Barometric Pressure (BAROMETRIC PRESS) to calcu-
late the Outdoor Enthalpy (OUTDOOR ENTHALPY). The
control uses the Return Air Temperature (RETURN AIR
TEMP), Return Relative Humidity (RARH LEVEL), and
Barometric Pressure (BAROMETRIC PRESS) to calculate the
Econo Max
Position
(DAMPMAX)
Damper Position
Indoor Fan Speed
Maximum Speed
(SPEEDMAX)
10%
MINP_
MAX
(MP_SPD3,
MP_POS3)
(MP_SPD2,
MP_POS2)
(MP_SPD1,
MP_POS1)
Key
:
Minimum Position Curve
IAQPMAX
(AQP_SPD3,
AQP_POS3)
(AQP_SPD2,
AQP_POS2)
(AQP_SPD1,
AQP-POS1)
IAQ Purge Position Curve
IAQMINP
(AQ_SPD3,
AQ_POS3)
(AQ_SPD2,
AQ_POS2)
(AQ_SPD1,
AQ_POS1)
IAQ Minimum Position Curve
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