22
The shape of the Economizer Minimum Position vs. Fan Speed
curve is determined by the configuration parameters: Econ Min at
25% Fan speed (ConfigurationECONMP.25), Econ Min at
50% Fan speed (ConfigurationECONMP.50), Econ Min at
75% Fan speed (ConfigurationECONMP.75) and Econ Min
at Max Fan speed (ConfigurationECONMP.MX). These
configurations are preset at the factory for default purposes. The
Econ Min at Max Fan Speed (MP.MX) should be changed based
on the air balance of the unit for proper ventilation. The Econ Min
at 25% Fan speed (MP.25), Econ Min at 50% Fan speed (MP.50)
and Econ Min at 75% Fan speed (MP.75) damper positions will be
calculated and changed automatically after changing the Econ Min
at Max Fan Speed (MP.MX) and Supply Fan Maximum Speed
(ConfigurationI.FANFS.MX).
The damper position curve can be field adjusted per application, if
needed. The Econ Min at 25% Fan speed (Configuration
ECONMP.25), Econ Min at 50% Fan speed (Configuration
ECONMP.50) and Econ Min at 75% Fan speed
(ConfigurationECONMP.75) damper position are user
configurable and can be determined by setting the fan speed at 25,
50 and 75% and determining the damper position required to
maintain the Econ Min at Max Fan Speed (Configuration
ECONMP.MX) outside air CFM through the economizer
outside air dampers. The default calculations programmed into the
HC controls are based on a side shot economizer at 400 CFM/TON
Supply Air flow with negative 0.25 in H2O pressure in the return
duct. Econ Min at Max Fan Speed (FS.MN) is set by user based
on minimum required outside air ventilation CFM required for the
application. This procedure would be the same as if this were a CV
unit with the unit running at the design point CFM. This
determines the minimum position amount of outside air CFM
required when the fan is running at maximum speed.
Free Cooling
The economizer will be allowed to help with cooling (Run Status
MODEOK.EC = Yes) if the supply air temperature sensor
reading is valid, there are no applied lockouts, and there is not a
dehumidification demand. There are four economizer lockouts that
can be applied at any time. Econo Cool OAT Lockout (Operating
ModesECONE.LOC) occurs when the Outdoor Air
Temperature (OAT) is greater than the configured Econo Cool Hi
Temp Limit (ConfigurationECONEH.LO)orlessthanthe
configured Econo Cool Lo Temp Limit (ConfigurationECON
EL.LO). Econo Diff DBulb Lockout (Operating Modes
ECOND.LOC) occurs when Diff Dry Bulb Control is enabled
(ConfigurationECONDF.DB = Enable) and the accessory
return air temperature (RAT) is lower then the outdoor air
temperature (OAT). Econo Cool Enth Lockout (Operating
ModesECON EN.LO) occurs when an enthalpy sensor is
installed and the Outdoor Enthalpy is HIGH. OAQ Lockout Mode
(Operating ModesECONAQ.LO) occurs when the outdoor
air quality sensor is configured for lockout and the value is greater
then the OAQ Lockout Limit (ConfigurationAIR.QOAQ.L).
Any one of these lockouts will disable economizer free cooling.
Thermostat Control
If the unit is in cooling, operating under thermostat control, Y1 = ON,
and the economizer is available for cooling, the economizer will
control the supply-air temperature to the low cool set point
(SetpointsLCSP). When Y2 = ON, the economizer will control the
supply-air temperature to high cool set point (SetpointsHCSP). If a
compressor is also being used during Free Cooling, the damper will
be locked at the economizer maximum position (Configuration
ECONEC.MX).
Space Sensor Control
If the unit is in cooling, operating under space temperature control,
the economizer is available for cooling, and no compressors are
operating, the economizer will control the SAT to either
SetpointsLCSP or SetpointsHCSP (See Table 6.) If a
compressor is ON, the economizer will be at the economizer
maximum cooling position (ConfigurationECONEC.MX).
If the control senses low suction pressure for any active refrigerant
circuit when the economizer is also providing cooling, the
maximum allowable economizer position will be reduced. Factory
default configurations have been qualified over a large range of
conditions and should only be changed with care. For unit
troubleshooting, factory default maximum economizer limits for
this condition are provided in Table 7.
Table 6 – LCSP and HCSP Transitions for
Space Temperature Mode
CURRENT SAT
SET POINT
COOL DEMAND
(F)
NEXT SAT
SET POINT
LCSP >0.5 HCSP
HCSP <0 LCSP
LCSP <–0.5 Exit Cooling
LEGEND
HCSP --- High Cool Set Point
LC SP --- L o w C o o l S e t P o i n t
SAT --- S u p p l y --- Ai r Te m p e r a t u r e
Table 7 – Maximum Economizer Limits
During Low Suction Pressure
COOLING STAGE S I Z E S 0 4 --- 1 4 S I Z E S 1 7 --- 2 8
Bottom 50 50
Top 25 0
Unoccupied Free Cooling
The unoccupied free cooling algorithm attempts to maintain the
building space half way between the Occupied Cool Set Point
(SetpointsOCSP) and Occupied Heat Set Point
(SetpointsOHSP) using only the economizer when the conditions
in the building and the outdoors are suitable, during unoccupied
periods. Three different configurations define this algorithm:
Unoccupied Free Cooling (ConfigurationECONUEFC), Free
Cooling Preoccupancy Time (ConfigurationECONFC.TM),
and FreeCoolLow Temp Limit(ConfigurationECONFC.LO).
UEFC = 0 (Disabled)
When UEFC = 0, unoccupied free cooling is disabled. Cooling
will only occur if the space exceeds the unoccupied setpoints.
UEFC = 1 (Unoccupied)
When UEFC is set to 1, unoccupied free cooling can occur
throughout the entire unoccupied period. The space temperature
must be higher than the mid--point between the occupied cooling
and heating setpoints.
UEFC = 2 (Preoccupancy)
When UEFC is set to 2, unoccupied free cooling can only occur
when the time until the next occupied period is less than the Free
Cool PreOcc Time (FC.TM) in minutes.
Free Cool PreOcc Time (FC.TM)
FC.TM is the configuration that determines how many minutes
before occupancy that free cooling can occur when set for
Preoccupancy (UEFC = 2).
Free Cool Low Temp Limit (FC.LO)
Unoccupied free cooling cannot occur if the Outdoor Air
Temperature (TemperatureAIR.TOAT)islessthanFC.LO.
Power Exhaust (CV units)
To enable power exhaust, ConfigurationECONPE.EN must
be set to ENBL. If power exhaust is enabled, Power Exhaust 1 will
turn on when the economizer position is greater than the value of
ConfigurationECONPE.1. Power Exhaust 2 will turn on
when the economizer position is greater than the value of
ConfigurationECONPE.2. There are small time delays to
ensure that rapid cycling does not occur.
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