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For all units with Economizers:
1. 1.The outdoor temperature is below the Econo Cool Hi
Temp Limit (Configuration→ECON→EH.LO) and above
the Econo Cool Lo Temp Limit
(Configuration→ECON→EL.LO).
For units with Enthalpy Contro l
2. 1.The outdoor enthalpy is low and the outdoor temperature
is below the Econo Cool Hi Temp Limit
(Configuration→ECON→EH.LO) and above the Econo
Cool Lo Temp Limit (Configuration→ECON→EL.LO).
If the conditions above are true and the economizer is available for
cooling the economizer outside air and return air dampers will
modulate to allow proportions of outside air and return air to mix
and produce a temperature equal to the Cool Supply Air Set Point
(Setpoint→SASP). If reset is applied to the Supply Air
Temperature Set Point (Setpoint→SASP) the dampers w ill control
to the Supply Air Control Point (Run Status→COOL→SA.CP).
If the conditions above are true but the economizer dampers are not
able to satisfy the Cool Supply Air Set Point (Setpoint→SASP)
with Reset (if applied), the compressor will turn on and modulate
to provide additional cooling to satisfy the Cool Supply Air Set
Point (Setpoint→SASP) with Reset if applied.
If the conditions above are not true, the economizer will move to a
user Configurable Econ Min at Max Fan Speed
(Configuration→ECON→MP.MX) where the minimum
economizer damper position is based on the Supply Fan Maximum
Speed (Configuration→UNIT→FS.MX). This is done to
maintain a constant airflow through the economizer by opening the
dampers more as the indoor fan speed is reduced or closing the
dampers more as the indoor fan speed increases.
The shape of the Economizer Minimum Position vs. Fan Speed
curve is determined by t he configuration parameters: Econ Min at
25% Fan speed (Configuration→ECON→MP.25), Econ Min at
50% Fan speed (Configuration→ECON→MP.50), Econ Min at
75% Fan speed (Configuration→ECON→MP.75) and Econ Min
at Max Fan speed (Configuration→ECON→MP.MX).
The Econ Min at 25% Fan speed
(Configuration→ECON→MP.25), Econ Min at 50% Fan speed
(Configuration→ECON→MP.50) and Econ Min at 75% Fan
speed (Configuration→ECON→MP.75) damper positions are
calculated based on the Econ Min at Max Fan Speed
(Configuration→ECON→MP.MX) and Supply Fan Maximum
Speed (Configuration→UNIT→FS.MX).
The Econ Min at 25% Fan speed
(Configuration→ECON→MP.25), Econ Min at 50% Fan speed
(Configuration→ECON→MP.50) and Econ Min at 75% Fan
speed (Configuration→ECON→MP.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→ECON→MP.MX) Outside air CFM through the
economizer outside air dampers. The default calculations
programmed into the PD controls is based on a side shot
economizer at 400 CFM/TON Supply Air flow with negative 0.25
in H
2
O pressure in the return duct.
Econ Min at Max fan Speed (Configuration→UNIT→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.
Configure the Indoor Fan Max Speed CFM
(Configuration→ECON→IDF.C) either to the supply CFM
determined from the fan performance tables or air balance reports.
If using fan performance tables make corrections for all installed
accessory pressure loss and use measured RPM, power and static
pressure readings to determine the actual Indoor Fan Max Speed
CFM (Configuration→ECON→IDF.C).
MINIMUM
IAQ
DAMPER
POSITION
(AQ. MN)
ECON MIN
AT MAX
FANSPEED
POSITION
(MINP_MAX)
INCREASING VENTILATION
VENTILATION FOR PEOPLE
VENTILATION FOR SOURCES
100
700
INSIDE/OUTSIDE CO
2
AQ
DIFFERENTIAL
LOW (AQD.L)
AQ
DIFFERENTIAL
HIGH (AQD.H)
DIFFERENTIAL
C09145
Fig. 8 -- Economizer Minimum Position
If the indoor fan is not operating, the economizer will be fully
closed, 0% open.
Ventilation
Mode
If the space temperature falls 0.5F below the Occupied Cool Set
Point (Setpoints→OCSP) the compressor will turn off, the indoor
fan speed will be set to the Vent Mode Fan Speed
(Configuration→UNIT→FS.VM) and the economizer will open
to the position that will supply minimum ventilation air at the
configured Vent Mode Fan Speed
(Configuration→UNIT→FS.VM). Default Vent Mode Fan Speed
(Configuration→UNIT→FS.VM) is 50%. Configuring the Vent
Mode Fan Speed (Configuration→UNIT→FS.VM) lower will
provide additional energy savings in vent mode but may not
supply the required ventilation air to the conditioned space at lower
fan speeds.
Demand Control V
entilation
The 48PD units can also be equipped with optional CO
2
sensors
for additional indoor air quality control. When unit is equipped
with a return duct CO
2
sensor or return duct CO
2
sensor and
outside air CO
2
sensor the Economizer minimum position vs. fan
speed curve will be recalculated based on the CO
2
level of the
return and/or outside air as shown in Fig. 8. See the Indoor Air
48/50PD