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 Control
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 will 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 the 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 H2O 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).
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AT MAX |
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IAQ |
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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 Ventilation
The 48PD units can also be equipped with optional CO2 sensors for additional indoor air quality control. When unit is equipped with a return duct CO2 sensor or return duct CO2 sensor and outside air CO2 sensor the Economizer minimum position vs. fan speed curve will be recalculated based on the CO2 level of the return and/or outside air as shown in Fig. 8. See the Indoor Air
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