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Table 37. Economizer high-limit decision methods
Fixed Dry Bulb
Reference Enthalpy + Fixed Dry
Bulb
Comparative Enthalpy + Fixed Dry
Bulb
Differential Dry Bulb
When conditions are suitable for economizer operation, the
outdoor air damper modulates between Economizer
Minimum Position (or Flow) Setpoint and 100% open.
Economizing will not allow additional mechanical cooling
until the damper position is 100% and supply fan has
reached 100% capacity. If economizer cooling becomes
disabled, damper will revert to minimum position (or Flow)
control, and transition to mechanical cooling.
Given the economizer decision methods above, if
Comparative Enthalpy + Fixed Dry Bulb is configured and
the return air or humidity sensor goes out of range or fails,
a fallback method of Fixed Dry Bulb or Reference Enthalpy
+ Fixed Dry Bulb is selectable in the user interface Feature
Settings screen.
A building automation system can directly command
economizer operation via Economizer Airside Enable
(auto, enable, disable). If commanded Enable, the
controller will start economizer cooling, regardless of
outdoor air conditions. If commanded Disable, economizer
cooling will be disabled (except if a mode of Night Purge is
commanded to the controller). If commanded Auto, the
controller will use the configured Economizer high limit
method and input values to determine if economizer
cooling is available.
Outdoor Air Damper Fault Detection and
Diagnostics (FDD)
The outdoor air damper fault will have two fault detection
components, faults that are generated when operating in
minimum ventilation mode and another set when the
damper is being controlled by the economizer:
FDD: Excessive Outdoor Air
In minimum ventilation mode and the damper feedback
position is >10% of the damper commanded value for 5
continuous minutes.
FDD: Outdoor Air Damper Not Modulating
RT-SVX073A-EN
Enable
Disable
Enable
Disable
Enable
Disable
Enable
Disable
Outdoor Air Temperature < Economizer Outdoor Air Enable Setpoint – 5°F
Outdoor Air Temperature > Economizer Outdoor Air Enable Setpoint
Outdoor Air Enthalpy < Reference Enthalpy Setpoint – 3 BTU/lb
AND
Outdoor Air Temperaure < Economizer Outdoor Air Enable Setpoint – 5°F
Outdoor Air Enthalpy > Reference Enthalpy Setpoint
OR
Outdoor Air Temperaure > Economizer Outdoor Air Enable Setpoint
Outdoor Air Enthalpy < Return Air Enthalpy – 3 BTU/lb
AND
Outdoor Air Temperaure < Economizer Outdoor Air Enable Setpoint – 5°F
Outdoor Air Enthalpy > Return Air Enthalpy
OR
Outdoor Air Temperaure > Economizer Outdoor Air Enable Setpoint
Outdoor Air Temperature < Return Air Temperature
Outdoor Air Temperature > Return Air Temperature
In minimum ventilation mode and the damper feedback is
<10% of the damper commanded value for 5 continuous
minutes.
FDD: Unit Economizing When It Should Not
In economizer cooling mode and the damper feedback is
>10% of the damper commanded value for 5 continuous
minutes.
FDD: Unit Not Economizing When It Should
In economizer cooling mode and the damper feedback is
<10% of the damper commanded value for 5 continuous
minutes.
Dehumidification Control - Occupied
Dehumidification control is a sub-mode of Cool when hot
gas reheat is configured and the dehumidification feature is
enabled. There are two methods for controlling
dehumidification: Space Relative Humidity or Space Dew
Point. A valid Evaporator Leaving Air Temperature sensor
(s) is required for either method.
For both methods upon demand for dehumidification, the
unit will drive the outdoor damper to minimum position (or
closed during unoccupied periods) and will drive
mechanical cooling capacity to an evaporator leaving air
setpoint and modulate the reheat valve based on discharge
air reheat setpoint which is reset based on space
temperature or directly by a commanded reheat setpoint.
Relative Humidity
When a valid space relative humidity value is above the
Space Dehumidification Setpoint, dehumidification control
is invoked. Economizer cooling disables. The sub-mode
Cool – Dehumidification is reported on the user interface
Operating Modes screen.
Dehumidification control disables when space relative
humidity is less than Space Dehumidification Setpoint
minus the Occupied Dehumidification Offset, Heat Cool
Unit Startup
83
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