RT-SVX063E-EN
83
Table 33. Economizer high-limit decision methods
Fixed Dry Bulb
Enable
Outdoor Air Temperature < Economizer Outdoor Air Enable Setpoint – 5°F
Disable
Outdoor Air Temperature > Economizer Outdoor Air Enable Setpoint
Reference Enthalpy + Fixed Dry
Bulb
Enable
Outdoor Air Enthalpy < Reference Enthalpy Setpoint – 3 BTU/lb
AND
Outdoor Air Temperaure < Economizer Outdoor Air Enable Setpoint – 5°F
Disable
Outdoor Air Enthalpy > Reference Enthalpy Setpoint
OR
Outdoor Air Temperaure > Economizer Outdoor Air Enable Setpoint
Comparative Enthalpy + Fixed
Dry Bulb
Enable
Outdoor Air Enthalpy < Return Air Enthalpy – 3 BTU/lb
AND
Outdoor Air Temperaure < Economizer Outdoor Air Enable Setpoint – 5°F
Disable
Outdoor Air Enthalpy > Return Air Enthalpy
OR
Outdoor Air Temperaure > Economizer Outdoor Air Enable Setpoint
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:
FFDDDD:: EExxcceessssiivvee OOuuttddoooorr AAiirr
In minimum ventilation mode and the damper
feedback position is >10% of the damper commanded
value for 5 continuous minutes.
FFDDDD:: OOuuttddoooorr AAiirr DDaammppeerr NNoott MMoodduullaattiinngg
In minimum ventilation mode and the damper
feedback is <10% of the damper commanded value for
5 continuous minutes.
FFDDDD:: UUnniitt EEccoonnoommiizziinngg WWhheenn IItt SShhoouulldd NNoott
In economizer cooling mode and the damper feedback
is >10% of the damper commanded value for 5
continuous minutes.
FFDDDD:: UUnniitt NNoott EEccoonnoommiizziinngg WWhheenn IItt SShhoouulldd
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. A valid Evaporator Leaving Air
Temperature sensor(s) are required. Two circuit, split
coil units will have an Evaporator Leaving Air
Temperature sensor installed in each airstream. 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
Mode Status is not Cool or a sensor required for
dehumidification control is invalid.
Discharge Air Control Units
When Dehumidification is active, cooling capacity is
controlled by Evaporator Leaving Air Temperature
relative to Evaporator Leaving Air Temperature
Setpoint. This setpoint is suitable to satisfy the space
humidity load. Cooling capacity will first load on circuit
1 to provide hot gas reheat capacity. The Hot Gas
SSeeqquueennccee ooff OOppeerraattiioonn
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