Two-Speed Fans; Variable-Speed Fans; Economizer Control; Economization Enable - Emerson E2 Installation And Operation Manual

ters. Whether in Continuous or Auto mode, the fan will
be ON when Fan Control calls for it to be ON, and OFF
when it calls for it to be OFF.
11.6.4.2

Two-Speed Fans

Two-speed fans have a LOW and a HIGH speed set-
ting. You must specify the number of the stage that, when
activated, will switch the fan from LOW to HIGH.
For example, if there are four cooling stages and you
wish to have the fan switch from LOW to HIGH when
stage 3 becomes active, enter a "3" as the switch-over set-
point. This will cause the fan to switch to HIGH speed
when stage 3 is activated. Likewise, when stage 3 is deac-
tivated, the fan speed will switch from HIGH to LOW.
You may choose a different switch-over setpoint for
occupied heating, occupied cooling, unoccupied heating,
and unoccupied cooling modes.
When the AHU is in dehumidification mode, the AHU
Control application will ignore the fan speed settings of
the active stages and switch to a user-specified dehumidi-
fication speed (usually LOW).
Continuous Two-Speed Fan Operation When All
Stages Are OFF
When the fan is operating in Continuous mode and all
heating and cooling stages are OFF, the fan speed will
default to a user-specified speed. You may choose a differ-
ent default speed for both occupied and unoccupied opera-
tion.
11.6.4.3

Variable-Speed Fans

Variable-speed fans may operate at any percentage of
its maximum speed. The method AHU Control uses to
determine the speed percentage is dependent on whether
the heat and cool outputs are staged or modulating.
Variable-Speed Fan Operation
For staged AHU outputs, each stage must be pro-
grammed with its own speed percentage setpoint. The
AHU Control application looks at all of the active stages,
takes the highest speed percentage setpoint, and operates
the fan at this speed.
For example, if cool stage 1 is active with a 30% speed
percentage setpoint, the fan will likewise operate at 30%
speed. If a second stage with a setpoint of 50% were to
become active, the fan speed would increase to 50%.
For each heating and cooling stage, you may specify
both occupied and unoccupied speed percentage setpoints.
Dehumidification with VS Fans
When in dehumidification mode, a user-defined slow-
down percentage is subtracted from the variable-speed fan
percentage. This percentage will continue to be subtracted
until the AHU exits dehumidification mode.
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11-22 E2 RX/BX/CX I&O Manual

11.6.5 Economizer Control

Economizer dampers on AHUs are used to bring out-
side air into the building for use in cooling. When temper-
ature and humidity conditions are favorable, the
economization dampers are opened, and outside air is
allowed to flow into the AHU. Economization is generally
used by the AHU just as a cool stage would be in Temper-
ature Control; if cooling is needed, and conditions are
favorable for economization, the dampers will open and
economization will begin. If more cooling is needed, the
cooling stages would then cycle on as normal.
The E2 supports control of both two-position (digital)
and variable-position (analog) economizer dampers.
11.6.5.1

Economization Enable

Before the AHU Control application may open econo-
mization dampers, it must first determine if the outdoor air
conditions are favorable for economization. There are six
possible ways the AHU Control may do this:
1. Enthalpy Switch - An enthalpy switch is a digital
device that is pre-set to detect when the tempera-
ture and humidity levels are favorable for econo-
mization. When the conditions are favorable, this
switch sends an OK (ON) signal to the AHU
Control application. Otherwise, the switch sends
a NOT OK (OFF) signal, and economization is
disabled.
2. Dewpoint Setpoint - A dewpoint probe measur-
ing the dewpoint of the outside air is compared to
a setpoint. If the outside air dewpoint is less than
the setpoint, economization is enabled. If it is
higher than the setpoint, economization is dis-
abled.
3. Calculated Enthalpy - The AHU Control appli-
cation calculates the outside air enthalpy by read-
ing the value of a relative humidity sensor and an
outdoor air temperature sensor. If the enthalpy is
lower than the setpoint, economization is
enabled. If the enthalpy is greater than the set-
point, economization is disabled.
4. Dewpoint Fail-Safe - This is similar to method
#2, except an outdoor temperature sensor value is
compared to the setpoint instead of a dewpoint
probe's value. This comparison is a poor substi-
tute for actual dewpoint readings and is recom-
mended for use as a fail-safe only. When
possible, use humidity or dewpoint sensors.
5. Temperature Comparison - The AHU Control
application simply compares the temperature of
the inside air with the temperature of the outside
air. If the outside air is cooler than the inside air,
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