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Air Handling Units (AHU) Software Overview • 11-21
11.6.4 Fan Control
The E2 BX can control three different types of AHU
fans. However, only one fan can be controlled per AHU.
The fan types include: single-speed, two-speed, and vari
-
able-speed. Controls for all fans are similar in that they
operate in either of three modes:
• Continuous - The fan is always on, even
when the AHU is not in heating, cooling, or
dehumidification mode.
• Auto - The fan is only on when the AHU is
in heating, cooling, or dehumidification
mode.
• Summer ON, Winter Auto - This mode
allows the AHU fan to operate in Continuous
mode during the summer months and in
Auto mode during the winter months.
The operation of the fans during Continuous and Auto
modes is largely dependent on the fan type.
11.6.4.1 Single-Speed Fans
Single-speed fans require no advanced control parame-
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.