Emerson E2 operation manual Single-Speed Fans, Two-Speed Fans, Variable-Speed Fans

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.

11.6.4.1Single-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.

11.6.4.2Two-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.

11.6.4.3Variable-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.

Air Handling Units (AHU) Software Overview • 11-21