Emerson E2 Economizer Control, Two-Speed Fans, Variable-Speed Fans, Economization Enable

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.

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.

11.6.5.1Economization 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,

11-22 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011