Emerson E2 Manuel D’Utilisation

Economizer dampers on AHUs are used to bring out-

side air into the building for use in cooling. When tem-
perature 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 Tem-
perature 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.

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, 

economization is enabled.

6.

In vs. Out Enthalpy - This strategy requires 
indoor and outdoor humidity sensors and also 
indoor and outdoor temperature sensors. The 
enthalpy of the outdoor air is calculated and com-
pared to the enthalpy of the indoor air. If the out-
door air enthalpy is less than the indoor air 
enthalpy, economization is enabled. Otherwise, 
economization is disabled.

You may choose a different economization checking 

method for use in summer and winter months.

In addition to the methods listed above, there are two 

economization lockout features that apply to all AHUs 
using economization.
Max Outside Air Humidity

The Max Outside Air Humidity is the highest allow-

able humidity level for the outside air. If the outside rela-
tive humidity is higher than this setpoint, economization 
will not be allowed to occur.
Minimum Supply Temp

 The Minimum Supply Temp is a user-defined setpoint 

that locks out economization if the supply air temperature 
falls below a minimum supply temperature setpoint. This 
feature ensures that the air coming from outside is not too 
cold.

Control of digital or two-position economizers is rela-

tively simple. When conditions are favorable for econo-
mization, the dampers will be opened when cooling is 
needed. If not favorable, the dampers will be closed.

For variable-position dampers, economization is gen-

erally used by the AHU just as a cool stage would be in 
Temperature 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 position of the analog economizer damper is 

important only for the purposes of keeping the mixed air 
temperature (a combination of the outdoor air coming 
through the dampers and the return air temperature) near 
the user-specified setpoint. The damper is controlled using 
PID control.

Dehumidification control uses the AHU’s existing cool 

stages (and a separate dehumidification device such as a 
desiccant wheel, if available) to remove moisture from the