Emerson E2 User Manual

An AHU controls all aspects of an air handling unit, 

including up to eight stages of auxiliary or reclaim heat, 
six cooling stages, dehumidification, analog or digital 
economization, and support for single-, two-, or variable-
speed fans.

Normally, since AHUs are designed to cover a wide 

area of space, AHU Control applications operate on their 
own and are not associated with Zone applications (they 
are large enough to be “zones” within themselves). How-
ever, if desired, an AHU may be associated with a Zone 
application, which will allow the AHU to use the Zone’s 
setpoints, occupancy state, summer/winter state, and dehu-
midification and economization enabling.

As mentioned, Zone applications do not “control” tem-

perature themselves. Zone applications simply pass along 
the setpoints an HVAC unit will use, and the individual 
unit is responsible for controlling to the setpoint using its 
own temperature input.

The Zone application passes along eight different set-

points, which are shown in Table 11-12. Of these set-
points, the application receiving the setpoints will only use 
one cooling and one heating setpoint. The pair the applica-
tion will use is determined by whether the current season 
is SUMMER or WINTER and whether the building is 
OCCUPIED or UNOCCUPIED (both of which is supplied 
by the Zone). 

SUMMER COOL OCC

SUMMER HEAT OCC

SUMMER COOL UOC

SUMMER HEAT UOC

WINTER COOL OCC

WINTER HEAT OCC

WINTER COOL UOC

WINTER HEAT UOC

Each MultiFlex RTU and AHU application has a space 

temperature output that is equal to the unit’s current Con-
trol Temperature. When associated with a Zone applica-
tion, this space temperature output is connected to one of 
sixteen Zone Temperature inputs in the Zone application. 
These sixteen inputs are then combined using a user-
defined combination method to yield the Zone Tempera-
ture.

Zone Temperature provides an at-a-glance view of 

how well the units within the Zone are doing in heating 
and cooling. If desired, Zone Temperature may also be 
used as a temperature control input by one or all of the 

Zone’s HVAC applications. 

A Zone application is responsible for analyzing the 

outside air conditions and determining if the conditions are 
favorable for bringing in outside air. If the air condition is 
favorable, the Zone sends a signal to its associated HVAC 
units telling them economization is OK. If not, it sends a 
signal to disable economization.

It is up to the associated HVAC unit to process the 

economization information and open the dampers.

There are five possible ways a Zone application may 

determine when conditions are favorable for economiza-
tion:

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 Zone 
application. Otherwise, the switch sends a NOT 
OK (OFF) signal.

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 Zone application cal-
culates the outside air enthalpy by reading the 
value of a relative humidity sensor and an out-
door air temperature sensor. This calculated 
enthalpy is compared to a setpoint. If the 
enthalpy is less than the setpoint, economization 
is enabled. If greater, 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