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Zone Control Software Overview • 11-27
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.
11.7.4 Temperature Control
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).
11.7.5 Zone Temperature
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.
11.7.6 Economizer Control
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.
11.7.7 Economization Enable
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
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 method for use in summer
and winter months. Also, an alternate method may be
specified that will be used as a fail-safe when the primary
method is not available (due to sensor failure, etc.).
Cooling Heating
SUMMER COOL OCC SUMMER HEAT OCC
SUMMER COOL UOC SUMMER HEAT UOC
WINTER COOL OCC WINTER HEAT OCC
WINTER COOL UOC WINTER HEAT UOC
Table 11-12
- Possible Heating/Cooling Setpoints