Emerson E2 operation manual Zone Temperature, Economization Enable, AHUs

11.7.3.3AHUs

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.

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).

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

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.

11.7.7Economization 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

Zone Control Software Overview • 11-27