Airside economizer (ASE)
Optional airside economizer mixing box is designed to attach to the return air (RA) side of the
evaporator. Mixing comes with low-leakage opposed blade dampers. The economizer is used to
provide ventilation and free cooling for the DSH series of horizontal air handlers.
The economizer is setup through the LCD keypad display and joystick located on the unit SE
control (SEC) board or through a recommended mobile access portal (MAP) gateway, available for
purchase from the factory– see
Mobile Access Portal Gateway Product Bulletin (LIT-12011884)
. The
control board is mounted inside of the unit main electrical box. Access to the mixing box is not
required to setup ASE sequence of operation. Communication between SEC board and ASE board
automatically establishes through an SA bus connection.
The factory supplied economizer includes: economizer module, mixing box, actuator, enthalpy
sensor, temperature sensor, dampers, and wiring harness. See Appendix A: Wiring Diagrams for
the economizer electrical wiring schematics or
Airside Economizer Kit Installation and Operation
Instructions (Form 145.10-NO2)
for more details on installation and application.
Sequences of operation
Several functions can drive the economizer, including minimum position, free cooling, economizer
loading, and minimum outdoor air supply.
Economizer minimum position
The outside air (OA) damper minimum position is set during occupied mode when outside air (OA)
is not suitable for free cooling and the unit is in economizer mode. The position of the damper is set
proportionally between the Economizer Minimum Position and the Economizer Minimum Position
Low Speed Fan setpoints in relation to the VFD output percentage.
Note:
On a constant volume single speed supply fan system, both setpoints should be set to
the same value.
Free cooling
Four types of free cooling options are available: dry bulb changeover, single enthalpy, dual enthalpy
changeover, and auto.
Dry bulb changeover
For dry bulb economizer operation, the outside air is suitable for free cooling if the outside air
temperature (OAT) is 1.0°F below the Economizer OAT Enable setpoint and 1.0°F below the RA
temperature (RAT). Free cooling is no longer available if the OAT rises above either the Economizer
OAT Enable setpoint or the RAT.
Single/dual enthalpy changeover
For single enthalpy economizer operation, the outside air is suitable for free cooling if the OA
enthalpy is at least 1 Btu/lb below the economizer OA enthalpy setpoint and the OAT is no greater
than the RAT plus 9.0°F.
If the OAT rises above the RAT plus 10.0°F, free cooling is no longer available. The OAT must drop to
no greater than RAT plus 9.0°F to enter free cooling again.
Free cooling is no longer available if the OA enthalpy rises above the economizer OA enthalpy
setpoint. For dual enthalpy economizer operation, the OA enthalpy must be lower than the RA
enthalpy by 1 Btu/lb and the OAT is no greater than the RAT plus 9.0°F.
Auto
The control determines the type of free cooling changeover based on which sensors are present
and reliable. Conditions include:
•
Return and outside air dry bulb = dry bulb changeover
Model DSH024C–120C Horizontal R-410A Air Conditioning Units C-Generation Smart Equipment Controller
38
Johnson Controls
