Economizer Cycle Decision-Outdoor Air/Return Air Dry Bulb Temperature Comparison - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

Item
No. Function
1 Sensor senses OA temperature.
2 Sensor senses OA humidity.
3 OA enthalpy calculated from OA temperature
and humidity.
4 Sensor senses RA temperature.
5 Sensor senses RA humidity.
6 RA enthalpy calculated from OA temperature
and humidity.
7 Economizer mode selector and command
schedule.
8 Setpoint for minimum OA damper position.
9 Actuator positions OA and RA dampers.
10 Actuator positions EA dampers.
11 Control program coordinates occupancy,
temperature, smoke control, and ventilation
controls.
FEATURES
1. Outdoor air is used for cooling (or to supplement the
chilled water system) anytime the OA enthalpy is less
than the RA enthalpy.
2. Enthalpy considers total heat and will consider variations
in OA and RA moisture content, thus saving more energy
than dry-bulb based and OA enthalpy economizer loops.
3. The OA enthalpy calculation should be global and
broadcast over the digital system network to other AHU
systems.
4. Operator options for overriding the basic decision and
for reverting to an OA DB economizer decision.
5. The test-and-balance minimum OA damper position value
is provided as text. If the operator adjusts the minimum
OA damper value, there is no longer a point of reference
as to what it should be without this note.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
AIR HANDLING SYSTEM CONTROL APPLICATIONS
CONDITIONS FOR SUCCESSFUL OPERATION
1. High quality RH sensors with at least 3% accuracy and
long term stability should be selected.
2. Periodic maintenance of the humidity sensors is provided.
3. In some cases, only certain AHUs (which have varying
latent loads) need RA enthalpy sensors and calculations,
and others will be perform satisfactorily with OA enthalpy
only. If RA moisture varies similarly on several AHUs, a
single comparison and decision may be globally shared
among them.
NOTE: The OA/RA enthalpy comparison decision is
best on systems where the return air
experiences wide swings in humidity when the
OA temperatures are between approximately
16 C and 27 C. The size of the AHU should
also be considered since savings will vary with
fan airflow.
SPECIFICATIONS
An economizer decision function shall be provided to switch
the AHU from OA cooling to minimum OA based upon an
OA/RA enthalpy calculation and comparison. Anytime the OA
enthalpy is below the RA enthalpy, the system shall switch to
the economizer mode, except the system shall be locked out of
the economizer mode anytime the OA DB is higher than 27.5 C.
Software shall also be provided to allow the user to switch the
economizer mode to an OA dry bulb base (with an appropriate
commandable setpoint), or to lock the system into or out of the
economizer mode.
ECONOMIZER CYCLE DECISION—
OUTDOOR AIR/RETURN AIR DRY BULB
TEMPERATURE COMPARISON
This rarely used economizer decision is similar to the enthalpy
comparison but considers dry bulb temperatures only. This
scheme is best on small systems if return air temperatures vary
significantly when the OA temperatures are between
approximately 16 C and 27 C.
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