MICROPROCESSOR-BASED/DDC FUNDAMENTALS
139
ENGINEERING MANUAL OF AUTOMATIC CONTROL
Night Cycle
The night cycle program (Fig. 7) maintains a low temperature
limit (heating season) or high temperature limit (cooling
season) during unoccupied periods by cycling the air handling
unit while the outdoor air damper is closed. Digital control
systems often reduce fan capacity of VAV AHU systems to
accomplish this and reduce energy usage.
Fig. 8. Enthalpy Decision Ladder.
Load Reset
The load reset program (Fig. 9) assures that only the
minimum amount of heating or cooling energy is used to satisfy
load requirements. Samples of zone demands are taken and
the zone with the greatest load is used to reset the temperature
of the heating or cooling source.
C2439
NO NO
SELECT
MINIMUM
OA
OPTIONAL CONSIDERATIONS
YES YES
OADB
>
RADB
LEGEND:
OAh = OUTDOOR AIR ENTHALPY
RAh= RETURN AIR ENTHALPY
OA = OUTDOOR AIR
OADB = OUTDOOR AIR DRY BULB
RADB = RETURN AIR DRY BULB
OAh > RAh
1
1
SELECT
MINIMUM
OA
SELECT
LOCAL
CONTROL
ZONE WITH GREATEST
DEMAND FOR HEATING
HEATING LOAD RESET COOLING LOAD RESET
C2435
ZONE WITH GREATEST
DEMAND FOR COOLING
FULL
HEATING
UPPER
BOUNDRY
LOWER
BOUNDRY
100
60
HOT DECK
TEMPERATURE (F)
NO
HEATING
UPPER
BOUNDRY
LOWER
BOUNDRY
60
53
COLD DECK
TEMPERATURE (F)
FULL
COOLING
NO
COOLING
AHU ON
AHU OFF
5:00 PM
12:00 MIDNIGHT
8:00 AM
OPTIMUM
STOP
PERIOD
C2432
UNOCCUPIED PERIOD
INDOOR
TEMPERATURE (F)
72
65
60
5:00 PM 12:00 MIDNIGHT 8:00 AM
4:30 PM
OPTIMUM
START
PERIOD
Fig. 7. Night Cycle.
Night Purge
The night purge program uses cool, night outdoor air to
precool the building before the mechanical cooling is turned
on. Digital control systems often reduce fan capacity of VAV
AHU systems during Night Purge to reduce energy usage.
Outdoor temperature, outdoor RH or dewpoint, and space
temperature are analyzed. One hundred percent outdoor air is
admitted under the following typical conditions:
1. Outdoor air above a summer-winter changeover point,
such as 50F.
2. Outdoor temperature below space temperature by a
specified RH or determined differential.
3. Outdoor air dewpoint less than 60F.
4. Space temperature above some minimum for night purge
such as 75F.
Enthalpy
The enthalpy program (Fig. 8) selects the air source that
requires the least total heat (enthalpy) removal to reach the
design cooling temperature. The selected air source is either
the return air with a selectable minimum amount of outdoor
air or a mixture of outdoor and return air as determined by
local control from discharge-air or space temperature
measurement. Measurements of return-air enthalpy and return-
air dry bulb are compared to outdoor air conditions and used
as criteria for the air source selection. A variation of this is
comparing the outside air enthalpy to a constant (such as 27.5
Btu per pound of dry air) since the controlled return air enthalpy
is rather stable.
Fig. 9. Typical Load Reset Schedules.
Load reset used with digital controllers is the application
that most sets digital control apart from pneumatic and
traditional control. The application uses VAV box loadings to
determine VAV AHU requirements for air static pressure and
temperature, uses reheat valve loadings to determine hot water
plant requirements for temperature and pressure, and uses
chilled water valve positions to determine chilled water plant
requirements for temperature and pressure. These adjustments
to the various requirements reduce energy costs when all VAV
dampers are less than 70 percent open and AHU design
conditions (for example, 55F at two inch static pressure) are
not required.
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