ASHRAE Journal - September 2022 - 40

TECHNICAL FEATURE
TABLE 2 System descriptions.
MZ
AHU
CERL 1
CERL 2
Bragg 1
Bragg 2
Bragg 3
LOCATION
TYPE
Central Ill.
Central lll.
Central N.C.
Central N.C.
Central N.C.
Conventional
Conventional
Neutral Deck
Neutral Deck
Neutral Deck
*5 HP supply fan and 3 HP return fan.
was adjusted using a linear reset schedule based on the
zone with the highest CO2 level. Similar to the " fixed
flow setpoint " control described above, the system was
operated at no less than a designer selected minimum
fan speed and, during economizer operation, the OA
damper was prevented from closing more than required
to maintain minimum outside airflow setpoint.
Coil Valve Control
The control scheme adjusts coil output capacity by
enabling/disabling coil water valves and using a binary
hot deck reset.
Enabling/Disabling Valves. The hot/cold deck (heating
and cooling) coil control valves are disabled (closed)
when no heating (or cooling) is called for by any zone
and otherwise enabled to modulate to the fixed deck air
temperature setpoint.
Figures 4 and 5 show how the coil valve control is
enabled and disabled by the zone damper command
signals issued by the digital control system. Figure 4 presumes
that each zone damper is normally open (N.O.) to
the hot deck; therefore, damper fail-safe position (upon
loss of control signal or power) is to full open to heating.
Similarly, Figure 4 presumes that each zone damper
is normally closed (N.C.) to the cold deck;, therefore,
damper fail-safe (upon loss of control signal or power) is
full closed to cooling.
The control system sequencing diagram in Figure 4
shows that the system polls all the zones served by the
AHU and enables the hot deck valve when any of the
system's multiple zone dampers receive an actuation
command between 0% to 25% indicating a zone with a
fairly significant call for heat. The valve remains enabled
until all spaces are sufficiently satisfied. When all of the
zone damper signals are commanded to be between 60%
and 100%, none of the zones are calling for much heat,
so the hot deck control valve is disabled (or " off " ). The
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ASHRAE JOURNAL ashrae.o rg
S E PTEM B E R 2022
valve remains in its present state (now closed) until any
one zone indicates a need for heating. In this way zone
damper commands between 25% and 60% result in the
hot deck valve enable or disable command remaining
unchanged from its last state.
The threshold values of 25% and 60% in Figure 4 are
intended to improve performance and save energy by
minimizing standby losses and the amount of conditioned
air that leaks through zone dampers into the
supply air to the zone. The triggering values for enabling
and disabling the coils can be adjusted as needed to
accommodate site-specific factors such as local climate.
Note that for a bypass MZ unit there is no hot deck, so
there is no hot deck enable/disable control.
The sequencing diagram in Figure 5 shows that the
cold deck valve is similarly enabled and disabled as the
hot deck valve shown in Figure 4, but the thresholds of
percent signal from the zone dampers are different.
Here, if all zone damper signals are less than 5%, there
is essentially no need for cooling, so the cold deck valve
is disabled and closed. At 15% signal, there is sufficient
need for cooling that the valve is enabled to modulate.
Because every system is unique, all threshold values are
configurable to meet the specific needs of the MZ unit,
such as dehumidification needs, which may require
the cold deck threshold to be turned off and set to 0%
instead of 5%.
Using a Binary Hot Deck Temperature Reset to
modulate to one of two fixed temperature setpoints
instead of using the more common proportional hot
deck temperature reset. A binary " reset " was used where
the setpoint was adjusted to either a higher setpoint
(e.g., 90°F [32°C]) or a lower (e.g., 80°F [27°C]) setting,
depending on whether the outside air temperature was
below or above 50°F [10°C], respectively. Without proportional
reset, the upper binary setpoint provides for
a hotter deck air temperature, allowing a lower airflow
AGE OF UNIT
(YR)
40
40
10
10
10
END USE
Office
Conference Rooms
Classrooms
Office
Office
UNIT SIZE (cfm)
15,190
3,475
4,620
4,670
5,930
FLOOR AREA (ft2)
8,800
2,400
2,983
4,328
4,837
FAN MOTOR SIZE (HP)
8*
3
5
5
7.5
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ASHRAE Journal - September 2022

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