ASHRAE Journal - September 2022 - 38

TECHNICAL FEATURE
FIGURE 3 Fan capacity control loop (for conventional multizone system) Notes: A = Analog Signal, ZN = Zone, D = Damper, C = Command, SYS = System,
ENA = Enable, EFF = Effective, SP = Setpoint, DIR = Direct Acting
A
A
A
A
A
ZN1-D-C
ZN2-D-C
ZN3-D-C
ZN4-D-C
ZN5-D-C
MAX
Cooling
100%
MINUS
MIN
Heating
SF-C-MIN
Fan Capacity Control Loop
Zone Heating/Cooling Load Determination
still getting adequate heat transfer to provide the needed
conditioning to the zone. If a damper opens beyond the
target (typically 95%), that indicates that more conditioning
is needed, and fan speed is ramped up. The most
" needy " damper is determined with the control logic
comparing the percent open (to either deck) from the
whole collection of zone damper signals (Figure 3). The
fan speed modulates between its high and low limits,
typically set at 50% minimum and 100% maximum. The
low limit may be reduced by the designer to about 30%
if the motor is relatively new (e.g., a premium effi ciency
motor) and can accommodate that speed reduction
without overheating.
Figure 3 shows the control diagram for the supply fan
VFD of a conventional MZ unit, which determines the
zone with the greatest load to inform the capacity control
of the fan. On the left, the zone damper commands from
each of the zones are polled for the maximum and minimum
signal. The maximum signal indicates the most
open cooling damper position. The minimum signal
indicates the most open heating damper position. The
control logic compares the heating and cooling damper
positions to determine which is most open where the
minimum signal is subtracted from 100% to determine
the heating damper " amount open " position. The most
open damper position (either most open to heating or
most open to cooling) is then used as the process variable
for the PI#
control loop, with a target damper open
setpoint (typically 95%) to control fan speed. The desired
(50%)
Supply Fan Capacity Control Loop
(_ _) Entries Are For Example Only
fan speed from the PI control loop is compared to the
minimum fan speed command, and the greater value is
used as the supply fan command (thus ensuring the fan
doesn't go below the minimum permitted speed).
Outside Air Requirements were met through measurement
of outside airfl ow and modulation of the
outside air damper (or mixed air dampers) to maintain
ventilation requirements (e.g., ASHRAE Standard 62.1,
Ventilation for Acceptable Indoor Air Quality, at the time of the
demonstrationII
) and makeup air requirements. In the
demonstration project, each system used either a fi xed
airfl ow setpoint or a demand-controlled ventilation
(DCV) sequence of operation.
In the case of fi xed fl ow setpoint, the outside air
requirements were met using a fl ow setpoint in the control
logic along with a designer selected minimum fan
speed (Figure 3).
For the demonstration systems, the outside air ducts
had such ample capacity (since they were originally
sized for an economizer ) that the ventilation requirements
were met at the minimum fan speed that prevents
motor overheating. During economizer mode, the
control logic keeps the outdoor air damper suffi ciently
open to maintain minimum outside airfl ow.**
In the case of demand-controlled ventilation (DCV),
the outside air requirements were accommodated using
dynamic reset of the outdoor airfl ow setpoint. This
included reducing outdoor airfl ow when spaces were
at reduced or no occupancy. Either occupancy or CO2
#Although a PID controller is often used in this application, only the proportional and integral features of the controller are used.
IIImplementers should consider current ASHRAE guidelines to ensure acceptable indoor environmental quality.
**For systems with lower outside air capacity (e.g., those without airside economizer), control logic would be needed to ensure that the
minimum fan speed is increased if ventilation requirements are not met when the outside air damper is fully open.
38
ASHRAE JOURNAL ashrae.o rg S E P T E M B E R 2 0 2 2
ZN-D-MIN
MAX
ZN-DC-EFF
ZN-DC-SP
PID
(95%)
DIR
0%
MAX
SF-C
A
ZN-D-MAX
SYS-ENA

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ASHRAE Journal - September 2022

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