Auto-Zone Plus
Section 1
Design Guide
1-11
Step #2 - Sizing the Central Unit
Because the zones are controlled with variable air volume, it is unlikely that all zones
will be at design load at the same time. The zoning allows for the diversity of loads to be
taken into account and will often provide better comfort with a smaller HVAC unit.
In sizing the system, the individual zone loads should be calculated using any dependable
load estimating program. Because of diversity, the central unit should be selected for the
instantaneous peak load, not the sum of the peak loads, as would be done with a constant
volume single zone system. Consider the following when sizing the central unit.
•
Size the peak cooling load based on the month and hour of the greatest total
building/system load.
•
Heating should be sized for the lowest design temperature with an additional margin
for morning "pickup". This margin is generally recommended to be 20 to 25 percent
of base design.
Step #3 - Duct Design Considerations
The AZ Plus system uses a typical low pressure duct design. To reduce noise problems
duct pressures should not exceed 1 inch W.C.
Primary trunk ducts should not be "undersized." This is especially true for "pressure
dependent" systems. Pressure dependent refers to the typical Plus Zone Controller without
the airflow sensor. With larger trunk ducts, it is easier to assure relatively constant
pressure to each zone. Runs should be as short as possible, and the trunk duct system kept
as symmetrical as possible to facilitate system balancing. Wherever possible, run the trunk
ducts above corridors and locate the zone dampers above corridors to reduce the noise in
the space and facilitate service of the units. Trunk ducts should be sized for no more than
0.1 inch W.C. drop per 100 feet, and a maximum duct velocity of 2000 FPM.
Note:
For pressure independent terminal units with velocity sensors and
conventional "VAV" boxes properly selected for "quiet" operation, this 2000
FPM rule can be exceeded by up to 50 percent. The designer, however, should
be very experienced in VAV system design before considering modification
of this general rule
.
Typical VAV systems with pressure independent terminals use the static regain method
for sizing ducts. The typical Auto-Zone Plus system is a low-pressure, pressure
dependent system that utilizes conventional unitary air-conditioning units. These systems
should use the equal-friction method of sizing the ducts, and use the maximum loss of 0.1
inch per 100 feet as described above.
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Summary of Contents for Auto-Zone Plus
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