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036-21478-001 Rev. A (0802)
Step #2 - Sizing the Central Unit
Because the zones are controlled with variable air vol-
ume, 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 pro-
gram. 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 vol-
ume single zone system. Consider the following when
sizing the central unit.
•
Size the peak cooling load based on the
month day 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 Meridian
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 Meridian
Zone Controller without the airflow sensor. With larger
trunk ducts, it is easier to assure relatively constant pres-
sure to each zone. Runs should be as short as possible,
and the trunk duct system kept as symmetrical as pos-
sible 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.
Meridian
Systems
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 termi-
nals use the static regain method for sizing ducts. The
typical Meridian
Zoning 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.
Step #4 - Air Motion/Diffuser Selection
Air motion is a consideration for occupant comfort. The
selection of diffusers for an Meridian
requires more care than a constant volume system due
to varying flow of air into the zones. Slot diffusers are
recommended due to their superior performance at low
airflows. Because the zone airflow is variable volume,
lower cost round or rectangular diffusers that were sat-
isfactory for constant volume may prove unsatisfactory
with an Meridian
Zoning system. These diffusers may
result in "dumping" of the cold air at low flows in the
cooling mode, and insufficient room air motion at low
air flows in the heating mode. Although high air mo-
,
tion in the heating mode can be undesirable, a slot dif-
fuser with a high induction ratio generally helps to re-
duce room air "stratification" when the heating comes
from a ceiling diffuser. Linear slot diffusers should be
properly selected for the airflow and "throw" suited to
the specific installation or zone.
Additional factors to consider in diffuser selection are
sound level and throw at design flow. Generally, mul-
tiple diffusers will result in lower sound levels in the
space, but this must be balanced with the additional
hardware and installation costs. It is commonly recom-
Zoning Design Guide
Zoning system
15
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