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Master
Bedroom
Large
WIndow
You can mark the supply duct layout and damper location on the
Floor Plan Worksheet for the home. Fig. 7 shows an example of
supply duct layout and damper locations marked on a floor plan
worksheet.
TASK 8—DETERMINE APPROPRIATE DAMPER AND
DUCT SIZES
Airflow is the way that a zoning system moves heating or cooling
Btu's to the load (the demand for heating or cooling in the home)
and back to the heating or cooling equipment. For the equipment
to perform properly, the system must maintain a minimum load on
the equipment. In a residential system, the necessary airflow
usually is measured in cubic feet per minute (CFM) exiting the
duct system. For most zoning systems, the system must maintain a
minimum of 300 CFM per ton exiting the duct system (not
recirculated).
A zoning system distributes air based on the differing load in each
zone. Most of the time, some of the dampers are partially closed;
some dampers may be completely closed. By using a larger duct
size for a zoning system that you would use in a non-zoned system,
you help assure that air flows at an appropriate velocity where it is
needed and that the sound level remains acceptable.
Because Zone Perfect Plus controls airflow based on the propor-
tional needs of each zone, the logic used for selecting appropriate
damper and duct sizes is different than the logic you would use for
other zoning systems or a non-zoned system. It is not reasonable to
attempt to design an air distribution system that can handle 100
percent of the system's airflow capacity through 1 zone. However,
it is reasonable to design the air distribution system to be able to
move the minimum airflow with one-third of the zones' dampers
fully open.
NOTE: The use of multi-stage heating and/or cooling equipment
can significantly reduce the size of dampers and ducts required for
an installation. For more information, see the section "Using
Multi-Stage Equipment with Zoning."
Determining Duct Size Based on Damper Size
Bath
Bath
Bedroom
Bedroom
Zone Damper
Fig. 7—Example of Supply Duct Layout and Damper Locations
Kitchen
Informal
Dining
Room
Living Room
Shaded
The most logical way to determine the damper and duct size
requirements for a zoning system is to first determine the zone duct
size required for each zone. Select the damper size that will move
500 CFM per ton at the desired velocity as you normally would.
Do not undersize these dampers! Then, size ducts based on 500
CFM per ton of load to each zone. Zone ducts should always be as
large or larger than the selected damper.
NOTE: For zones that need quiet operation and fast setback
recovery, use 600 CFM per ton to size zone dampers and ducts.
Larger ducts are always better with an ICM blower system. With
a non-ICM blower in the system and excessively large ducts, one
or both of the following problems might occur:
• Humidity removal might suffer.
• Register "throw" might be reduced. Continuous fan use and
good register and return placement can solve this problem.
The combination of larger ducts and an ICM blower eliminates
many problems that can occur in a zoning installation.
It is wise to oversize the ducts for small zones so that the heating
and cooling equipment can continue to operate when only the
small zones need conditioning. The zoning system will maintain
proper temperature control even if the ducts are oversized.
When determining the duct size for a zoned system, keep in mind
that duct balancing, which is an important concept for designing
non-zoned systems, is not necessary in a zoned system. The zoning
system itself will balance the ducts.
Why Oversized Ducts Are Not a Problem
with Zone Perfect Plus
Example 1: When the air handling equipment is used on a system
with oversized ducts, it delivers less air through the ductwork. This
reduction in volume and velocity can cause conditioned air to
stratify and not reach the zone sensor in a zone. When this situation
occurs, the system assumes there is a greater demand in that zone
and opens the zone damper to increase airflow to that zone.
—8—
N
Unit in
Utility
Basement
Room
Family Room
Shaded
Zone 1
Zone 2
Zone 3
Zone 4
A98343
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