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System Design Guidelines
Designing a duct system for a Harmony
tion is similar to non−zoned duct design, but with a few extra
considerations. Certain decisions must be made before
ducts, dampers, supply registers, return grilles, and ther-
mostats are installed. This section is a general guide to de-
termine how to group rooms into zones. Refer to ACCA
Manual D for detailed duct design guidelines. ACCA
Manual G helps in selecting which design is best for a given
application.
A−Zoning Considerations
Structural/Architectural
Overheating may occur in the second story of multi−level
houses that use regular HVAC systems. Because heated
air rises to the second floor, these second story rooms be-
come too hot. Zoning will remedy this situation by splitting
the conditioned space into two specific zones (first and
second floors). The second floor temperature is now con-
trolled by its own thermostat, not that of the first floor.
never group rooms on different levels into the same
D
zone
all zones should be roughly equal in load
D
Climate
Overheating and undercooling because of weather
changes are problems associated with homes that are
conditioned by one thermostat. Zoning reduces overheat-
ing and undercooling by grouping rooms with the same di-
rectional exposure into one zone. Therefore, a room with a
northern exposure and a room with a southern exposure
should not be zoned together. During the winter, the north-
ern room may be too cool while the southern room may be
too hot. In order to keep a balance of conditioned air, the
zone must contain rooms with the same directional expo-
sure.
Never group rooms with opposite directional expo-
D
sures into the same zone.
®
II zoning applica-
Page 8
Occupancy
A single structure contains different types of living spaces.
Without zoning, it is difficult to condition those areas so that
everyone will be comfortable. Zoning a home or office al-
lows the user to control the zones which need to be condi-
tioned.
Rooms that are used or occupied at the same time should be
zoned together. For instance, bedrooms which are generally
occupied at night, should not be zoned with a living room that
is used only during the day or evening.
Never group rooms that are not usually occupied at the
D
same time into the same zone.
B−Standard Zone Duct Systems
®
The Harmony
II zone control system allows independent
zone conditioning for up to four areas in a house or build-
ing. Ideally, a four zone system will use four trunks, each
with one damper. However, the existing duct system in
some retrofit applications may not have been designed for
zoning. In these cases, the duct system must be modified
either with new duct or extra dampers to ensure that each
zone is isolated from the others.
In new construction, it is recommended that each zone
have its own return air register. This will better isolate each
zone and prevent air from mixing. In retrofit applications
with few return registers, there may be a tendency for air to
mix between zones. If this is the case, install extra return
registers.
Designing Ducts for the Harmony
System
1− Perform a standard heat/cool load calculation of the
home or office.
2− Keeping the previous calculations in mind, determine
which rooms will be grouped into each zone.
3− From the load calculation, determine the cfm at each reg-
ister. For each zone, add the cfm from the registers. No
one zone should have less than 20−25% of total cfm.
4− Using ACCA guidelines and work sheets, choose the best
duct system design. Duct layout will depend on the struc-
ture of the building and the region of the country. Figures
3 and 4 show applications of reduced radial and extended
plenum systems.
®
II Zone Control
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