AIR DISTRIBUTION SYSTEM DESIGN
the perimeter where heat loss occurs is becoming less
important. In hot climates, ceiling diffusers or high wall outlets
that discharge air parallel to the ceiling are typically installed. In
moderate climates, outlet location is less critical. Outlet locations
near interior walls can significantly reduce duct lengths
(decreasing costs), thermal losses (if ducts are located outside
the conditioned space), and blower requirements. To prevent
supply air from being swept directly up by kitchen, bathroom, or
other exhaust fans, the distance between supply registers and
exhaust vents should be kept as large as possible.
The location of the return register has only a secondary effect
on room air motion. However, returns can help defeat
stratification and improve mixing of room air if they are placed
high when cooling is the dominant space-conditioning need
and low when heating is dominant. In multi-story homes with
both heating and cooling, upper-level returns should be placed
high and lower-level returns should be placed low. Otherwise,
the location of the return register can be determined by what
will minimize duct runs, improve air circulation and mixing of
supply air, and impact other considerations such as aesthetics.
D U C T D E S I G N M E T H O D
The air distribution system should be designed at the same time
the house plans are being developed, following the procedures
in the Air Conditioning Contractors of America's (ACCA's)
Manual D: Residential Duct Systems. Planning locations for
ductwork, structural framing, plumbing, and electrical wiring
simultaneously avoids conflicts between these systems.
The following eight steps should be followed in the design of
an air distribution system to ensure efficiency and comfort:
1. Select the general type of heating and cooling equipment
(e.g., furnace, heat pump, air conditioner). The heating and
cooling equipment should be selected based on occupant
preferences, availability of different fuels (e.g., natural gas,
electricity), installation costs, and operating costs.
2. Select the general type of air distribution system (supply and
return duct systems). The general designs and duct materials
for the supply and return duct systems should be selected
after considering the type of equipment selected and its
location, the local climate, the architectural and structural
features of the house, zoning requirements, and installation
and operating costs. ACCA's
Manual G: Selection of
Distribution Systems and Manual RS: Comfort, Air Quality, and
Efficiency by Design can assist in this selection.
3. Calculate the design heating and cooling loads of each room
of the house and the loads that are associated with the entire
house using ACCA's
Manual J: Residential Load Calculation
(eighth edition). Room loads are used to determine the air flow
needed for each room, and the house loads are used to size
and select specific heating and cooling equipment models.
4. Size and select the specific models of the heating and cooling
equipment using ACCA's
Manual S: Residential Equipment
Selection. This precedes the duct sizing calculations because,
in residential applications, the blower (fan) data of the selected
equipment establish the duct design criteria. In addition,
identify any component or device (e.g., filter, humidifier,
electric resistance heater, cooling coil) that was not included
when the blower data and their associated pressure drops
5. Develop a scale drawing or rough sketch of the air distribution
system showing the location of the air handling equipment,
supply outlets, return openings, loads and air flow rates
associated with each supply and return register, location of
duct runs, lengths of straight duct runs, fitting types, and
equivalent lengths of the fittings. Be sure to account for the
direction of joists, roof hips, and other potential obstructions
such as two-story foyers or rooms.
6. Determine the size of all the ducts based on the room loads,
blower data, pressure drops of additional components or
devices, and equivalent duct lengths following the procedures
Manual D: Residential Duct Systems. Several duct
layouts may need to be examined before reaching a final design.
7. Select and size the air distribution system devices (return
grilles and supply air diffusers, grilles, and registers) using
ACCA's Manual T: Air Distribution Basics for Residential and
Small Commercial Buildings. These must be selected to
maintain air velocities below values that will cause noise but,
in the case of supply outlets, sufficiently high so that air is
disbursed to exterior walls or ceilings as desired.
8. Select the insulation levels for the duct system in accordance
with the 2000 International Energy Conservation Code.