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AIR DUCTS
Many states, provinces and localities are considering or have
implemented
standards
and/or
restrictions
on
duct
sizing
practices, ductwork leakage, and/or ductwork thermal, airflow
and
electrical
efficiencies.
CONSULT
LOCAL
CODE
OFFICIALS
for
ductwork
design
and
performance
requirements in your area.
General Requirements
The
duct
system
should
be
designed
and
sized
according
to
accepted
national
standards
such
as
those
published
by:
Air
Conditioning Contractors Association (ACCA
Manual D), Sheet
Metal
and
Air
Conditioning
Contractors
National
Association
(SMACNA)
or American
Society of Heating, Refrigerating and
Air
Conditioning
Engineers
(ASHRAE)
or
consult
The
Air
Systems Design
Guidelines reference tables available from your
local distributor. The duct system should be sized to handle the
required system design CFM at the design external static pressure.
The
furnace
airflow
rates
are
provided
in
Table
9-Air
Delivery-CFM (With Filter). When
a furnace is installed so that
the supply ducts carry air circulated by the furnace to areas outside
the space
containing
the furnace,
the return
air shall
also be
handled by duct(s) sealed to the furnace casing and terminating
outside the space containing the furnace.
Secure ductwork with proper fasteners for type of ductwork used.
Seal supply-
and return-duct connections to furnace with code
approved tape or duct sealer.
NOTE:
Flexible connections should be used between ductwork
and furnace to prevent transmission of vibration.
Ductwork
passing
through
unconditioned
space
should
be
insulated to enhance system performance. When air conditioning is
used, a vapor barrier is recommended.
Maintain a 1-in. (25 mm) clearance from combustible materials to
supply air ductwork for a distance of 36-in. (914 mm) horizontally
from
the
furnace.
See
NFPA
90B
or local
code
for further
requirements.
Return Duct Sizing
Refer
to
the
Filter
Selection
and
Duct
Sizing
section
for
information on the proper selection of filter sizes and the associated
ductwork
and
duct
transitions.
Improperly
designed
filtering
systems
and
return
ductwork
are
the
most
common
causes
of
airflow and/or noise complaints in HVAC systems.
Ductwork Acoustical Treatment
NOTE:
Metal duct systems that do not have a 90 degree elbow
and 10 ft. (3 M) of main duct to the first branch take-off may
require
internal
acoustical
lining.
As
an
alternative,
fibrous
ductwork may be used if constructed and installed in accordance
with
the latest edition
of SMACNA
construction
standard
on
fibrous glass ducts. Both acoustical lining and fibrous ductwork
shall comply with NFPA
90B as tested by UL Standard 181 for
Class 1 Rigid air ducts.
NOTE:
For horizontal applications, the top most flange may be
bent past 90° to allow the evaporator coil to hang on the flange
temporarily while the remaining attachment and sealing of the coil
are performed.
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