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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 ram) clearance
from combustible
materials
to
supply air ductwork
for a distance
of 36-in.
(914 ram) horizontally
from
the
furnace.
See
NFPA
90B
or local
code
for
further
requirements.
Return Duet 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.
Duetwork
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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