29
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.
NOTICE
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.
59SC
5
A
