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All installations and services must be performed by qualified service personnel.
H. Airflow Requirements and Sizing of Duct Work
The duct system must be sized and installed by a qualified installer or service
person, following the design standards of the Air Conditioning Contractors of
America (ACCA) or the American Society of Heating, Refrigeration, and Air
Conditioning Engineers (ASHRAE). This furnace has been designed to
operate against a maximum external static pressure of 0.4 in. W.G. This is
equivalent to .2 in. W.G. supply and .2 in. W.G. return pressure drops.
1. Supply and return air ducts have to be furnished by the installer and run
between the appliance, which must be installed outdoors, and the interior of
the structure the appliance serves. These ducts must be sealed to the casing
of the appliance.
2. To reduce the transmission of vibration and noise to the duct system and to
reduce flexure of the duct system due to thermal expansion and contraction, it
is recommended that flexible joints be installed at the supply and return duct
connections to the unit.
3. The return air duct system must equal the supply air duct system in the flow
capacity (CFM) for a given pressure drop. Use a supplier's catalog for proper
sizing of outlet and return air registers and grills to ensure that they meet the
flow requirements of the run to which they are connected.
4. The duct system shall be sized to provide the maximum air flowrate (CFM)
required of the installation. Two common rules for determining minimum
airflow in heating and cooling systems follow:
a. For heating, 14 CFM of airflow are required per 1000 BTU/hr of heat
output, based on steady state operation and a 50° to 80° temperature rise.
b. For cooling, 400 CFM of airflow are required per ton of air conditioning.
(For reference, a ton of A/C = 12,000 BTU/hr removed from the space.)
Refer to Example 1, in Appendix B: Calculations of this manual, for a
sample calculation of how to determine the required minimum air flowrate.
5. Duct sizing is based upon both air velocity and pressure drop considerations.
When possible, current practice favors designing ductwork for lower air
velocities. (For residences, a maximum air velocity of 800 FPM is suggested.)
This results in quieter duct systems, systems which require less fan power
(reduced operating costs), and less carefully constructed ductwork (lower
initial costs).
However, lower air velocities also result in larger duct sizes than necessary at
higher velocities. In some cases, space restrictions may limit the ductwork to
smaller than optimal sizes.
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