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Table 3: Suggested Duct Sizes for Homes, Quiet Offices, Or Similar Installations
(Based on a 0.1 in. W.G. static pressure drop per 100 ft. of duct.)
7. The supply and return air ducts, or flexible joints, should be carefully secured and sealed to the
appliance housing to prevent air leakage from, or into, the duct system. For best performance,
insulate the outside surfaces of the ducts to reduce heat loss from, or heat gain to, the ducts.
8. As a final step in the installation, the appliance must be adjusted to deliver a temperature rise within
the range of 50° to 80°F. Adjust the blower motor speed to obtain a temperature rise within the
acceptable range. The required blower speed will depend on the airflow resistance of a supply and
return air duct systems.
Fig. 9: Supply air duct sizing Example
The RETURN AIR DUCT SYSTEM
should equal the warm air duct system in airflow capabilities.
NOTE: When a return register is located in the same room as the furnace, the register must be at least 20
feet away from the furnace.