Thermo Pride MODEL OPA - Manual (Page 23)

All installations and services must be performed by qualified service personnel.
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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. The insulation system
should be carefully selected and installed to minimize water and moisture
absorption by the insulation.