Thermo Pride GMD1-60N Installation And Service Manual page 18

All installations and services must be performed by qualified service personnel.
plate is supplied with the furnace for adapting the 3-inch round flex combustion air duct
to the 2-inch x 7-inch combustion air duct.
Recommended materials for the combustion air duct are smooth, round, galvanized
steel duct or schedule 20 PVC pipe. The maximum intake duct length allowed is
equivalent to 40-feet of straight duct. Reduce this length by 6-feet for the intake hood
and each 90-degree elbow used, and by 3-feet for each 45-degree elbow used the
combustion air duct.
Optional combustion air - a 4" round duct may be used by connecting a 3 x 4 reducer
to the 3" flex duct supplied with the furnace.
Figure 6
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