Thermo Pride OMD-70 - Manual (Page 19)

All installations and services must be performed by qualified service personnel.
The preferred location of the outside combustion air intake termination (e.g. an optional stainless
steel intake hood, part no.370183) is through the side of the structure, skirting or enclosure. An
alternate termination location is under the structure in the skirted or crawlspace area providing a
minimum of 50 square inches of free area exists around the perimeter for outside combustion air
to be drawn through. NOTE: Combustion air cannot be drawn from the occupied space of the
home.
The furnace is shipped with a combustion air duct that must be installed in the bottom of the
burner enclosure, before the burner is installed. This 2-inch by 7-inch duct is shipped in two
pieces. High temperature silicone sealant should be applied to both vertical seams, before it is
snapped together with four flanges on one end. The duct must then have high temperature
silicone applied to the flanges. Then, the combustion air duct can be inserted through the
opening in the floor base model no.70. When the combustion air duct has been inserted
completely, secure the duct to the combustion adapter with self-tapping sheet metal screws, refer
to Figure 6. Make certain that the flanges, duct, and adapter are completely sealed to the burner
enclosure base with high temperature silicone caulk.
NOTE: The adapter may be cut with sheet metal shears to the appropriate size and secured over
the inlet air duct opening. The adapter will convert the rectangular opening to a round connector
ring. The ring will allow the connection of 2-inch I.D. flexible hose (provided) between the
adapter and the inlet air boot, attached to the burner. Secure both ends of the flex hose with hose
clamps (provided).
A 2-inch round to 7-inch x 4-inch rectangular, sheet metal, transition boot is supplied with the
furnace for adapting a 4-inch round combustion air duct to the 2-inch x 7-inch combustion air
duct. This boot has a screen inserted into the 4-inch round portion of the boot to provide a degree
of protection from pests entering the furnace when the boot is terminated in the crawl space, or
skirted area, directly below the furnace.
The material recommended for the combustion air duct is smooth, round, galvanized steel duct
or schedule 20 PVC pipe. The maximum 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. Flexible, spiral, or corrugated duct is
not recommended, due to the relatively high pressure drop associated with air movement through
this type of passageway.
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