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Furnace input rate must be within ±2 percent of input on furnace rating plate.
1. Determine natural gas orifice size and manifold pressure for correct input.
a. Obtain yearly heat value average (at installed altitude) from local gas supplier.
b. Obtain yearly specific gravity average from local gas supplier.
c. Verify furnace model. Table 13 can only be used for model 355MAV furnaces.
d. Find installation altitude in Table 13.
NOTE: For Canada altitudes of 2000 to 4500 ft, use U.S.A. altitudes of 2001 to 3000 ft in Table 13.
e. Find closest natural gas heat value and specific gravity in Table 13.
f. Follow heat value and specific gravity lines to point of intersection to find orifice size and low- and high-heat manifold pressure settings
for proper operation.
EXAMPLE: (0-2000 ft altitude)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 45
Manifold pressure 3.6-in. wc for high heat
1.5-in. wc for low heat
* Furnace is shipped with No. 45 orifices. In this example, all main burner orifices are the correct size and do not need to be changed to
obtain the proper input rate.
g. Check and verify burner orifice size in furnace. NEVER ASSUME ORIFICE SIZE; ALWAYS CHECK AND VERIFY.
2. Adjust manifold pressure to obtain input rate.
a. Remove burner enclosure front.
NOTE: Manifold pressure MUST always be measured with the burner box cover REMOVED.
b. Remove caps that conceal adjustment screws for low- and high-heat gas valve regulators. (See Fig. 56 .)
c. Move setup switch SW-2 on control center to ON position. (See Fig. 31.) This keeps furnace locked in low-heat operation.
d. Jumper R and W/W1 thermostat connections on control center to start furnace.
e. Turn low-heat adjusting screw (5/64 hex Allen wrench) counterclockwise (out) to decrease input rate or clockwise (in) to increase input
rate.
Fig. 55—Filling Condensate Trap
A99119
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