Operating instructions

III. PURGE GAS LINES
If not previously done, purge lines after all connections have been
made and check for leaks.
WARNING: Never purge a gas line into a combustion
chamber. Never test for gas leaks with an open flame. Use
a commercially available soap solution made specifically
for the detection of leaks to check all connections. Failure
to follow this warning could result in fire, explosion,
personal injury, or death.
IV. ADJUSTMENTS
A. Set Gas Input Rate
Furnace gas input rate on rating plate is for installations at altitudes
up to 2000 ft.
In the U.S.A., the input ratings for altitudes above 2000 ft must be
reduced by 2 percent for each 1000 ft above sea level.
In Canada, the input ratings must be derated by 5 percent for
altitudes of 2001 ft to 4500 ft above sea level.
Furnace input rate must be within ± 2 percent of input on furnace
rating plate adjusted for altitude.
1. Determine natural gas orifice size and manifold pressure for
correct input.
a. Obtain average gas heat value (at installed altitude) from
local gas supplier.
b. Obtain average gas specific gravity from local gas
supplier.
c. Verify furnace model and size. Table 9 can only be used
for model 350MAV furnaces with heating sizes of 040
through 120. Table 10 can only be used for model
350MAV furnaces with a heating size of 140.
d. Find installation altitude in Table 9 or 10.
NOTE: For Canada altitudes of 2001 to 4500 ft, use U.S.A.
altitudes of 2001 to 3000 ft in Table 9 or 10.
e. Find closest natural gas heat value and specific gravity
on Table 9 or 10.
f. Follow heat value and specific gravity lines to point of
intersection to find orifice size and manifold pressure
settings for proper operation.
EXAMPLE: (0-2000 ft altitude using Table 9)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 45
Manifold pressure 3.6-in. wc
* 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.
Check and verify burner orifice size in furnace. NEVER ASSUME
ORIFICE SIZE; ALWAYS CHECK AND VERIFY.
2. Adjust manifold pressure to obtain input rate.
NOTE: Manifold pressure must always be measured with burner
enclosure front REMOVED. Gas meter must always be clocked
with burner enclosure front INSTALLED.
a. Remove burner enclosure front.
b. Remove cap that conceals adjustment screw for gas
valve regulator. (See Fig. 57.)
c. Jumper R and W thermostat connections on control to
start furnace.
d. Turn adjusting screw, counterclockwise (out) to decrease
manifold pressure or clockwise (in) to increase manifold
pressure.
Fig. 55—Inducer Housing Drain Tube
A99118
Fig. 56—Filling Condensate Trap
A99119
—40—