Bryant 373LAV - Manual (Page 15)

3. To operate furnace, follow procedures on operating instruc-
tion label attached to furnace.
4. With furnace operating, set thermostat below room tem-
perature and observe that furnace goes off. Set thermostat
above room temperature and observe that furnace restarts.
D. Adjustments
1. Set gas input rate
Furnace gas input rate on rating plate is for installations at
altitudes up to 2000 ft. Furnace input rate must be within
±
2
percent of input on furnace rating plate.
a. Determine natural gas orifice size and manifold pressure
for correct input.
(1.) Obtain average yearly gas heat value (at installed
altitude) from local gas supplier.
(2.) Obtain average yearly gas specific gravity from
local gas supplier.
(3.) Verify furnace model. Table 8 can only be used for
model 373LAV Furnaces.
(4.) Find installation altitude in Table 8.
NOTE: For Canada altitudes of 2000 to 4500 ft, use U.S.A.
altitudes of 2001 to 3000 ft in Table 8.
(5.) Find closest natural gas heat value and specific
gravity in Table 8.
(6.) 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)
Heating value = 1050 Btu/cu ft
Specific gravity = 0.62
Therefore: Orifice No. 43*
Manifold pressure 3.6-in. wc
* Furnace is shipped with No. 43 orifices. In this example
all main burner orifices are the correct size and do not need
to be changed to obtain proper input rate.
(7.) Check and verify burner orifice size in furnace.
NEVER ASSUME ORIFICE SIZE. ALWAYS
CHECK AND VERIFY.
NOTE: If orifice hole appears damaged or it is suspected to have
been redrilled, check orifice hole with a numbered drill bit of
correct size. Never redrill an orifice. A burr-free and squarely
aligned orifice hole is essential for proper flame characteristics.
b. Adjust manifold pressure to obtain input rate.
(1.) Remove regulator adjustment seal cap. (See Fig.
17.)
(2.) Turn adjusting screw, counterclockwise (out) to
decrease manifold pressure or clockwise (in) to
increase manifold pressure.
NOTE: This furnace has been approved for a manifold pressure
of 3.2-in. wc to 3.8-in. wc when installed at altitudes up to 2000 ft.
For altitudes above 2000 ft, the manifold pressure can be adjusted
from 2.0-in. wc to 3.8-in. wc.
CAUTION: DO NOT bottom out gas valve regulator
adjusting screw. This can result in unregulated manifold
pressure and result in excess overfire and heat exchanger
failures.
(3.) After correct manifold pressure is obtained, replace
gas valve regulator adjustment screw cap and verify
adjusted gas input rate using method outlined in
item c.
(4.) Burner flame should be clear blue, almost transpar-
ent. (See Fig. 18.)
c. Verify natural gas input rate by clocking gas meter.
NOTE: High-Altitude Adjustment
UNITED STATES
At installation altitudes above 2000 ft, this furnace has been
approved for a 4 percent derate for each 1000 ft above sea level.
See Table 7 for derate multiplier factor.
CAUTION: Do not redrill orifices. Improper drilling
(burrs, out-of-round holes, etc.) can cause excessive
burner noise and misdirection of burner flames. This can
result in flame impingement of burners and heat exchang-
ers, causing failures.
A93059
BURNER
ORIFICE
TABLE 7—ALTITUDE DERATE MULTIPLIER FOR U.S.A.
ALTITUDE
(FT)
% OF
DERATE
DERATE MULTIPLIER
FACTOR FOR U.S.A*
0—2000 0 1.00
2001—3000 8—12 0.90
3001—4000 12—16 0.86
4001—5000 16—20 0.82
5001—6000 20—24 0.78
6001—7000 24—28 0.74
7001—8000 28—32 0.70
8001—9000 32—36 0.66
9001—10,000 36—40 0.62
* Derate multiplier factor is based on midpoint altitude for altitude range.
Fig. 17—Redundant Automatic Gas Control Valve
A95618
MANIFOLD
PRESSURE TAP
GAS
PRESSURE
REGULATOR
ADJUSTMENT
INLET
PRESSURE TAP
ON AND
OFF SWITCH
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