Bryant 986TB Installation, Start-Up, Operating And Service And Maintenance Instructions page 70

5. 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.
6. Check and verify burner orifice size in furnace. NEVER
ASSUME ORIFICE SIZE. ALWAYS CHECK AND
VERIFY.
NOTICE
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.
7. Replace orifice with correct size, if required as shown in
Table 21. Use only factory−supplied orifices. See
EXAMPLE 1.
EXAMPLE 1
EXAMPLE: 0 − 2000 ft. (0 − 609.6M) altitude
Heating value = 1050 Btu/cu ft.
Specific gravity = 0.62
Therefore: Orifice No. 44
(Furnace is shipped with No. 44 orifices. In this example, all main
burner orifices are the correct size and do not need to be changed to
obtain proper input rate.)
Manifold pressure: 3.4−in. w.c. (847 Pa) for high heat, 1.4−in. w.c.
(349 Pa) for low heat
NOTE: To convert gas manifold Table pressures to Pascals,
multiply the in. w.c. value by 249.1 Pa/in. w.c. (1 in. w.c. = 249.1
Pa).
Check Inlet Gas Pressure
The inlet gas pressure must be checked with the furnace operating
in maximum heat. This is necessary to make sure the inlet gas
pressure does not fall below the minimum pressure of 4.5 in. w.c.
for natural gas. The maximum inlet gas pressure is 13.6 in. of water
column. If the inlet pressure is too low, you will not be able to
adjust the manifold pressure to obtain the proper input rate. To
check the inlet gas pressure:
1. Make sure the gas supply is turned off to the furnace and at
the electric switch on the gas valve.
2. Loosen set screw on inlet tower pressure tap no more than
one full turn with a 3/32−in. hex wrench, or remove the 1/8
in. NPT plug from the inlet pressure tap on the gas valve.
3. Connect a manometer to the inlet pressure tap on gas valve.
4. Turn on furnace power supply.
5. Turn gas supply manual shutoff valve to ON position.
6. Turn furnace gas valve switch to ON position.
7. Jumper the R to W/W1 and W2 thermostat connections at
the furnace control board.
8. When main burners ignite, confirm inlet gas pressure is
Between 4.5 in. w.c. and 13.6 in. w.c.
9. Remove jumper across thermostat connections to terminate
call for heat. Wait until the blower off delay is completed.
10. Turn furnace gas valve electric switch to OFF position.
11. Turn gas supply manual shutoff valve to OFF position.
12. Turn off furnace power supply.
13. Remove manometer from the inlet pressure tap of the gas
valve.
WARNING
!
FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Inlet pressure tap set screw must be tightened and ⅛−in.
NPT pipe plug must be installed to prevent gas leaks.
14. Tighten set screw on inlet tower pressure tap with 3/32−in.
hex wrench, or if ⅛−in. NPT plug was removed, apply pipe
dope sparingly to end of plug and re−install in the gas
valve.
Adjust Manifold Pressure
1. Adjust manifold pressure to obtain low fire input rate. See
Fig. 59.
a. Turn gas valve ON/OFF switch to OFF.
b. Loosen set screw on manifold tower pressure tap with
3/32−in. hex wrench, or remove the 1/8 inch NPT plug
from the manifold pressure tap on the gas valve.
c. Connect a water column manometer or similar device to
manifold pressure tap.
d. Turn gas valve ON/OFF switch to ON.
e. Move setup SW1−2 on furnace control to ON position to
lock furnace in low−heat operation. See Fig. 64 and Fig.
38.
f. Manually close blower door switch.
g. Jumper R and W/W1 thermostat connections on control to
start furnace. See Fig. 38.
h. Remove regulator adjustment cap from low heat gas valve
pressure regulator and turn low−heat adjusting screw (3/16
or smaller flat−tipped screwdriver) counterclockwise (out)
to decrease input rate or clockwise (in) to increase input
rate. See Fig. 59.
NOTICE
DO NOT set low−heat manifold pressure less than 1.3−in. w.c.
(324 Pa) or more than 1.7 in. w.c. (423 Pa) for natural gas. If
required manifold pressure is outside this range, change main
burner orifices to obtain manifold pressure in this range.
i. Install low−heat regulator adjustment cap.
j. Move setup switch SW1−2 to OFF position after complet-
ing low−heat adjustment.
k. Leave manometer or similar device connected and proceed
to Step 2.
2. Adjust manifold pressure to obtain high fire input rate See
Fig. 59.
a. Jumper R to W/W1 and W2 thermostat connections on fur-
nace control. This keeps furnace locked in high−heat oper-
ation.
b. Remove regulator adjustment cap from high−heat gas
valve pressure regulator and turn high heat adjusting screw
(3/16−in. or smaller flat−tipped screwdriver) counterclock-
wise (out) to decrease input rate or clockwise (in) to in-
crease input rate. See Fig. 59.
70