Bryant 986TB Installation, Start-Up, Operating And Service And Maintenance Instructions page 71

NOTICE
DO NOT set high−heat manifold pressure less than 3.2−in.
w.c. (797 Pa) or more than 3.8 in. w.c. (947 Pa) for natural
gas. If required manifold pressure is outside this range,
change main burner orifices to obtain manifold pressure in this
range.
c. When correct input is obtained, replace caps that conceal
gas valve regulator adjustment screws. Main burner flame
should be clear blue, almost transparent. See Fig. 66.
d. Remove jumpers R to W/W1 and R to W2.
Clocking the Meter
1. Verify natural gas input rate by clocking meter.
NOTE: Contact your HVAC distributor or gas supplier for metric
gas meter tables, if required.
a. Turn off all other gas appliances and pilots served by the
meter.
b. Move setup switch SW1−2 to ON position. This keeps fur-
nace locked in low−heat operation when only W/W1 is en-
ergized..
c. Jumper R to W/W1.
d. Run furnace for 3 minutes in low−heat operation.
e. Measure time (in sec) for gas meter to complete one revolu-
tion and note reading. The 2 or 5 cubic feet dial provides
a more accurate measurement of gas flow.
f. Refer to Table 20 for cubic ft. of gas per hr.
g. Multiply gas rate cu ft./hr by heating value (Btuh/cu ft.) to
obtain input rate.
h. If clocked rate does not match required input from Step 1,
increase manifold pressure to increase input or decrease
manifold pressure to decrease input. Repeat steps b through
e of Step 1 until correct low−heat input is achieved. Re−in-
stall low heat regulator seal cap on gas valve.
i. Jumper R to W/W1, and W2. This keeps furnace locked
in high−heat operation when both W/W1 and W2 are ener-
gized.
j. Repeat items d through g for high−heat operation, repeat-
ing Step 2 and adjusting the high−heat regulator screw as
required.
2. Restore furnace to normal operating condition.
a. Turn gas valve ON/OFF switch to OFF.
b. Remove water column manometer or similar device from
manifold pressure tap.
WARNING
!
FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Manifold pressure tap set screw must be tightened or ⅛−in.
NPT pipe plug must be installed to prevent gas leaks.
c. Tighten set screw on manifold pressure tap with 3/32−in
hex wrench, or if 1/8−in. NPT plug was removed, apply
pipe dope sparingly to end of plug and reinstall in the gas
valve.
d. Turn gas valve ON/OFF switch to ON.
e. Move setup SW1−2 on furnace control to position required
for attached thermostat (OFF for single-stage thermostats,
ON for two-stage thermostats).
f. Check for gas leaks and verify furnace operation.
Adjust Temperature Rise
NOTE: Blower door must be installed when taking temperature
rise reading. Leaving blower door off will result in incorrect
temperature measurements, due to possible changes in duct static
pressure and airflow.
CAUTION
!
FURNACE DAMAGE HAZARD
Failure to follow this caution may result in:
S Overheating the heat exchangers or condensing flue gases
in heat exchanger areas not designed for condensate.
S Shortened furnace life
S Component damage.
Temperature rise must be within limits specified on furnace
rating plate. Recommended operation is at midpoint of rise
range or slightly above.
When setup switch SW1−4 is ON, operation will be near the high
end of the rise range for improved comfort.
Furnace must operate within ranges of temperature rise specified
on the furnace rating plate. Determine air temperature rise as
follows:
a. Place thermometers in return and supply ducts as near fur-
nace as possible. Be sure thermometers do not see heat ex-
changer so that radiant heat does not affect readings. This
practice is particularly important with straight−run ducts.
b. When thermometer readings stabilize, subtract return−air
temperature from supply−air temperature to determine air
temperature rise.
NOTE: Temperature rise can be determined for low−heat and
high−heat operation by locking the furnace in each mode of
operations. The mode of operation is based on the position of
Setup Switch SW1−2 on the furnace control board. (NEW)
c. This furnace is capable of automatically providing proper
airflow to maintain the temperature rise within the range
specified on furnace rating plate. If temperature rise is out-
side this range, proceed as follows:
(1.) Check gas input for low− and high−heat operation.
(2.) Check derate for altitude, if applicable.
(3.) Check all return and supply ducts for excessive
restrictions causing static pressure greater than
0.5−In. W.C.
(4.) Ensure Low Heat Rise Adjust switch SW1−3 on
furnace control is in ON position when a bypass
humidifier is used. See Fig. 38 for switch location.
(5.) Make sure proper model plug is installed.
To lock the furnace in low heat:
1. Turn SW1−2 ON at the furnace control.
2. Connect a jumper across R and W/W1 at the thermostat
terminals at the furnace control.
3. Allow the burners to ignite and the blower to turn on.
4. Allow the supply temperature to stabilize and verify the
proper rise range.
If the temperature rise is too high or too low in low heat:
1. Remove jumpers from R and W/W1.
2. Wait until the blower off delay is completed.
3. Turn 115 VAC power off.
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