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j. Reinstall manifold pressure tap plug from gas valve.
k. Move setup switch SW- -1 (LHT) to off position after com-
pleting low- -heat adjustment.
l. Leave manometer or similar device connected and proceed
to Step 2.
2. Adjust manifold pressure to obtain high fire input rate. See
Fig. 54.
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 See Fig. 54 and turn high heat ad-
justing screw (3/16- -in. or smaller flat- -tipped screwdriver)
counterclockwise (out) to decrease input rate or clockwise
(in) to increase input rate.
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. 59.
d. Remove jumpers R to W/W1 and R to W2.
3. 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- -1 (LHT) to ON position. This
keeps furnace locked in low- -heat operation when only
W/W1 is energized..
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 re-
volution 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 h for high- -heat operation, repeat-
ing Step 2 and adjusting the high- -heat regulator screw, as
required.
4. 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.
c. Replace manifold pressure tap plug to gas valve.
d. Turn gas valve ON/OFF switch to ON.
e. Move setup SW-1 (LHT) on furnace control to position re-
quired for attached thermostat (OFF for single-stage ther-
mostats, ON for two-stage thermostats).
f. Check for gas leaks and verify furnace operation.
!
WARNING
FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Reinstall manifold pressure tap plug in gas valve to prevent
gas leak.
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.
Place SW- -1 (LHT) in ON position. Jumper R to W/W1 and W2 to
check high- -gas- -heat temperature rise. To check low- -gas- -heat
temperature rise, remove jumper to W2. Determine air temperature
rise for both high and low gas heat. Do not exceed temperature rise
ranges specified on unit rating plate for high and low gas heat.
This furnace must operate within the temperature rise ranges
specified on the furnace rating plate. Determine the air temperature
as
follows:
1. Place duct thermometers in return and supply ducts as close
to furnace as possible. Be sure thermometers do not "see"
heat exchangers so that radiant heat does not affect
thermometer readings. This is particularly important with
straight run ducts.
2. When thermometer readings stabilize, subtract return- -air
temperature from supply- -air temperature to determine
temperature rise.
If the temperature rise is outside this range, check the following:
1. Gas input for low- -and high gas heat operation.
2. Derate for altitude if applicable.
3. Return and supply ducts for excessive restrictions causing
static pressures greater than 0.50- -in. w.c. (125 Pa).
4. Adjust temperature rise by adjusting blower speed.
S Increase blower speed to reduce temperature rise.
S Decrease blower speed to increase temperature rise.
64
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