TABLE 4
ADJUSTING OR CHECKING
FURNACE INPUT
NATURAL GAS:
The maximum gas supply pressure to
the furnace should be 10.5” W.C. for
natural gas. The minimum gas supply
pressure for purposes of input adjust-
ment to the furnace should be 5” W.C.
A properly calibrated manometer or
gauge is required for accurate gas
pressure readings.
1. When adjusting the furnace input,
the high fire input should be
checked. The high fire manifold pres-
sure should be 3.5' W.C. Follow
these steps to be sure the furnace is
high fire mode:
a. With a single stage thermostat,
the furnace runs for 12 minutes on
low fire before shifting to high fire.
To be certain that it is on high fire,
jump terminals “W” and “W2” on
the control board in the blower
compartment.
b. With a two stage thermostat, set
the thermostat to its highest set-
ting to keep the furnace operating
in the high fire mode.
2. To adust high fire manifold pressure,
remove the adjustment cover screw
on the outlet end of the gas valve
and turn the adjustment screw clock-
wise to increase the pressure and
counterclockwise to reduce the pres-
sure. Replace the cover screw
securely.
3. The low fire manifold pressure
should be 1.7" W.C. As mentioned
above, the furnace remains in the
low fire mode for 12 minutes upon a
heat call with a single stage thermo-
stat. With a two stage thermostat,
disconnect the thermostat lead to the
“W2” terminal on the control board
and the furnace will remain in the low
fire mode. To adjust the pressure,
remove the regulator cover, on top of
the valve, and adjust as noted under
Step 2, above. After the adjustment
replace the screw cover securely.
NOTE: Use a 3/32⬙ allen wrench for
making the pressure adjustment.
LP GAS:
Furnaces for use on LP gas, the LP gas
supply pressure must be set between
11.0” and 13.0” W.C. by means of the
tank or branch supply regulators. The
furnace manifold pressure should be
set at 10” W.C. at the gas control valve.
For elevations up to 7,000 feet, rating
plate input ratings apply. For high alti-
tudes (elevations 7,000 and over) and
for any necessary major changes in the
gas flow rate the orifice spud must be
changed.
TO CHANGE ORIFICE SPUDS:
1. Shut off the manual main gas valve
and remove the gas manifold.
2. Replace the orifice spuds.
3. Reassemble in reverse order.
4. Turn the gas supply back on and
check for leaks.
5. Check for proper operation and set
to proper manifold pressure.
Check of input is important to pre-
vent over firing of the furnace
beyond its design-rated input.
NEVER SET INPUT ABOVE THAT
SHOWN ON THE RATING PLATE.
TO CHECK FURNACE INPUT:
1. Make certain that all other gas
appliances are shut off, with the
exception of pilot burners.
2. Start the furnace
3. Time the meter to measure the
time required to burn one cubic
foot of gas.
4. Use Table 4 to determine input
rate.
METER TIME IN MINUTES AND SECONDS FOR NORMAL
INPUT RATING OF FURNACES EQUIPPED FOR NATURAL OR LP GAS
INPUT
BTU/HR
METER
SIZE
CU. FT.
HEATING VALUE OF GAS BTU PER CU. FT.
900 1000 1040 1100 2500
MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC. MIN. SEC.
ONE 15112 1 15 1 18 3 20
50,000
TEN 10 50 12 00 12 30 13 12 30 00
ONE 0 44 0 48 0 50 0 53 20
75,000
TEN 7 12 80819 8 48 20 0
ONE 0 33 0 36 0 38 0 40 1 30
100,000
TEN 5 24 60615 6 36 15 0
ONE 0 26 0 29 0 30 0 32 1 12
125,000
TEN 4 19 4 48 50517 12 0
ONE 0 31 0 24 0 25 0 26 10
150,000
TEN 3 36 40410 4 20 10 0
Heating Value of Gas (BTU/Ft
3
) x 3600 x correction factor
Input BTU/HR =
Time in Seconds (for 1 cu.ft.) of Gas
22
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