106
Field Selections
TIMING DIAGRAM
TIMING DIAGRAM AND FIELD ADJUSTMENTS
TABLE 17
T
IMING DIAGRAM
ST-A1205-10-X0
A
D
J
U
S
T
I
N
G
O
R
C
H
E
C
K
I
N
G
F
U
R
N
A
C
E
I
N
P
U
T
The maximum gas supply pressure to the furnace should
be 10.5" w.c. for natural gas and 13.0" w.c. for L.P. The
minimum gas supply pressure for purposes of input ad-
justment to the furnace should be 5" w.c. for natural gas
and 11" w.c. for L.P.
A calibrated manometer is required for accurate gas pres-
sure readings.
The manifold pressure should be set at 3.5" w.c. high fire,
1.8" low fire, for natural gas and 10” w.c. high fire and 4.9”
w.c. low fire for L.P. Only small variations in the gas flow
should be made by means of the pressure regulator ad-
justment. In no case should the final manifold pressure
vary more than plus or minus 0.3" w.c. from the above-
specified pressures. To adjust the pressure regulator, re-
move the regulator cap and turn the adjustment screw
clockwise to increase pressure or counterclockwise to de-
crease pressure. Then replace the regulator cap securely.
Any necessary major changes in the gas flow rate should
be made by changing the size of the burner orifices.
To change orifice spuds, shut off the manual gas valve
and remove the gas manifold. On LP gas furnaces, the LP
gas supply pressure must be set between 11" and 13" w.c.
by means of the tank or branch supply regulators. The fur-
nace manifold pressure should be set at 10" w.c. at the
gas control valve. For elevations up to 2,000 feet, rating
plate input ratings apply. For high altitudes (elevations
over 2,000 ft.), see conversion kit index for derating and
orifice spud sizes.
Checking furnace input is important to prevent over firing
beyond its design-rated input. NEVER SET INPUT
ABOVE THAT SHOWN ON THE RATING PLATE. Use the
following table or formula to determine input rate. Start the
furnace and measure the time required to burn one cubic
foot of gas. Prior to checking the furnace input, make cer-
tain that all other gas appliances are shut off, with the ex-
ception of pilot burners. Time the meter with only the
furnace in operation.
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