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441 01 2121 01
Furnace gas input rate on rating plate is for installations at
altitudes up to 2000 Ft. (610 M). Furnace input rate must be
within +/−2 percent of furnace rating plate input. For altitudes
above 5500 Ft. (1676 M), a field−supplied high altitude
pressure switch is required.
1. Determine the correct gas input rate.
The input rating for altitudes above 2,000 ft. (610 M)
must be reduced by 4 percent for each 1,000 ft. (305
M) above sea level.
For installations below 2000 Ft. (610 M), refer to the
unit rating plate. For installations above 2000 Ft. (610
M), multiply the input on the rating plate by the de−rate
multiplier in Table 10 for the correct input rate.
2. Determine the correct orifice and manifold pressure ad-
justment. All models in all positions except Low NOx
models in downflow and horizontal positions use Table
14 (22,000 Btuh per burner.) Low NOx models in down-
flow or horizontal positions must use Table 15 (21,000
Btuh per burner.) See input listed on rating plate.
a. Obtain average yearly gas heat value (at installed
altitude) from local gas supplier.
b. Obtain average yearly gas specific gravity from loc-
al gas supplier.
c. Find installation altitude in Table 14 or 15.
d. Find closest natural gas heat value and specific
gravity in Table 14 or 15.
e. 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 opera-
tion.
f. Check and verify burner orifice size in furnace.
NEVER ASSUME ORIFICE SIZE. ALWAYS CHECK
AND VERIFY.
NOTE: 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.
g. Replace orifice with correct size, if required by
Table 14 or 15. Use only factory−supplied orifices.
See EXAMPLE 1.
Table 11 – Altitude Derate Multiplier for U.S.A.
ALTITUDE
FT. (M)
PERCENT
OF DERATE
DERATE MULTIPLIER
FACTOR*
0–2000
(0−610)
0 1.00
2001–3000
(610−914)
8–12 0.90
3001–4000
(914−1219)
12–16 0.86
4001–5000
(1219−1524)
16–20 0.82
5001–6000
1524−1829)
20–24 0.78
6001–7000
(1829−2134)
24–28 0.74
7001–8000
(2134−2438)
28–32 0.70
8001–9000
(2438−2743)
32–36 0.66
9001–10,000
(2743−3048)
36–40 0.62
Table 12 – Blower Off Delay Setup Switch
DESIRED HEATING MODE
BLOWER OFF DELAY (SEC.)
SETUP SWITCH (SW−7 AND −8)
POSITION
SW1−7 SW1−8
90 OFF OFF
120 ON OFF
150 OFF ON
180 ON ON
EXAMPLE 1: 0–2000 ft. (0−610 M) altitude
For 22,000 Btuh per burner application use Table 14.
Heating value = 1000 Btuh/cu ft.
Specific gravity = 0.62
Therefore: Orifice No. 43*
Manifold pressure: 3.7−In. W.C. for high−heat
1.6−In. W.C. for low−heat
* Furnace is shipped with No. 43 orifices. In this example
all main burner orifices are the correct size and do
not need to be changed to obtain proper input rate.
3. Adjust manifold pressure to obtain low fire input rate.
(See Figure 48)
a. Turn gas valve ON/OFF switch to OFF.
b. Remove manifold pressure tap plug from 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 posi-
tion to lock furnace in low−heat operation. (See
Table 10 and Figure 24)
f. Manually close blower door switch.
g. Jumper R and W/W1 thermostat connections on
control to start furnace. (See Figure 24)
h. Remove regulator adjustment cap from low heat
gas valve pressure regulator (See Figure 48) and
turn low−heat adjusting screw (3/16 or smaller flat−
tipped screwdriver) counterclockwise (out) to de-
crease input rate or clockwise (in) to increase input
rate.
NOTE: DO NOT set low−heat manifold pressure less than
1.4−In. W.C. or more than 1.7−In. W.C. for natural gas. If
manifold pressure is outside this range, change main burner
orifices.
i. Install low−heat regulator adjustment cap.
j. Move setup switch SW1−2 to off position after com-
pleting low−heat adjustment.
k. Leave manometer or similar device connected and
proceed to Step 4.
4. Adjust manifold pressure to obtain high fire input rate.
(See Figure 48)
a. Jumper R to W/W1 and W2 thermostat connections
on furnace control. This keeps furnace locked in
high−heat operation.
b. Remove regulator adjustment cap from high−heat
gas valve pressure regulator (See Figure 48) and
turn high heat adjusting screw (3/16−in. or smaller
flat−tipped screwdriver) counterclockwise (out) to
decrease input rate or clockwise (in) to increase
input rate.
NOTE: DO NOT set high−heat manifold pressure less than
3.2−In. W.C. or more than 3.8 In. W.C. for natural gas. If
manifold pressure is outside this range, change main burner
orifices to obtain manifold pressure in this range.
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