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g. Reconnect R lead to furnace control board, release blower
door switch and re- -install blower door.
4. Operate furnace per instruction on door.
5. Verify furnace shut down by lowering thermostat setting
below room temperature.
6. Verify furnace restarts by raising thermostat setting above
room temperature.
Adjustments
WARNING
!
FIRE HAZARD
Failure to follow this warning could result in injury, death
and/or property damage.
DO NOT bottom out gas valve regulator adjusting screw.
This can result in unregulated manifold pressure and result in
excess overfire and heat exchanger failures.
CAUTION
!
FURNACE DAMAGE HAZARD
Failure to follow this caution may result in reduced furnace
life.
DO NOT redrill orifices. Improper drilling (burrs,
out- -of- -round holes, etc.) can cause excessive burner noise
and misdirection of burner flames. This can result in flame
impingement of heat exchangers, causing failures. (See Fig.
42.)
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.
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 adjust-
ment.
There are two different orifice and manifold adjustment ta-
bles. All models in all positions, except Low NOx models
in downflow or horizontal positions, use Table 12 (22,000
Btuh/Burner).
Low NOx models in the downflow or horizontal positions
must use Table 13 (21,000 Btuh/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 local gas
supplier.
c. Find installation altitude in Table 12 or 13.
d. Find closest natural gas heat value and specific gravity
in Table 12 or 13.
e. Follow heat value and specific gravity lines to point of
intersection to find orifice size and manifold pressure set-
tings for proper operation.
f. Check and verify burner orifice size in furnace. NEVER
ASSUME ORIFICE SIZE. ALWAYS CHECK AND
VERIFY.
g. Replace orifice with correct size if required by Table 10,
11, 12 and 13. Use only factory- -supplied orifices. See
EXAMPLE 1.
Table 10 – Altitude Derate Multiplier for U.S.A.
ALTITUDE
(FT. / M)
0–2000 (0--- 610)
2001–3000 (610--- 914)
3001–4000 (914--- 1219)
4001–5000 (1219--- 1524)
5001–6000 (1524--- 1829)
6001–7000 (1829--- 2134)
7001–8000 (2134--- 2438)
8001–9000 (2438--- 2743)
9001–10,000 (2743--- 3048)
* Derate multiplier factors are based on midpoint altitude for altitude
range.
EXAMPLE 1: (0 to 2000 ft. (0 to 610 M) altitude)
For 22,000 Btuh per burner application use Table 12.
Heating value = 1000 Btuh/cu ft.
Specific gravity = 0.62
Therefore: Orifice No. 43*
Manifold pressure: 3.7- -In. W.C.
*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 correct input rate.
a. Turn gas valve ON/OFF switch to OFF.
b. Remove manifold pressure tap plug from gas valve. (See
Fig. 41.)
c. Connect a water column manometer or similar device to
manifold pressure tap.
d. Turn gas valve ON/OFF switch to ON.
e. Manually close blower door switch.
f. Set thermostat to call for heat.
g. Remove regulator seal cap and turn regulator adjusting
screw counterclockwise (out) to decrease input rate of
clockwise (in) to increase input rate.
h. Install regulator seal cap.
i. Leave manometer or similar device connected and pro-
ceed to Step 4.
NOTE: DO NOT set manifold pressure less than 3.2- -In. W.C. or
more than 3.8- -In. W.C. for natural gas at sea level. If manifold
pressure is outside this range, change main burner orifices. Refer
to Table 10, 11, 12 and 13.
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.
4. Verify natural gas input rate by clocking meter.
NOTE: Gas valve regulator adjustment cap must be in place for
proper input to be clocked.
a. Turn off all other gas appliances and pilots served by the
meter.
b. Run furnace for 3 minutes in heating operation.
c. Measure time (in sec) for gas meter to complete 1 revolu-
tion and note reading. The 2 or 5 cubic feet dial provides
a more accurate measurement of gas flow.
d. Refer to Table 12 for cubic ft. of gas per hr.
31
PERCENT
DERATE
OF
MULTIPLIER
DERATE
FACTOR*
0
1.00
8–12
0.90
12–16
0.86
16–20
0.82
20–24
0.78
24–28
0.74
28–32
0.70
32–36
0.66
36–40
0.62
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