CAUTION: UNIT DAMAGE HAZARD
Failure to follow this caution may result in component
damage due to flame impingement of burners and heat
exchangers.
DO NOT redrill orifices. Improper drilling (burrs, out-of-
round holes, etc.) can cause excessive burner noise and
misdirection of burner flames.
3. Verify Natural Gas Input Rate By Clocking Gas Meter.
NOTE: Be sure all pressure tubing, combustion-air and vent
pipes, and burner enclosure front are in place when checking input
by clocking gas meter.
a. Calculate high-altitude adjustment (if required).
UNITED STATES
At altitudes above 2000 ft, this furnace has been ap-
proved for a 2 percent derate for each 1000 ft above sea
level. See Example and Table 11 for derate multiplier
factor.
CANADA
At installation altitudes from 2001 to 4500 ft, this
furnace must be derated 5 percent by an authorized Gas
Conversion Station or Dealer. To determine correct input
rate for altitude, see example above and use 0.95 as
derate multiplier factor.
b. Reinstall burner box cover.
NOTE: Clocking gas input rate MUST always be performed with
the burner box cover INSTALLED.
c. Check that gas valve adjustment caps are in place for
proper input to be clocked.
d. Obtain yearly heat value average for local gas supply.
NOTE: Be sure heating value of gas used for calculations is
correct for your altitude. Consult local gas utility for altitude
adjustment of gas heating value.
e. Check and verify orifice size in furnace. NEVER AS-
SUME THE ORIFICE SIZE. ALWAYS CHECK AND
VERIFY.
f. Turn off all other gas appliances and pilots.
g. Move setup switch SW-1 to ON position. This keeps
furnace locked in low-heat operation.
h. Jumper R-to-W/W1.
i. Let furnace run for 3 minutes in low-heat operation.
j. Measure time (in sec) for gas meter to complete 1
revolution. Note reading.
k. Refer to Table 11 for cubic ft of gas per hr.
l. Multiply gas rate cu ft/hr by heating value (Btu/cu ft).
m. Move setup switch SW-1 to OFF position and jumper R
and W1 and W2 thermostat connections. This keeps
furnace locked in high-heat operation. Repeat items ’i’
through ’l’ for high-heat operation.
EXAMPLE: (0-2000 ft altitude)
Furnace input from rating plate is 100,000 Btuh.
Btu heating input = Btu/cu ft X cu ft/hr
Heating value of gas = 975 Btu/cu ft
Time for 1 revolution of 2-cu ft dial = 70 sec
Gas rate = 103 cu ft/hr (from Table 11)
Btu heating input = 103 X 975 = 100,425 Btuh. In this
example, the orifice size and manifold pressure adjustment
is within ±2 percent of the furnace input rate.
NOTE: Measured gas inputs (high-heat and low-heat) must be
within ±2 percent of that stated on furnace rating plate when
installed at sea level or derated per that stated above when installed
at higher altitudes.
n. Remove jumper across R, W/W1, and W2 thermostat
connections to terminate call for heat.
Fig. 54—Burner Flame
A89020
BURNER FLAME
BURNER
MANIFOLD
EXAMPLE:
100,000 Btuh input furnace installed at 4300 ft.
Derate Furnace Input Rate
Furnace Input Rate X Multiplier = at Installation
at Sea Level Factor Altitude
100,000 X 0.91 = 91,000
Fig. 55—Burner Orifice
A93059
BURNER
ORIFICE
TABLE 11—ALTITUDE DERATE MULTIPLIER FOR U.S.A.
ALTITUDE
(FT)
PERCENT
OF DERATE
DERATE MULTIPLIER
FACTOR FOR U.S.A.*
0–2000 0 1.00
2001–3000 4–6 0.95
3001–4000 6–8 0.93
4001–5000 8–10 0.91
5001–6000 10–12 0.89
6001–7000 12–14 0.87
7001–8000 14–16 0.85
8001–9000 16–18 0.83
9001–10,000 18–20 0.81
* Derate multiplier factor is based on midpoint altitude for altitude range.
—47—
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