Operating instructions
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 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.
c. Gas valve regulator seal MUST be on.
d. Turn off all other gas appliances and pilots.
e. Start furnace and let operate for 3 minutes.
f. Measure time (in sec) for gas meter test dial to complete
1 revolution.
g. Refer to Table 12 for cu ft of gas per hr.
h. Multiply gas rate (cu ft/hr) X heating value (Btu/cu ft)
using natural gas heating value from local gas
utility/supplier.
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 12)
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.
B. Set Temperature Rise
CAUTION: Temperature rise must be within limits
specified on furnace rating plate. Recommended opera-
tion is at midpoint of rise range or slightly above. Failure
to follow this caution may result in primary and second-
ary heat exchanger failure.
TABLE 9—MODEL 350MAV ORIFICE SIZE AND MANIFOLD PRESSURE FOR CORRECT INPUT (CONTINUED)
FOR USE WITH 040 THROUGH 120 SIZE FURNACES ONLY
(TABULATED DATA BASED ON 20,000 BTUH PER BURNER, DERATED 2 PERCENT FOR EACH 1000 FT ABOVE SEA LEVEL)
ALTITUDE
RANGE
(FT)
AVG GAS
HEAT VALUE
(BTU/CU FT)
SPECIFIC GRAVITY OF NATURAL GAS
0.58 0.60 0.62 0.64 0.66
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
U.S.A. Only
7001
to
8000
15%
derate
650 43 3.5 43 3.7 43 3.8 42 3.2 42 3.3
675 44 3.8 43 3.4 43 3.5 43 3.6 43 3.7
700 44 3.5 44 3.6 44 3.8 43 3.4 43 3.5
725 44 3.3 44 3.4 44 3.5 44 3.6 44 3.7
750 45 3.7 45 3.8 44 3.3 44 3.4 44 3.5
775 45 3.5 45 3.6 45 3.7 45 3.8 44 3.3
800 45 3.3 45 3.4 45 3.5 45 3.6 45 3.7
825 45 3.1 45 3.2 45 3.3 45 3.4 45 3.5
850 45 2.9 45 3.0 45 3.1 45 3.2 45 3.3
ALTITUDE
RANGE
(FT)
AVG GAS
HEAT VALUE
(BTU/CU FT)
SPECIFIC GRAVITY OF NATURAL GAS
0.58 0.60 0.62 0.64 0.66
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
U.S.A. Only
Altitudes
8001
to
9000
17%
derate
625 43 3.5 43 3.6 43 3.8 42 3.2 42 3.3
650 44 3.7 43 3.4 43 3.5 43 3.6 43 3.7
675 44 3.5 44 3.6 44 3.7 44 3.8 43 3.4
700 44 3.2 44 3.3 44 3.4 44 3.6 44 3.7
725 45 3.6 45 3.8 44 3.2 44 3.3 44 3.4
750 45 3.4 45 3.5 45 3.6 45 3.8 44 3.2
775 45 3.2 45 3.3 45 3.4 45 3.5 45 3.6
800 45 3.0 45 3.1 45 3.2 45 3.3 45 3.4
ALTITUDE
RANGE
(FT)
AVG GAS
HEAT VALUE
(BTU/CU FT)
SPECIFIC GRAVITY OF NATURAL GAS
0.58 0.60 0.62 0.64 0.66
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
Orifice
no.
Manifold
Pressure
U.S.A. Only
9001
to
10,000
19%
derate
600 43 3.5 43 3.6 43 3.7 42 3.2 42 3.3
625 44 3.7 44 3.8 43 3.5 43 3.6 43 3.7
650 44 3.4 44 3.5 44 3.7 44 3.8 43 3.4
675 44 3.8 44 3.3 44 3.4 44 3.5 44 3.6
700 45 3.6 45 3.7 45 3.8 44 3.3 44 3.4
725 45 3.3 45 3.4 45 3.6 45 3.7 45 3.8
750 45 3.1 45 3.2 45 3.3 45 3.4 45 3.5
775 45 2.9 45 3.0 45 3.1 45 3.2 45 3.3
*Orifice numbers 45 are factory installed.
Percents of derate are based on midpoints of U.S. altitude ranges.
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