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TABLE 8—MODEL 373LAV ORIFICE SIZE† AND MANIFOLD PRESSURE FOR CORRECT INPUT
(Tabulated Data Based on 23,000 BTUH Per Burner, Derated 4 Percent per 1000 Ft Above Sea Level)*
AVG GAS
ALTITUDE
HEAT VALUE
RANGE
AT ALTITUDE
(FT)
(BTU/CU FT)
575
600
9001
625
650
to
675
700
10,000
725
750
775
* For size 060135 only, input is 22,500 Btuh per burner. Deduct 0.1-in. wc from manifold pressure shown in table. Change orifice size if manifold pressure falls below 3.2-in.
wc on altitudes up to 2000 ft, otherwise change orifice size if manifold pressure falls below 2.0-in. wc.
† Orifice sizes shown in BOLD are factory installed.
(4.) Burner flame should be clear blue, almost transpar-
ent. (See Fig. 18.)
c. Verify natural gas input rate by clocking gas meter.
NOTE: High-Altitude Adjustment
UNITED STATES
At installation altitudes above 2000 ft, this furnace has been
approved for a 4 percent derate for each 1000 ft above sea level.
See Table 7 for derate multiplier factor.
EXAMPLE:
92,000 Btuh input furnace installed at 4300 ft.
Furnace Input
Derate
Rate at
X
Multiplier
Sea Level
Factor
92,000
0.82
CANADA
At installation altitudes from 2000 to 4500 ft, this furnace must be
derated 10 percent by an authorized Gas Conversion Station. To
determine correct input rate for altitude, see example above and
use 0.90 as derate multiplier factor.
a. Turn off all other gas appliances and pilots.
b. Start furnace and let operate for 3 minutes.
c. Measure time (in sec) for gas meter test dial to complete
1 revolution.
d. Refer to Table 9 for cu ft of gas per hr.
e. 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)
Btu heating input = Btu/cu ft X cu ft/hr
Heating value of gas = 1050 Btu/cu ft
Time for 1 revolution of 2-cu ft dial = 82 sec
Gas rate = 88 cu ft/hr (from Table 9)
Btu heating input = 88 X 1050 = 92,400 Btuh
In this example, the orifice size and manifold pressure
adjustment is within ±2 percent of the furnace input rate.
2. Set temperature rise.
Furnace must operate within range of temperature rise
specified on the unit rating plate. Determine the air tem-
perature rise as follows.
0.58
0.60
Orifice
Manifold
Orifice
No.
Pressure
No.
43
3.0
43
43
2.7
43
43
2.5
43
43
2.3
43
43
2.1
43
48
3.8
43
48
3.5
48
49
3.8
48
49
3.6
49
Furnace Input Rate
=
at Installation
Altitude
75,440
—18—
SPECIFIC GRAVITY OF NATURAL GAS
0.62
Manifold
Orifice
Manifold
Pressure
No.
Pressure
3.1
43
3.2
2.8
43
2.9
2.6
43
2.7
2.4
43
2.5
2.2
43
2.3
2.1
43
2.1
3.6
48
3.7
3.4
48
3.5
3.7
49
3.8
TABLE 9—GAS RATE (CU FT/HR)
SECONDS
SIZE OF TEST DIAL
FOR 1
1
REVOLUTION
cu ft
10
360
11
327
12
300
13
277
14
257
15
240
16
225
17
212
18
200
19
189
20
180
21
171
22
164
23
157
24
150
25
144
26
138
27
133
28
129
29
124
30
120
31
116
32
113
33
109
34
106
35
103
36
100
37
97
38
95
39
92
40
90
41
88
42
86
43
84
44
82
45
80
46
78
47
76
48
75
49
73
a. Place duct thermometers in return and supply ducts as
near furnace as possible. Be sure thermometers do not
"see" heat exchangers so that radiant heat does not affect
thermometer readings. This is particularly important
with straight-run ducts.
b. When thermometer readings stabilize, subtract return-air
temperature from supply-air temperature to determine
temperature rise.
c. Adjust air temperature rise by adjusting blower speed.
Increase blower speed to reduce temperature rise. De-
crease blower speed to increase temperature rise.
(Continued)
0.64
Orifice
Manifold
Orifice
No.
Pressure
No.
43
3.3
43
43
3.0
43
43
2.8
43
43
2.6
43
43
2.4
43
43
2.2
43
43
2.1
43
48
3.6
48
48
3.4
48
SECONDS
SIZE OF TEST DIAL
2
5
FOR 1
1
cu ft
cu ft
REVOLUTION
cu ft
720
1800
50
72
655
1636
51
71
600
1500
52
69
555
1385
53
68
514
1286
54
67
480
1200
55
65
450
1125
56
64
424
1059
57
63
400
1000
58
62
379
947
59
61
360
900
60
60
343
857
62
58
327
818
64
56
313
783
66
54
300
750
68
53
288
720
70
51
277
692
72
50
267
667
74
48
257
643
76
47
248
621
78
46
240
600
80
45
232
581
82
44
225
563
84
43
218
545
86
42
212
529
88
41
206
514
90
40
200
500
92
39
195
486
94
38
189
474
96
38
185
462
98
37
180
450
100
36
176
439
102
35
172
429
104
35
167
419
106
34
164
409
108
33
160
400
110
33
157
391
112
32
153
383
116
31
150
375
120
30
147
367
0.66
Manifold
Pressure
3.4
3.1
2.8
2.6
2.4
2.3
2.1
3.7
3.5
2
5
cu ft
cu ft
144
360
141
355
138
346
136
340
133
333
131
327
129
321
126
316
124
310
122
305
120
300
116
290
112
281
109
273
106
265
103
257
100
250
97
243
95
237
92
231
90
225
88
220
86
214
84
209
82
205
80
200
78
196
76
192
75
188
74
184
72
180
71
178
69
173
68
170
67
167
65
164
64
161
62
155
60
150
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