035-17477-002 Rev. D (1202)
Unitary Products Group
27
CHECKING GAS INPUT (NATURAL GAS)
NOTE:
Front door of burner box must be secured when
checking gas input.
1.
Turn off all other gas appliances connected to the gas
meter.
2.
With the furnace turned on, measure the time needed for
one revolution of the hand on the smallest dial on the
meter. A typical domestic gas meter usually has a 1/2 or
1 cubic foot test dial.
3.
Using the number of seconds for each revolution and the
size of the test dial increment, find the cubic feet of gas
consumed per hour, (Table 9 on Page 27).
NOTE:
To find the Btuh input, multiply the number of cubic
feet of gas consumed per hour by the BTU content of the gas
in your particular locality. Contract your gas company for this
information, as it varies widely from city to city.
EXAMPLE:
It is found by measurement that it takes 26 sec-
onds for the hand to turn on the 1 cubic foot dial to make a
revolution with only a 120,000 Btuh furnace running. Using
this information, locate 26 seconds in the first column of.
Read across to the column headed
1 Cubic Foot
where you
will see that 138 cubic feet of gas per hour are consumed by
the furnace at that rate. Multiply 138 by 850 (the BTU rating
of the gas obtained from the local gas company). The result is
117,300 Btuh, which is close to the 120,000 Btuh rating of the
furnace.
If the actual input is not within
+
2% of the furnace rating, with
allowance being made for the permissible range of the regu-
lator setting (0.3 inches W.C.), replace the orifice spuds with
spuds of the proper size.
FIGURE 37 :
White Rodgers 36G Gas Valve
O
FF
OFF
O
N
ON
INLET
WRENCH
BOSS
ON/OFF
SWITCH
MAIN
REGULATOR
ADJUST
OUTLET
Be sure to relight any gas appliances that were
turned off at the start of this input check.
Table 9:
GAS RATE (CUBIC FEET PER HOUR)
SECONDS FOR
ONE REVOLUTION
SIZE OF TEST DIAL
1/2 CUBIC
FOOT
1 CUBIC FOOT
10
180
360
12
150
300
14
129
257
16
113
225
18
100
200
20
90
180
22
82
164
24
75
150
26
69
138
28
64
129
30
60
120
32
56
113
34
53
106
36
50
100
38
47
95
40
45
90
42
43
86
44
41
82
46
39
78
48
37
75
50
36
72
52
35
69
54
34
67
56
32
64
58
31
62
60
30
60
