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TABLE 9 - GAS RATE (CUBIC FEET PER HOUR)
Seconds for
one Revol.
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
NOTE: Front door of burner box must be secured when check-
ing 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 from Table 9.
To find the BTUH input use the following formula:
Cu. Ft. Gas x meter correction factor x Gas BTU Content =
BTU Per Hour Input
The gas meter is affected by both the temperature and also the
barometric pressure. The meter should be correct at 60° F, and
30.0" barometric pressure. At all other conditions it will be
inaccurate, although the correction factor is easily calculated.
Higher temperatures (over 60° F) will speed up the meter and
make it read high.
Temperatures under 60° F will slow it and make it read low. The
barometric pressure above 30.0" will slow the meter and below
30.0" speed up the meter. At some conditions the meter may
be off significantly. An error of ± 5% is not uncommon. The gas
meter correction factor is calculated as follows:
(Barometric Pressure) x 520
(Temperature ° F + 460) x 30
Unitary Products Group
Size of Test Dial
1/2 cu. ft.
1 cu. ft.
180
360
150
300
129
257
113
225
100
200
90
180
82
164
75
150
69
138
64
129
60
120
56
113
53
106
50
100
47
95
45
90
43
86
41
82
39
78
37
75
36
72
35
69
34
67
32
64
31
62
30
60
= Meter Correction Factor
Example 1: 28.9" Barometric Pressure, 80° F
(28.9) x 520
= 15028
(80 + 460) x 30
= 16200
Contact your gas supplier for actual BTU content of the gas.
EXAMPLE - CHECKING GAS INPUT
It is found by measurement that it takes 26 seconds for the hand
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 Table 9. 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.
With the barometer at 28.9" and a 70° F temperature, the
correction factor will be .945. If the local gas heating value is
935 BTU per cubic foot the calculations will be as follows:
138 cu. ft/hr x .945 correction factor x 935 BTU/cu. ft. =
121,933 BTU/Hr.
The calculated firing rate of 121,933 BTU per hour is within the
± 2% tolerance of our nominal 120,000 furnace.
If the actual input is not within 2% of the furnace rating, with
allowance being made for the permissible range of the regula-
tor setting (0.3 inches W.C.), replace the orifice spuds with spud
of the proper size.
CAUTION: Be sure to relight any gas appliances that
were turned off at the start of this input check.
ADJUSTMENT OF MANIFOLD GAS PRES-
SURE
FIGURE 32 - GAS VALVES
TOP - WHITE-RODGERS 36E24
BOTTOM - HONEYWELL VR8205
650.75-N4U
= .928 Correction Factor
21
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