035-17477-001 Rev. A (801)
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 :
Gas Valves - (Top) White-Rodgers
(Bottom) Honeywell (Approved for Field
Replacement)
INLET
WRENCH
BOSS
ON/OFF SWITCH
MAIN REGULATOR
ADJUST
OUTLET
ON
OFF
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
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