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279550-YIM-A-0207
Unitary Products Group 27
Adjustment of Temperature Rise
After about 20 minutes of high heat operation, determine the
furnace temperature rise. Take readings of both the return air
and the heated air in the ducts about six feet from the furnace
where they will not be affected by radiant heat.
The temperature rise (or temperature difference between the
return air and the heated air from the furnace) must lie within
the range shown on the rating plate and the data in Tables 9
and 11.
After the temperature rise has been determined, the CFM can
be calculated as follows:
Direct Drive Blower
All units have direct drive, constant CFM blower motors.
Checking Gas Heat Input
Natural Gas
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 gas meter usually has a 1/2 or a 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 19.
If the actual input is not within 5% of the furnace rating with
allowance being made for the permissible range of the regulator
setting, replace the orifice spuds with spuds of the proper size.
NOTE: To find the BTU input, multiply the number of cubic feet
of gas consumed per hour by the BTU content of the gas
in your particular locality. (Contact your gas company for
this information since it varies widely from city to city.)
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Table 19: Gas Rate Cubic Feet Per Hour
1
1. EXAMPLE: By actual measurement, it takes 38 seconds for the
hand on the 1-cubic foot dial to make a revolution with just a
100,000 BTUH furnace running. Using this information, locate
38 seconds in the first column of Table 19. Read across to the
column headed “1 Cubic Foot,” where you will see that 95 cubic
feet of gas per hour are consumed by the furnace at that rate.
Multiply 95 x 1050 (the BTU rating of the gas obtained from the
local gas company). The result is 99,750 BTUH, which is close
to the 100,000 BTUH rating of the furnace.
Seconds for
One Rev.
Size of Test Dial
1/2 cu. ft. 1 cu. ft.
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