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:
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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.)
Table 19: Gas Rate Cubic Feet Per Hour1
Seconds for | Size of Test Dial | |
One Rev. | 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 |
1.EXAMPLE: By actual measurement, it takes 38 seconds for the hand on the
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