NOTE: The tubes must be inserted and held in a position per- pendicular to the air flow so that velocity pressure will not affect the static pressure readings.
3.Using an inclined manometer, determine the pressure drop across a dry evaporator coil. Since the moisture on an evaporator coil may vary greatly, measuring the pressure drop across a wet coil under field conditions would be inac- curate. To assure a dry coil, the compressors should be de- energized while the test is being run.
| 0.7 |
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| 0.6 |
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| 060 |
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(IWG) | 0.5 |
| 036 |
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| 048 |
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DROP | 0.4 |
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PRESSURE | 0.3 |
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| 0.2 |
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| 0.1 |
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| 0 |
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| 750 | 1250 | 1750 | 2250 | 2750 | 3250 | 3750 |
NOMINAL CFM
FIG. 15 - PRESSURE DROP ACROSS A DRY EVAPORATOR COILVS SUPPLY AIR CFM, WITHOUT AIR FILTERS
4. Knowing the pressure drop across a dry coil, the actual CFM through the unit can be determined from the curve in Figure 15.
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 UL rating plate and the data in Table 2.
=
If the CFM is above or below the specified value, the supply air motor speed may have to be changed by moving the motor speed tap to another terminal.
WARNING:Failure to properly adjust the total system air quan- tity can result in poor system performance.
5.After readings have been obtained, remove the tubes and seal the holes with the 5/16" dot plugs removed in Step 1.
NOTE:
CHECKING GAS INPUT
NATURAL GAS
1.Turn off all other gas appliances connected to the gas me- ter.
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 con- sumed per hour from Table 14.
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 - it varies widely from city to city.)
TABLE 14 - GAS RATE - CUBIC FEET PER HOUR
Seconds | Size of Test Dial | ||
for One | 1/2 cu. ft. | 1 cu. ft. | |
Rev. | |||
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| ||
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 | |
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 in the table above. 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 re- sult is 99,750 Btuh, which is close to the 100,000 Btuh rating of the furnace.
SECURE OWNER'S APPROVAL: When the system is functioning properly, secure the owner's approval. Show him the loca- tion of all disconnect switches and the thermostat. Teach him how to start and stop the unit and how to adjust temperature settings within the limitations of the system.
16 | Unitary Products Group |
