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. Usingan inclined manometer, determine the pressuredrop
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 conditionswould be inac
-
curate. To assurea dry coil, thecompressors should be de-
energized while the test is being run.
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:Failureto properly adjustthe total systemair quan
-
tity can result in poor system performance.
5. After readings have been obtained, remove the tubes and
sealthe holeswith the5/16" dotplugs removedinStep 1.
NOTE: DE-ENERGIZE THE COMPRESSOR BEFORE TAK
-
ING ANY TEST MEASUREMENTS TO ASSURE A
DRY INDOOR COIL.
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
onerevolution ofthehand onthesmallest dialonthe 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
sizeof thetest dialincrement, findthe cubicfeet ofgascon
-
sumed per hour from Table 14.
If theactual input isnot within5% of thefurnace 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
yourparticular locality(contactyour gascompany forthis
information - it varies widely from city to city.)
16 Unitary Products Group
035-14832-003-A-0204
Seconds
for One
Rev.
Size of Test Dial
1/2 cu. ft. 1 cu. ft.
10
12
14
16
18
180
150
129
113
100
360
300
257
225
200
20
22
24
26
28
90
82
75
69
64
180
164
150
138
129
30
32
34
36
38
60
56
53
50
47
120
113
106
100
95
40
42
44
46
48
45
43
41
39
37
90
86
82
78
75
50
52
54
56
58
60
36
35
34
32
31
30
72
69
67
64
62
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.
TABLE 14 - GAS RATE - CUBIC FEET PER HOUR
=
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.
FIG. 15 - PRESSURE DROP ACROSS A DRY
EVAPORATOR COIL VS SUPPLY AIR CFM,
WITHOUT AIR FILTERS
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
750 1250 1750 2250 2750 3250 3750
036
048
NOMINAL CFM
PRESSURE DROP (IWG)
060
