evaporator coil may vary greatly, measuring the pressure
drop across a wet coil under field conditions would be
inaccurate. To assure a dry coil, the scroll compressors
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 or CGA 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: DE-ENERGIZE THE SCROLL COMPRESSORS BE-
FORE TAKING 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 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 18.
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.)
SECURE OWNER’S APPROVAL
:
When the system is functioning properly, secure the owner’s approval. Show him the
location 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.
CFM
=
Btuh Input x 0.8
1.08 x
o
F Temp. Rise
FIG. 15 PRESSURE DROP vs SUPPLY AIR CFM
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 result is 99,750 Btuh,
which is close to the 100,000 Btuh rating of the furnace.
TABLE 18 - GAS RATE - CUBIC FEET PER HOUR
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
750
1250
1750
2250
2750
3250
3750
036
048
072
NOMINAL CFM
P
R
ES
SURE
DROP
(
IWG)
060
530.18-N8Y
18
Unitary Products Group
