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
inaccurate. To assure a dry coil, the compressors should
be deactivated while the test is being run.
4. Knowing the pressure drop across a dry coil, the actual
CFM through the unit and clean 2" filters, can be deter-
mined from the curve in Figure 17.
WARNING: Failure to properly adjust the total system air quan-
tity can result in extensive blower damage.
After readings have been obtained, remove the tubes and
reinstall the two 5/16" dot plugs that were removed in Step 1.
NOTE:
DE-ENERGIZE THE COMPRESSORS BEFORE TAKING ANY TEST
MEASUREMENTS TO ASSURE A DRY INDOOR COIL.
ADJUSTMENT OF TEMPERATURE RISE
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 ETL rating plate and the data in Table 2.
After the temperature rise has been determined, the cfm can
be calculated as follows:
After about 20 minutes of 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. Increase the blower
cfm to decrease the temperature rise; decrease the blower cfm
to increase the rise. Refer to Table 10 for blower motor and
drive data.
BELT DRIVE BLOWER
All units have belt drive single-speed blower motors. The
variable pitch pulley on the blower motor can be adjusted to
obtain the desired supply air CFM. Tighten belts enough to
prevent slipping. but do not over tighten. Belt deflection should
be between 1/4" and 1/2" per foot. Refer to Table 8 for blower
motor and drive data.
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 13.
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.)
CFM
=
Btuh Input x 0.8
1.08 x
o
F Temp. Rise
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.
FIG. 17 - PRESSURE DROP ACROSS A DRY INDOOR
COIL VS SUPPLY AIR CFM
Seconds
for One
Rev.
Size of Test Dial
1/2 cu. ft.
1 cu. ft.
4
6
8
10
450
300
228
180
900
600
450
360
12
14
16
18
20
150
129
113
100
90
300
257
225
200
180
22
24
26
28
82
75
69
64
164
150
138
129
Example: By actual measurement, it takes 13 seconds for the hand on the 1-cubic
foot dial to make a revolution with just a 300,000 Btuh furnace running. Read
across to the column in the table above, headed “1 Cubic Foot”, where you will
see that 278 cubic feet of gas per hour are consumed by the furnace at that rate.
Multiply 278 x 1050 (the Btu rating of the gas obtained from the local gas com-
pany). The result is 292,425 Btuh, which is close to the 300,000 Btuh rating of the
furnace.
TABLE 13 - GAS RATE - CUBIC FEET PER HOUR
530.18-N11Y
Unitary Products Group
19
