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 U.L. or CGA rating plate.
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 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 TGas Rate table.
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.
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.
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 19 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, interpolate for 19 seconds in the first column in the table above. Read
across to the column headed “1 Cubic Foot”, where you will see that 190 cubic
feet of gas per hour are consumed by the furnace at that rate. Multiply 190 x 1050
(the Btu rating of the gas obtained from the local gas company). The result is
199,500 Btuh, which is close to the 204,000 Btuh rating of the furnace.
TABLE 13 - GAS RATE - CUBIC FEET PER HOUR
530.18-N10Y
Unitary Products Group
17
