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284812-YIM-D-1008
FIGURE 26 - TYPICAL FLAME APPEARANCE
Burners are now accessible for service.
Reverse the above procedure to replace the assem-
blies. Make sure that burners are level and seat at the
rear of the heat exchanger.
BURNER AIR SHUTTER ADJUSTMENT
Adjust burner shutters so no yellow flame is observed
in the heat exchanger tubes.
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 small-
est 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 the Gas
Rate - Cubic Feet Per Hour Table 35.
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.
To find the Btu input, multiply the number of
NOTE:
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.)
66
TABLE 35: GAS RATE - CUBIC FEET PER HOUR
Seconds
for One
1/2 cu. ft.
Rev.
4
6
8
10
12
14
16
18
20
22
24
26
28
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 company). The result is 292,425 Btuh, which is close to the 300,000 Btuh
rating of the furnace.
ADJUSTMENT OF TEMPERATURE RISE
The temperature rise (or temperature difference
between the return air and the heated air from the fur-
nace) must lie within the range shown on the rating
plate and the data in the Gas Heat Application Table 4.
Btuh Input x 0.8
=
CFM
108
.
After the temperature rise has been determined, the
cfm can be calculated as follows:
After about 20 minutes of operation, determine the fur-
nace 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 temper-
ature rise; decrease the blower cfm to increase the
rise. Refer to the Blower Motor and Drive Data
Table 30.
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.
Johnson Controls Unitary Products
Size of Test Dial
1 cu. ft.
450
900
300
600
228
450
180
360
150
300
129
257
113
225
100
200
90
180
82
164
75
150
69
138
64
129
o
x F Temp Rise
.
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