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127408-YIM-B-0606
Unitary Products Group 47
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 as it varies widely from
area to area).
EXAMPLE:
By actual measurement, it takes 19 seconds for the hand on
a 1 cubic foot dial to make a revolution with a 192,000 Btuh
furnace running. To determine rotations per minute, divide 60
by 19 = 3.16. To calculate rotations per hour, multiply 3.16 •
60 = 189.6. Multiply 189.6 • 1 (0.5 if using a 1/2 cubic foot
dial) = 189.6. Multiply 189.6 • (the Btu rating of the gas). For
this example, assume the gas has a Btu rating of 1050
Btu/ft.
3
. The result of 199,000 Btuh is within 5% of the
192,000 Btuh rating of the furnace.
ADJUSTMENT OF TEMPERATURE RISE
The temperature rise (the difference of temperature between
the return air and the heated air from the furnace) must lie
within the range shown on the CSA rating plate and the data
in Table 47.
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 6 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 (See ’SUPPLY AIR DRIVE
ADJUSTMENT’).
NOTE: Each gas heat exchanger size has a minimum
allowable CFM. Below this CFM, the limit will open.
BURNERS/ORIFICES INSPECTION/SERVICING
Before checking or changing burners, pilot or orifices,
CLOSE MAIN MANUAL SHUT-OFF VALVE AND SHUT OFF
ALL POWER TO THE UNIT.
1. Open the union fitting just upstream of the unit gas valve
and downstream from the main manual shut-off valve in
the gas supply line.
2. Remove the screws holding each end of the manifold to
the manifold supports.
3. Disconnect wiring to the gas valves and spark igniter(s).
Remove the manifold & gas valve assembly. Orifices can
now be inspected and/or replaced.
To service burners, complete step 4.
4. Remove the heat shield on top of the manifold supports.
Burners are now accessible for inspection and/or
replacement.
NOTE: Reverse the above procedure to replace the assem-
blies.
Make sure that burners are level and seat at the rear of the
gas orifice.
TABLE 49: GAS RATE CUBIC FEET PER HOUR
Seconds
for One
Rev.
Size of Test Dial
1/2 cu. ft. 1 cu. ft.
10
180 360
12
150 300
14
129 257
16
113 225
18
100 200
20
90 180
22
82 164
24
75 150
26
69 138
28
64 129
30
60 120
32
56 113
34
53 106
36
50 100
38
47 95
40
45 90
42
43 86
44
41 82
46
39 78
48
37 75
50
36 72
52
35 69
54
34 67
56
32 64
58
31 62
60
30 60
CFM Btu Input
0.8
1.08 Δ°F⋅()
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