1183746-UIM-D-1015
18
Johnson Controls Unitary Products
EXTERNAL STATIC PRESSURE SETUP
To measure external static pressure:
• Measure the supply air static pressure
• Record this positive number
• Measure the return air static pressure
• Record this negative number
• Treat the negative number as a positive and add the two numbers
together
• This is total system static
CHECKING GAS HEAT INPUT
Natural Gas
1.
Turn off all other gas appliances connected to the gas meter.
2.
With the unit 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 15.
If the actual input is not within 5% of the unit heating rating with allow-
ance being made for the permissible range of the regulator setting,
replace the orifice spuds with spuds of the proper size.
FIGURE 10:
Measuring External Static Pressure
A0435-001
RETURN AIR
DUCT
SUPPLY AIR
DUCT
NOTICE
To find the BTU input, multiply the number of cubic feet of gas con-
sumed per hour by the BTU content of the gas in your particular
locality. (Contact your gas company for this information since it varies
widely from city to city.)
Table 15:
Gas Rate Cubic Feet Per Hour
1
1.
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 of
Table 15. 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 unit.
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
