t
J
J
FURNACE ORIFICE
SIZING
To change orifice spuds, shut off the
manual main gas valve and remove
the gas manifold. Replace the orifice
spuds and reassemble in reverse
order. Consult Table 32 for orifice
sizing.
CHECKING FURNACE
INPUT
Checking furnace input is important
to prevent overtiring the furnace.
NEVER SET THE INPUT ABOVE
THAT SHOWN ON THE RATING
PLATE. Use the following table or
formula to determine the input rate.
Prior to checking furnace input, make
certain that all other gas appliances
are shut off with the exception of pilot
burners.
Start the furnace and note the time
needed to burn one cubic foot of gas.
Time the meter with only the furnace
in operation.
Do not make any attempt to derate
the furnace by changing gas orifices
or the gas pressure. Allow the natural
derate of the gas to occur. The
natural derate is approximately 1.8%
per 1000 feet.
No orifice sizing or other derate
procedures are necessary unless the
natural gas BTU content exceeds
1075 BTU/ft
3
at sea level or the LP
gas BTU content exceeds 2500
BTU/ft
3
at sea level. If the heating
values exceed these levels, consult
the manufacturer for specific derate
procedures.
TABLE 32
ORIFICE SIZES
UNITED STATES
RATING PLATE
I
ELEVATION
INPUT
BTU/HR. 0 TO 7,999 FT.
8,000 FT. AND ABOVE
:
I
HEATING VALUE@ 1,000 BTU/FT
3
,
SPECIFIC GRAVITY 0.62
NATURAL GAS
I
MANIFOLD PRESSURE @ 3.5" W.C.
45.000 43 45
50.000 42 43
67,500
43 45
75,000 42 43
100.000 42 43
125,000
42 43
150.000
42 43
HEATING VALUE@ 2,475 BTU/FT3, SPECIFIC GRAVITY 1.52/
L.P. GAS
MANIFOLD PRESSURE@ 10" W.C.
45,000
54 55
50,000 54
55
67,500 54
55
75,000
54
55
100.000
54
55
125,000 54 55
150,000 54
55
CANADA
RATING PLATE
ELEVATION
INPUT
BTU/HR. 0 TO 1,999 FT. 2,000 TO 4,500 FT.
HEATING VALUE@ 1,000 BTU/FT3, SPECIFIC GRAVITY 0.62
NATURAL GAS
MANIFOLD PRESSURE
@ 3.5" W.C.
45,000 45
50,000
43
42 43
67,500 45
75,000
43
42 43
100,000
42 43
125,000 43
150,000
42
42 43
HEATING VALUE@ 2,475 BTU/FT3, SPECIFIC GRAVITY 1.52/
LP.GAS
MANIFOLD PRESSURE@ 10" W.C.
45,000
54 55
50,000
54 55
67,500 54
55
75,000
54 55
100,000
54 55
125.000 54 55
150,000 54
55
TABLE33
METER TIME IN MINUTES AND SECONDS FOR NORMAL INPUT RATING
OF FURNACES EQUIPPED FOR NATURAL OR LP GAS
INPUT
BTU/HR.
METER
SIZE
CU.FT.
900
HEATING VALUE OF GAS BTU PER CU. FT.
1000
1040 1100 2500
MIN. SEC. MIN. SEC. MIN.
SEC. MIN. SEC.
MIN. SEC.
45,000
ONE
TEN
1
12
12
0
1
13
20
20
1
13
23
50
1
14
28
40
3
33
20
20
50,000
ONE
TEN
1
10
5
50
1
12
12
30
1
12
15
30
1
13
18
12
3
30
20
00
67,500
ONE
TEN
0
8
48
0
0
8
53
53
0
9
55
15
o
9
59
50
2
22
13
13
75,000
ONE
TEN
0
7
44
12
0
8
48
0
0
8
50
19
o
8
53
48
2
20
o
0
100,000
ONE
TEN
0
5
33
24
0
6
36
0
0
6
38
15
0
6
40
36
1
15
30
0
125,000
ONE
TEN
0
4
26
19
0
4
29
48
0
5
30
0
0
5
32
17
1
12
12
0
150,000
ONE
TEN
0
3
31
36
0
4
24
0
0
4
25
10
0
4
26
20
1
10
0
0
HEATINGVALUEOF GAS (BTU/FT
3
)
x3600
INPUTBTU/HR
=
TIME IN SECONDS (FOR 1 CU. FT.) OF GAS
97
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