25
GAS SUPPLY (cont.)
IMPORTANT: ENSURE that the furnace gas valve is not to be
subjected to high gas line supply pressures.
DISCONNECT the furnace and its individual manual gas stop
from the gas supply piping during any pressure testing that ex-
ceeds 1/2 PSIG. (3.48 kPa).
N
atural gas supply pressure must be 5" to 10.5" w.c. LP gas
s
upply pressure must be 11" to 13" w.c. This pressure must
be maintained with all other gas-fired appliances in opera-
tion.
The minimum gas supply pressure to the gas valve for proper fur-
nace input adjustments is 5" w.c. for natural gas, however 6" to 7"
is recommended. The minimum gas supply pressure is 11" w.c.
for LP gas.
GAS VALVE
This furnace has a 24-volt gas valve. It has ports for measuring
supply and manifold gas pressure. The valve body contains a
pressure regulator to maintain proper manifold gas pressure.
A control switch is on the valve body. It can be set to only the
“ON” or “OFF” positions. The gas valve is a slow-opening valve.
See Figure 20.
When energized, it takes 2 to 3 seconds to fully open.
GAS PRESSURE
!
CAUTION
ELEVATIONS ABOVE 2000 FT. REQUIRE THAT THE
FURNACE INPUT RATING BE ADJUSTED AND THAT
THE SIZE OF THE BURNER ORIFICES BE RECAL-
CULATED BASED ON ELEVATION AND GAS HEAT-
ING VALUE. THE BURNER ORIFICES MAY (OR MAY
NOT) NEED TO BE CHANGED. SEE THE SECTION
TITLED “HIGH ALTITUDE INSTALLATIONS” OF THIS
BOOK FOR INSTRUCTIONS.
FIGURE 20
T
YPICAL GAS VALVE (HONEYWELL)
REGULATOR CAP
!
WARNING
NE
V
E
R
P
URGE
A
GAS
LI
NE
I
NTO
THE
COMBUS
-
TI
ON
CHAMBE
R.
NE
V
E
R
US
E
MATCHE
S
,
FLAME
OR
ANY
I
GNI
TI
ON
S
OURCE
FOR
CHE
CKI
NG
LE
AK
-
A
GE
.
FAI
LURE
T
O
ADHE
RE
T
O
THI
S
W
ARNI
NG
CAN
CA
US
E
A
FI
RE
OR
E
X
P
LOS
I
ON
RE
S
ULTI
NG
I
N
P
ROP
E
RTY
D
AMA
GE
,
P
E
RS
ONAL
I
NJ
UR
Y
OR
DE
ATH.
TO
CHE
CK
FOR
GAS
LE
AKA
GE
,
US
E
AN
AP
-
P
ROV
E
D
CHLORI
DE
-FRE
E
S
OAP
AND W
ATE
R
S
O
-
LUTI
ON,
OR
OTHE
R
AP
P
ROV
E
D
ME
THOD.
TABLE 6
NATURAL GAS PIPE CAPACITY TABLE (CU. FT./HR.)
Capacity of gas pipe of different diameters and lengths in cu. ft. per hr. with pressure drop of 0.3 in. and specific
gravity of 0.60 (natural gas).
Nominal Length of Pipe, Feet
Iron Pipe
Size, Inches 10 20 30 40 50 60 70 80
1/2 132 92 73 63 56 50 46 43
3/4 278 190 152 130 115 105 96 90
1 520 350 285 245 215 195 180 170
1-1/4 1,050 730 590 500 440 400 370 350
1-1/2 1,600 1,100 890 760 670 610 560 530
After the length of pipe has been determined, select the pipe size which will provide the minimum cubic feet per hour
required for the gas input rating of the furnace. By formula:
Gas Input of Furnace (BTU/HR)
Cu. Ft. Per Hr. Required =
Heating Value of Gas (BTU/FT
3
)
The gas input of the furnace is marked on the furnace rating plate. The heating value of the gas (BTU/FT
3
) may be de-
termined by consulting the local natural gas utility or the LP gas supplier.
Gas Supply
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