Setting Gas Pressure - Rheem Gra Series Installation Instructions Manual

SETTING GAS PRESSURE

The maximum gas supply pressure
to the furnace should be 10.5" w.c.
natural gas, or 13" w.c. LP gas. The
minimum supply gas pressure to the
gas valve should be 5 w.c. natural
gas or 11 w.c. LP gas. A properly
calibrated U-Tube manometer is
required for accurate gas pressure
measurements.
CAUTION
!
ELEVATIONS ABOVE 2000 FT
REQUIRE THAT THE FURNACE
INPUT RATING BE ADJUSTED
AND THAT THE SIZE OF THE
BURNER ORIFICES BE RE-
CALCULATED BASED ON
ELEVATION AND GAS HEATING
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 41
MANIFOLD PRESSURE TAP
TABLE 4
LP GAS PIPE CAPACITY TABLE (CU. FT./HR.)
Maximum capacity of pipe in thousands of BTU per hour of undiluted liquefied petroleum gases (at 11 inches water
column inlet pressure).
(Based on a Pressure Drop of 0.5 Inch Water Column)
Nominal
Iron Pipe
Size, Inches 10 20 30
1/2 275 189 152
3/4 567 393 315
1 1,071 732 590
1-1/4 2,205 1,496 1,212 1,039
1-1/2 3,307 2,299 1,858 1,559
2 6,221 4,331 3,465 2,992
Example (LP): Input BTU requirement of unit, 120,000
Equivalent length of pipe, 60 ft. = 3/4" IPS required.
Supply Gas Pressure Measure-
ment. A line pressure tap is on the
input side of the gas valve. See
Figure 40.
1. With gas shut off to the furnace
at the manual gas valve outside
the unit, remove the input
pressure tap plug.
2. Connect a U-Tube manometer to
the pressure tap.
3. Turn on the gas supply and
operate the furnace and all other
gas-fired units on the same gas
line as the furnace.
4. Note or adjust the line gas
pressure to give:
A. 5 - 10.5 w.c. for natural gas.
B. 11 - 13 w.c. for LP gas.
5. Shut off the gas at the manual
gas valve and remove the
U-Tube manometer.
Length of Pipe, Feet
40 50 60 70 80
129 114 103 96 89
267 237 217 196 182
504 448 409 378 346
913 834 771 724
1,417 1,275 1,181 1,086 1,023
2,646 2,394 2,205 2,047 1,921
90 100 125 150
83 78 69 63
173 162 146 132
322 307 275 252
677 630 567 511
976 866 787
1,811 1,606 1,496
6. Replace the pressure tap plug
before turning on the gas.
If the supply gas line pressure is
above these ranges, install an in-line
gas regulator to the furnace for
natural gas units. With LP gas, have
the LP supplier reduce the line
pressure at the regulator.
If supply gas line pressure is below
these ranges, either remove any
restrictions in the gas supply piping or
enlarge the gas pipe. See Tables 3
and 4. With LP gas, have the LP
supplier adjust the line pressure at
the regulator.
CAUTION
!
ELEVATIONS ABOVE 2000 FT
REQUIRE THAT THE FURNACE
INPUT RATING BE ADJUSTED
AND THAT THE SIZE OF THE
BURNER ORIFICES BE RE-
CALCULATED BASED ON
ELEVATION AND GAS HEATING
VALUE. THE BURNER ORIFICES
MAY (OR MAY NOT) NEED TO BE
CHANGED. SEE THE SECTION
TITLED "HIGH ALTITUDE
INSTALLATIONS" OF THIS BOOK
FOR INSTRUCTIONS.
Manifold Gas Pressure
Measurement. Natural gas
manifold pressure should be 3.5"
w.c. LP gas manifold pressure
should be 10.0" w.c. Only small
variations in gas pressure should be
made by adjusting the pressure
regulator.
1. With the gas to the unit shut off
at the manual gas valve, remove
the pressure tap plug in the gas
manifold. See Figure 41.
2. Connect a U-Tube manometer to
this pressure tap. See Figure 42.
3. Turn on the gas supply and
operate the furnace.
4. Note or adjust the manifold gas
pressure to give:
A. 3.5 w.c. for natural gas.
B. 10.0 w.c. for LP gas.
5. To adjust the pressure regulator,
remove the regulator cap.
6. Turn the adjustment screw
clockwise to increase pressure,
or counterclockwise to decrease
pressure.
7. Securely replace the regulator
cap.
8. Shut off gas at the manual gas
valve and remove the U-Tube
manometer.
9. Replace the manifold pressure
tap plug before turning on
the gas.
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