Setting Gas Pressure - Rheem RGPH Installation Instructions Manual

SETTING GAS PRESSURE

The maximum gas supply pressure
to the furnace should be 7 w.c.
natural gas, or 14 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.
Supply Gas Pressure Measurement.
A line pressure tap is on the input side
of the gas valve.
1. With gas shut off to the furnace at
the manual gas valve outside the
unit, remove the input pressure tap
plug.
TABLE 2
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
Iron Pipe
Size, Inches 10 20
1/2 132 92
3/4 278 190
1 520 350
1-1/4 1,050 730
1-1/2 1,600 1,100
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:
Cu. Ft. Per Hr. Required
The gas input of the furnace is marked on the furnace rating plate. The heating value of the gas (BTU/FT3) may be
determined by consulting the local natural gas utility or the LP gas supplier.
2. Connect a U-Tube manometer to
the pressure tap. See Figure 15.
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 - 7 w.c. for natural gas.
B. 11 - 14 w.c. for LP gas.
5. Shut off the gas at the manual gas
valve and remove the
U-Tube manometer.
6. Replace the pressure tap plug
before turning on the gas.
Length of Pipe, Feet
30 40 50 60
73 63 56 50
152 130 115 105
285 245 215 195
590 500 440 400
890 760 670 610
Gas Input of Furnace (BTU/HR)
=
Heating Value of Gas (BTU/FT3)
TABLE 3
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
1/2 275 189
3/4 567 393
1 1,071 732
1-1/4 2,205 1,496
1-1/2 3,307 2,299
2 6,221 4,331
Example (LP): Input BTU requirement of unit, 150,000
Equivalent length of pipe, 60 ft. = 3/4" IPS required.
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 2 and
3. With LP gas, have the LP supplier
adjust the line pressure at the
regulator.
70 80
46 43
96 90
180 170
370 350
560 530
Length of Pipe, Feet
30 40 50 60
152 129 114 103
315 267 237 217
590 504 448 409
1,212 1,039 913 834
1,858 1,559 1,417 1,275 1,181
3,465 2,992 2,646 2,394 2,205
70 80 90 100 125
96 89 83 78 69
196 182 173 162 146
378 346 322 307 275
771 724 677 630 567
1,086 1,023 976 866
2,047 1,921 1,811 1,606 1,496
150
63
132
252
511
787
19