Rheem RGDG, Installation Instructions Manual

Page: 19 / 48

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 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/FT3)
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.
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 Length of Pipe, Feet
Iron Pipe
Size, Inches
10 20 30 40 50 60 70 80 90 100 125 150
1/2 275 189 152 129 114 103 96 89 83 78 69 63
3/4 567 393 315 267 237 217 196 182 173 162 146 132
1 1,071 732 590 504 448 409 378 346 322 307 275 252
1-1/4 2,205 1,496 1,212 1,039 913 834 771 724 677 630 567 511
1-1/2 3,307 2,299 1,858 1,559 1,417 1,275 1,181 1,086 1,023 976 866 787
2 6,221 4,331 3,465 2,992 2,646 2,394 2,205 2,047 1,921 1,811 1,606 1,496
Example (LP): Input BTU requirement of unit, 150,000
Equivalent length of pipe, 60 ft. = 3/4" IPS required.
19
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.
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.
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.