Reznor SCE Unit Installation Manual - Manual (Page 19)

Form RZ-NA I-SCE, Mfg #207697 Rev 7, Page 19
Sizing Gas Supply Lines
Measuring manifold gas pressure cannot be done until the heater is in operation. It is
included in the steps of the "Check-Test-Start" procedure in Paragraph 25. The following
warnings and instructions apply.
For Natural Gas: When the heater leaves the factory, the combination valve is set so that the
outlet gas pressure of a single-stage valve or high fire of a two-stage valve is regulated to
3.5" w.c. Low fire on a two-stage valve is set to 1.8" w.c. Inlet supply pressure to the valve
must be a minimum of 5" w.c. or as noted on the rating plate and a maximum of 14" w.c. NOTE:
Always check the rating plate for minimum gas supply pressure. Minimum supply pressure
requirements vary based on size of burner and the gas control option. Most units require a
minimum of 5" w.c. of natural gas as stated above, but Sizes 350 and 400 with electronic
modulation require a minimum of 6" w.c. natural gas supply pressure. Sizes 300 and 350 with
mechanical modulation require 7" w.c.
For Propane Gas: When the heater leaves the factory, the combination valve is set so that
the outlet gas pressure of a single-stage valve or high fire of a two-stage valve is 10" w.c.
Low fire on a two-stage valve is set to 5" w.c. Inlet pressure to the valve must be a minimum
of 11" w.c. and a maximum of 14" w.c.
Before attempting to measure or adjust manifold gas pressure, the inlet (supply) pressure
must be within the specified range for the gas being used both when the heater is in opera-
tion and on standby. Incorrect inlet pressure could cause excessive manifold gas pressure
immediately or at some future time.
Instructions to Check Valve Outlet (Manifold) Pressure:
1) With the manual valve (on the combination valve) positioned to prevent flow to the main
burners, connect a manometer to the 1/8" pipe outlet pressure tap in the valve. NOTE: A
manometer (fluid-filled gauge) is recommended rather than a spring type gauge due to the
difficulty of maintaining calibration of a spring type gauge.
2) Open the valve and operate the heater. Measure the gas pressure to the manifold. To
measure the low stage pressure on units equipped with a two-stage valve, disconnect the
wire from the "HI" terminal on the valve. (Be sure to reconnect the wire.)
Normally adjustments should not be necessary to the factory preset regulator. If adjustment
is necessary, set pressure to correct settings by turning the regulator screw IN (clockwise) to
increase pressure. Turn regulator screw OUT (counterclockwise) to decrease pressure. Con-
sult the valve manufacturer's literature provided with the furnace for more detailed informa-
tion.
Manifold or Orifice
(Valve Outlet) Pressure
Settings
Capacity of Piping
Cubic Feet per Hour based on 0.3" w.c. Pressure Drop
Specific Gravity for Natural Gas -- 0.6 (Natural Gas -- 1000 BTU/Cubic Ft)
Specific Gravity for Propane Gas -- 1.6 (Propane Gas -- 2550 BTU/Cubic Ft)
Length Diameter of Pipe
of 1/2" 3/4" 1" 1-1/4" 1-1/2" 2"
Pipe Natural Propan Natural Propan Natural Propan Natural Propan Natural Propan Natural Propan
20' 92 56 190 116 350 214 730 445 1100 671 2100 1281
30' 73 45 152 93 285 174 590 360 890 543 1650 1007
40' 63 38 130 79 245 149 500 305 760 464 1450 885
50' 56 34 115 70 215 131 440 268 670 409 1270 775
60' 50 31 105 64 195 119 400 244 610 372 1105 674
70' 46 28 96 59 180 110 370 226 560 342 1050 641
80' 43 26 90 55 170 104 350 214 530 323 990 604
90' 40 24 84 51 160 98 320 195 490 299 930 567
100' 38 23 79 48 150 92 305 186 460 281 870 531
125' 34 21 72 44 130 79 275 168 410 250 780 476
150' 31 19 64 39 120 73 250 153 380 232 710 433
175' 28 17 59 36 110 67 225 137 350 214 650 397
200' 26 16 55 34 100 61 210 128 320 195 610 372
Note: When sizing supply lines, consider possibilities of future expansion and increased requirements.
Refer to National Fuel Gas Code for additional information on line sizing.