5808011-JIM-A-0320
Johnson Controls Ducted Systems 37
Figure 23: Bottom entry gas piping
Note: Maximum capacity of pipe in cubic feet of gas per hour
based upon a pressure drop of 0.3 inch W.C. and 0.6
specific gravity gas.
Note: There may be a local gas utility requirement specifying a
minimum diameter for gas piping. All units require a 3/4
inch pipe connection at the entrance fitting. Line should
not be sized smaller than the entrance fitting size.
Gas connection
Route the gas supply line within the space and roof curb with the
exit through the unit’s basepan. See Figures 22 and 23 for the
gas piping inlet location. Typical supply piping arrangements are
shown in Figures 22 and 23. All pipe nipples, fittings, and the gas
cock are field supplied or may be purchased in the Ducted
Systems accessory kit #1GP0405.
Apply the following gas piping recommendations.
• You must install a drip leg and a ground joint union in the
gas piping.
• When required by local codes, install a manual shut-off
valve outside of the unit.
• Use wrought iron or steel pipe for all gas lines. Apply pipe
dope sparingly to male threads only.
• Clean all piping of dirt and scale. Hammer on the outside
of the pipe and blow out loose particles. Before initial
start-up, make sure that all gas lines external to the unit
are purged of air.
• The gas supply must be a separate line and installed in
accordance with all safety codes as prescribed under
Limitations.
Table 14: Gas pipe sizing - capacity of pipe
Length of
pipe (ft.)
Nominal iron pipe size
3/4 in. 1 in. 1-1/4 in.
10 278 520 1050
20 190 350 730
30 152 285 590
40 130 245 500
50 115 215 440
60 105 195 400
70 96 180 370
80 90 170 350
90 84 160 320
100 79 150 305
Table 15: Gas heat minimum supply air
Size (tons) Model Heat size
Supply air (CFM)
Heating
Minimum Maximum
J06
(6.5)
ZB
N12 1950 3250
N18 1950 3250
J07
(7.5)
ZB
N12 2250 3750
N18 2250 3750
J08
(8.5)
ZB
N12 2550 4250
N18 2550 4250
J10
(10)
ZB
N18 3000 5000
N24 3000 5000
J12
(12.5)
ZB
N18 3750 6250
N24 3750 6250
Natural gas may contain some propane. Propane is an
excellent solvent and will quickly dissolve white lead and
most standard commercial compounds. A special pipe
dope must be used when assembling wrought iron or
steel pipe. Shellac based compounds such as Gaskolac
or Stalastic, and compounds such as Rectorseal #5,
Clydes’s or John Crane may be used.
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