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MODU-FIRE® FORCED DRAFT
Gas-Fired Boiler Installation
Pipe Capacity for Natural Gas
Notice: See Pipe Capacity for Natural Gas chart for required pipe size, based on overall length of
pipe from meter plus equivalent length of all fittings. Approximate sizing may be based on 1 cu-
bic foot of natural gas per 1,000 Btu per hour input, i.e., 500,000 Btu per hour requires about 500
cubic feet per hour. (See "Typical Boiler Operating Conditions,"
Section 4.3, for more informa-
tion.)
3.8
BOILER WATER PIPING
3.8.1 Piping Design
Water Flow in System
Ideal operation of the MODU-FIRE
®
FORCED-
DRAFT boiler would consist of a 20º F temperature
differential across the heat exchanger at high fire.
Insufficient flow may result in excessive short cy-
cling of the boiler and eventual damage or prema-
ture failure of the equipment.
Notice: The closing of the flow switch does not
prove that flow is adequate. It only indicates that
some flow is present. Proper flow rates are 90-
150 gpm for N/D1500MFD and 120-150 gpm for
N/D2000MFD boilers.
Minimum Return Water Temperature should be
greater than 130º F to avoid problems of condensa-
tion on the outside of the heat exchanger or in the
flue passages.
Proper flow rates and return water temperature may
be achieved through a combination of primary and
secondary flow loops. Multiple zones and pumps
may result in different flow rates at different times.
Consideration must be given to all possible condi-
tions and their consequences.
Piping with Refrigeration Machines
When used with a refrigeration system, the boiler
shall be installed so that chilled medium is piped in
parallel with the boiler. Valves should be installed
to prevent chilled water from entering the boiler
when the system is operated in the cooling mode.
Piping with Air Handling Units
The boiler piping system of a hot water heating
boiler connected to heating coils located in air han-
dling units, where they may be exposed to refriger-
ated air circulation, must be equipped with flow
control valves or other automatic means to prevent
gravity circulation of the boiler water during the
cooling cycle.
Equivalent Pipe Length Maximum Capacity in Cubic Feet of Natural Gas per Hour
Pressure Drop of 0.5 inch Water Column/Equivalent Length of
Pipe (in feet)
Nominal
Iron
Pipe
Size
(Inches)
Internal
Diameter
(Inches)
90º Ell
(Feet)
Tee
(Feet)
20
40
60
80
100
150
200
1-1/4 1.380 3.45 6.9 950
1-1/2 1.610 4.02 8.04 1,460 990 810
2 2.067 5.17 10.3 2,750 1,900 1,520 1,300 1,150 950 800
2-1/2 2.469 6.16 12.3 4,350 3,000 2,400 2,050 1,850 1,500 1,280
3 3.068 7.67 15.3 7,700 5,300 4,300 3,700 3,250 2,650 2,280
4 4.026 10.1 20.2 15,800 10,900 8,800 7,500 6,700 5,500 4,600
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