5.4 Fuel Gas Pipe Sizing: Demand, Tables & Length Methods (IFGC 402)

Fuel Gas Pipe Sizing: Demand, Tables & Length Methods (IFGC 402)

The ICC P3 outline lists "Piping, Combustion Air & Venting (fuel gas sizing)" at 7 percent, the largest share of the Water Heaters domain, and names the 2024 IFGC as a reference. Replacing a 40,000 Btu/h tank water heater with a 199,000 Btu/h tankless unit can make the existing gas piping too small. This section covers the sizing rules a plans examiner checks.

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Step 1: Determine the Demand (IFGC 402.1 and 402.2)

• Supply requirement (402.1): Piping supplies enough gas for the maximum demand, and delivers gas to each appliance inlet at no less than the minimum supply pressure the appliance requires.
• Maximum demand (402.2): The flow is the sum of the maximum inputs of the appliances served. Pipe is sized on the total connected hourly load, assuming all appliances can run at full capacity at once. A diversity factor may be used only where it can be established.
• Altitude (402.2): The flow is adjusted for altitude where the installation is above 2,000 feet.

Converting Btu/h to cubic feet per hour (cfh): The sizing tables are in cfh. Divide the appliance input by the gas heating value, which the supplier provides. Designers commonly assume about 1,000 Btu per cubic foot for natural gas, so a 40,000 Btu/h water heater needs about 40 cfh.

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Step 2: Choose a Sizing Method (IFGC 402.3)

Gas piping is sized by one of these:

1. Tables or equations in Section 402.4 or 402.5.
2. Sizing tables in a listed piping system's installation instructions, such as CSST manufacturer tables.
3. Engineering methods.

Tables 402.4(1) through 402.4(37) cover Schedule 40 steel, copper tubing, CSST, polyethylene and other materials for natural gas and propane at several inlet pressures and pressure drops. The low-pressure equation (Equation 4-1) applies below 1-1/2 psi, and the high-pressure equation (Equation 4-2) applies at 1-1/2 psi and above. Both require pipe with smooth inside walls.

Reading Table 402.4(1): Schedule 40 Metallic Pipe

Natural gas, inlet pressure less than 2 psi, 0.3-inch w.c. pressure drop, specific gravity 0.60. Capacity in cfh.

Table 402.4(2) covers the same pipe at a 0.5-inch w.c. pressure drop, where capacities are higher. For example, 1-inch pipe at 60 feet carries 257 cfh instead of 195 cfh. The designer must still show the appliance gets its minimum inlet pressure (402.6).

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Step 3: Determine the Length (IFGC 402.4.1–402.4.3)

1. Longest length method (402.4.1): Every section is sized using the longest length from the point of delivery to the most remote outlet, together with the load carried by that section.
2. Branch length method (402.4.2): Sections on the longest run use the longest length. Each branch not already sized uses the length from the point of delivery to the most remote outlet on that branch.
3. Hybrid pressure (402.4.3): Higher-pressure piping is sized with the length to the most remote line pressure regulator. Piping downstream of the regulator is sized with the length from the regulator to its most remote outlet.

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Worked Example: Single-Family Gas Piping

   METER ──A (10 ft)── T1 ──B (20 ft)── T2 ──C (30 ft)── FURNACE 100 cfh
                        │                │
                   20 ft│           10 ft│
                        ▼                ▼
                  RANGE 65 cfh    WATER HEATER 40 cfh

   Meter to furnace = 60 ft (longest)   Meter to range = 30 ft
   Meter to water heater = 40 ft

Longest length method (use the 60-foot row for every section):

Branch length method: Sections A, B and C are unchanged. The range branch uses its own 30-foot length: 1/2-inch pipe carries 72 cfh at 30 feet, so it can be 1/2 inch. The water heater branch uses 40 feet, where 1/2-inch pipe carries 62 cfh, so it stays 1/2 inch. The branch length method can reduce branch sizes but never undersizes the main.

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Maximum Operating Pressure Inside Buildings (IFGC 402.7)

Gas piping inside buildings may not exceed 5 psig unless one of these applies:

• Joints are welded or brazed, or flanged with welded or brazed pipe-to-flange connections.
• Fittings are listed to ANSI LC-4/CSA 6.32 (press-connect).
• Piping is in a ventilated chase or otherwise enclosed.
• Piping is in areas used only for industrial processing or heating, research, warehousing, or boiler or mechanical rooms.
• The installation is temporary during construction or serves agricultural appliances.
• The system is LP-gas above 20 psi complying with NFPA 58.

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Plan Reviewer Verification Checklist: Gas Pipe Sizing

• Load Schedule: Every appliance input is listed in Btu/h and converted to cfh, and the total connected load is shown (402.2).
• Altitude: The flow is adjusted for sites above 2,000 feet (402.2).
• Method Stated: The table number, pressure drop and length method are identified (402.3 and 402.4).
• Lengths: The longest run is measured to the most remote outlet, and branch lengths are measured from the point of delivery (402.4.1 and 402.4.2).
• Inlet Pressure: Each appliance receives at least its listed minimum inlet pressure (402.6).
• Pressure Limit: Interior piping above 5 psig meets a 402.7 condition.