8.2 Gas Pipe Sizing and Sizing Table Methodology

Principles of Gas Pipe Sizing

Sizing residential fuel gas piping systems is one of the most critical steps in ensuring the safe and efficient operation of gas-burning appliances. Under IRC Section G2413.1, a gas piping system must be sized so that it can supply the maximum connected load of all appliances operating simultaneously, while maintaining the required pressure at the inlet of each appliance. If the piping is undersized, appliances will experience pressure drops that lead to incomplete combustion, burner instability, flame rollout, or safety shutdowns.

Key Sizing Parameters

To size a gas piping system using the IRC tables, the inspector must identify four key parameters:

1. Gas Type and Heating Value: The heating value of natural gas is typically assumed to be 1,000 Btu per cubic foot (Btu/ft³), while propane is typically assumed to be 2,500 Btu/ft³.
2. Inlet Pressure: Low-pressure systems operate at 0.5 psi (14 inches w.c.) or less. Systems operating at higher pressures (such as 2 psi) use different tables.
3. Pressure Drop: The allowable friction loss across the piping system. For low-pressure systems, a pressure drop of 0.5 inches w.c. is standard, though drops of 1.0 inch w.c. or 3.0 inches w.c. may be permitted depending on the appliance requirements.
4. Piping Material: Separate tables exist for Schedule 40 metallic pipe, copper tubing, and CSST (sized by Equivalent Hydraulic Diameter or EHD).

Flow Rate Calculation (CFH)

Appliance input ratings are listed on their nameplates in Btu/h. To use the sizing tables, these values must be converted to Cubic Feet per Hour (CFH) using the formula:

CFH = Appliance Input Rating (Btu/h) / Gas Heating Value (Btu/ft³)

For example, a 120,000 Btu/h natural gas furnace requires: 120,000 Btu/h / 1,000 Btu/ft³ = 120 CFH

Sizing Methodologies (G2413.4)

The IRC recognizes two primary sizing methods for residential gas piping:

1. Longest Length Method (G2413.4.1):
   • This is the most common and conservative method.
   • Step 1: Measure the actual developed length from the point of delivery (e.g., gas meter or second-stage regulator) to the most remote (farthest) appliance outlet in the system.
   • Step 2: This single longest length becomes the 'length row' for the entire system lookup. Every section of pipe in the system is sized using this same length row, regardless of how close it is to the meter.
   • Step 3: For each section of pipe, sum the load (CFH) of all appliances served by that section. Scan across the chosen length row in the table to find the smallest pipe size with a capacity equal to or greater than the section's load.
2. Branch Length Method (G2413.4.2):
   • This method allows for smaller branch lines.
   • The main run from the point of delivery to the most remote outlet is sized using the longest length.
   • Each branch line is sized using the developed length from the point of delivery to the outlet of that specific branch, along with the load of that branch.

Worked Example: Longest Length Method

Let's walk through a sizing scenario for a natural gas system operating at less than 0.5 psi, with a 0.5-inch w.c. pressure drop, using Schedule 40 black steel pipe.

• Appliances:
  • Furnace: 100,000 Btu/h (100 CFH)
  • Water Heater: 40,000 Btu/h (40 CFH)
  • Range: 60,000 Btu/h (60 CFH)
• Piping Layout:
  • Meter (Point of Delivery) to Tee A (Section 1): 15 feet. (Serves all: 200 CFH)
  • Tee A to Range (Section 2 - Branch): 25 feet. (Serves range only: 60 CFH)
  • Tee A to Tee B (Section 3 - Trunk): 15 feet. (Serves furnace and water heater: 140 CFH)
  • Tee B to Furnace (Section 4 - Branch): 10 feet. (Serves furnace only: 100 CFH)
  • Tee B to Water Heater (Section 5 - Branch): 5 feet. (Serves water heater only: 40 CFH)

Step 1: Determine the Developed Length to the Most Remote Outlet We calculate the developed length of all paths from the meter:

• Path to Range: 15 + 25 = 40 feet
• Path to Furnace: 15 + 15 + 10 = 40 feet
• Path to Water Heater: 15 + 15 + 5 = 35 feet

The longest developed length is 40 feet. Therefore, we use the 40-foot length row for all sizing lookups under the longest length method.

Step 2: Table Lookup using Table G2413.4(1) Excerpt (Natural Gas, 0.60 SG, 0.5 in. w.c. drop) Here is the capacity table excerpt for 40 feet:

• ½-inch pipe: 95 CFH
• ¾-inch pipe: 199 CFH
• 1-inch pipe: 374 CFH
• 1¼-inch pipe: 768 CFH

Step 3: Size Each Section

• Section 1 (Meter to Tee A): Load is 200 CFH. Looking at the 40-foot row, a ¾-inch pipe only has a capacity of 199 CFH. This is just 1 CFH short! We must size up to a 1-inch pipe (capacity 374 CFH).
• Section 2 (Tee A to Range): Load is 60 CFH. A ½-inch pipe (capacity 95 CFH) is sufficient.
• Section 3 (Tee A to Tee B): Load is 140 CFH. A ¾-inch pipe (capacity 199 CFH) is sufficient.
• Section 4 (Tee B to Furnace): Load is 100 CFH. A ½-inch pipe (95 CFH) is just short by 5 CFH. We must use a ¾-inch pipe (capacity 199 CFH).
• Section 5 (Tee B to Water Heater): Load is 40 CFH. A ½-inch pipe (capacity 95 CFH) is sufficient.

Sizing Tables Comparison and Selection

When preparing for the M1 exam, selecting the correct table is half the battle. If a candidate uses the wrong table, they will get the wrong pipe size even if their math is perfect. Here is a summary of commonly referenced tables in IRC Section G2413.4: