9.2 International Fuel Gas Code (IFGC): Pipe Sizing, Materials, Purging, and Testing

International Fuel Gas Code (IFGC) Scope & Approved Piping Materials

The International Fuel Gas Code (IFGC) governs the design, installation, testing, and purging of fuel gas piping systems, gas appliances, and gas vents in Texas. HVAC contractors must strictly adhere to IFGC Chapter 4 (Gas Piping Installations) when connecting gas furnaces, unit heaters, boilers, and rooftop package units to natural gas or liquefied petroleum gas (LP-gas / propane) supply systems.

1. Metallic Gas Pipe Specifications

• Schedule 40 Black Steel: Smooth metallic black steel pipe conforming to ASTM A53 or ASTM A106 is the primary approved material for indoor above-ground fuel gas distribution. Fittings must be malleable iron or steel rated for minimum 150 PSI working pressure.
• Galvanized Steel Pipe: Permitted for interior gas piping; however, threaded joints must be treated with approved gas pipe joint compound (pipe dope) or yellow PTFE tape rated for fuel gas.
• Prohibited Joints: Malleable iron unions with brass-to-iron seats must be used; cast iron fittings are strictly prohibited.

2. Corrugated Stainless Steel Tubing (CSST) & Mandatory Bonding

CSST (conforming to ASTM LC1 / CSA 6.26) provides flexible gas piping routes through wood framing. However, because thin-walled CSST is vulnerable to electrical arc perforation caused by transient lightning strikes, IFGC Section 310.1 and NEC 250.104 enforce strict bonding rules:

                      [ CSST BONDING SYSTEM ]

  Gas Meter / Service Entrance
       |
  Metallic Gas Pipe / Fitting
       |==== [ Brass / Bronze Bonding Clamp ]
       |               |
       |               +===> Bare / Insulated 6 AWG Copper Jumper
       v                                   |
  CSST Tubing                              v
  (ASTM LC1 Arc-Jacket)        Building Grounding Electrode
                               System (Ground Rod / Ufer)

• Mandatory Bonding Jumper: CSST gas piping systems MUST be bonded to the building electrical grounding electrode system using a minimum 6 AWG copper bonding jumper (or larger if specified by the CSST manufacturer).
• Bonding Connection: The bonding jumper must attach to a metallic pipe fitting or rigid steel pipe segment located adjacent to the gas meter or regulator, using a UL-listed brass or bronze grounding clamp. The jumper must connect directly to the electrical service panel ground bus or grounding electrode conductor.
• Arc-Resistant Jackets: Arc-resistant CSST systems featuring conductive black jackets must still be bonded according to manufacturer installation instructions.

3. Copper and Brass Pipe Restrictions (IFGC 403.4.3 pipe / 403.5.2 tubing)

Copper tubing for fuel gas distribution is subject to severe chemical restrictions:

• Hydrogen Sulfide Limit: Copper and brass pipe or tubing MUST NOT be used for natural gas if the gas supply contains more than 0.3 grains of hydrogen sulfide ($H_2S$) per 100 standard cubic feet of gas (0.7 mg/100 L).
• Chemical Corrosion Risk: Sulfur compounds in natural gas react with copper to form copper sulfide flakes. These flakes peel off internal pipe walls and clog gas valve orifices, pilot assemblies, and burner jets, causing severe fire hazards. Because natural gas in many Texas regions exceeds 0.3 grains $H_2S$, copper is widely prohibited for natural gas by local Texas utilities.
• LP-Gas Copper Exception: Seamless copper tubing (Type K or L conforming to ASTM B88 or ACR conforming to ASTM B280) IS PERMITTED for LP-gas (propane) distribution, provided joints are assembled using brazed fittings with filler metal melting above 1,000°F ($538^\circ\text{C}$). Soft solder (50/50 or 95/5) is strictly prohibited.

4. Polyethylene (PE) Underground Plastic Pipe

• Underground Application Only: Polyethylene plastic pipe (conforming to ASTM D2513) is approved ONLY for underground exterior gas service lines.
• Prohibited Locations: PE pipe is strictly prohibited inside buildings, above ground, or under structural slabs unless sleeved in an approved vented conduit.
• Tracer Wire Requirement: Underground PE piping must be installed with a continuous yellow insulated copper tracer wire (minimum 18 AWG) buried alongside the pipe to facilitate electronic pipe location.

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Gas Pipe Sizing Methodology: The Longest Length Method (IFGC 402)

Gas piping must be sized to deliver required volumetric flow rates to all connected appliances while maintaining system pressure within safe operating limits. For low-pressure natural gas systems (7 inches water column nominal supply pressure, ~0.25 PSI), the maximum allowable pressure drop from the gas meter outlet to the inlet of any appliance is 0.5 inches water column (0.5 in. w.g.).

Step-by-Step Longest Length Sizing Procedure

Under IFGC Section 402.4, contractors utilize the Longest Length Method to size piping networks:

1. Determine Peak Input Ratings: Obtain the maximum nameplate BTU/hr input rating for every gas appliance connected to the system. Convert BTU/hr to Cubic Feet per Hour (CFH) by dividing by the local gas heating value: $\text{CFH} = \frac{\text{Total BTU/hr Input}}{\text{1,000 BTU/CF (for Natural Gas)}}$ (For LP-gas, divide by 2,500 BTU/CF).
2. Measure Longest Length: Measure the total physical pipe length from the outlet of the gas meter (or 2nd-stage LP regulator) to the furthest appliance outlet in the system.
3. Select Sizing Table: Select the appropriate IFGC pipe sizing table matching the gas type (natural gas or LP), inlet pressure (e.g., 0.5 PSI), and allowable pressure drop (0.5 in. w.g.).
4. Apply Longest Length Column to ALL Sections: Locate the column in the IFGC table corresponding to the measured longest length. Use this single length column to size EVERY pipe section in the system, regardless of individual branch length.
5. Size Individual Sections: For each pipe section (mains, risers, and branches), calculate the cumulative CFH load passing through that specific segment, then select the minimum nominal pipe size from the longest length column that satisfies or exceeds that CFH demand.

                    [ LONGEST LENGTH SYSTEM EXAMPLE ]

  Gas Meter
     |
     +==== Main Section A (Serves Furnace 1 + Furnace 2 + Water Heater)
     |     Length to furthest outlet (Furnace 2) = 80 feet
     |
     +---> Branch 1: Water Heater (40,000 BTU/hr = 40 CFH)
     |
     +---> Main Section B (Serves Furnace 1 + Furnace 2)
     |         |
     |         +---> Branch 2: Furnace 1 (80,000 BTU/hr = 80 CFH)
     |
     +---> Branch 3: Furnace 2 [FURTHEST APPLIANCE - 80 FT]
                     (100,000 BTU/hr = 100 CFH)

  Rule: Sizing Column for 80 FT MUST be used for Main A, Main B, Branch 1, Branch 2, & Branch 3!

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Pressure Testing Requirements & Leak Detection (IFGC 406)

Before any new, extended, or altered fuel gas piping system is covered, concealed, or placed into operation, it must undergo a rigorous pressure test in accordance with IFGC Section 406 to verify structural integrity and gastight performance.

Test Pressure (IFGC 406.4.1)

• The Baseline Rule: The test pressure must be not less than 1.5 times the proposed maximum working pressure, but not less than 3 PSIG, irrespective of design pressure. There is no separate "low-pressure" and "medium-pressure" test value in the model code — 3 PSIG is the floor for ordinary fuel gas piping regardless of whether the system runs at 7 in. w.c. or 2 PSI.
• The High-Pressure Exception: Gas piping with distribution pressures above 5 PSIG must be tested to twice the maximum allowable operating pressure, but not less than 100 PSIG, for a minimum of 1 hour.
• Upper Limit: Where the test pressure exceeds 125 PSIG, it must not produce a hoop stress greater than 50 percent of the pipe's specified minimum yield strength.
• Test Medium: Test media must be air, nitrogen, carbon dioxide, or an inert gas. NEVER use oxygen or a combustible gas for pressure testing.

Open-Book Exam Trap — 3 PSIG vs. 10 PSIG. Many Texas jurisdictions amend this section. The North Central Texas Council of Governments (NCTCOG) model amendments used across the Dallas–Fort Worth region, for example, require 10 PSIG for welded piping and for piping carrying gas above 14 in. w.c., and Dallas amends the duration language separately. The PSI examination is graded against the unamended edition of the IFGC listed in the Candidate Information Bulletin, so answer exam questions with the code book in front of you — 3 PSIG — and apply the local amendment on the jobsite. Confusing the two is one of the most common ways experienced Texas installers lose code questions.

Test Duration (IFGC 406.4.2)

• Commercial (IFGC): Test duration must be not less than 1/2 hour for each 500 cubic feet of pipe volume, or fraction thereof.
• Residential (IRC): The residential counterpart, IRC Section G2417.4.2, allows a duration of not less than 10 minutes. Do not carry the 10-minute residential figure into an IFGC question.

Pressure Test Gauges and Leak Detection

• Gauge Scale Rule (IFGC 406.4): Mechanical gauges used to measure test pressure must have a range such that the highest end of the scale is not greater than five times the test pressure. A 3 PSIG test therefore caps the gauge at 15 PSI full scale, and a 0–100 PSI gauge is non-compliant because it cannot resolve a small pressure loss. There is no "25% to 75% of full scale" rule anywhere in the IFGC — that is a widespread misremembering, and it is the version most likely to appear as a distractor.
• Common Texas Amendment: Many Texas jurisdictions (the NCTCOG package, Dallas) go further and reject spring-type gauges as uncalibrated, specifying diaphragm gauges with a minimum 3-1/2 inch dial, a set hand, and defined increments. That refinement is a local amendment, not model-code text.
• Leak Isolation: Appliances must be isolated (valves shut off) or disconnected during testing if test pressure exceeds the appliance maximum valve inlet rating (typically 0.5 PSI / 14 in. w.g.).
• Approved Leak Detection Methods: Leak testing must be performed using an approved non-corrosive leak detection fluid (soap solution) or an electronic combustible gas detector. NEVER use open flames (matches, lighters) to search for gas leaks!

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Fuel Gas Piping Purging Safety Protocols (IFGC 406.7)

When placing a newly tested gas line into service, or after servicing existing piping, air inside the pipe must be completely purged with fuel gas to prevent explosive air-fuel mixtures from accumulating inside piping or appliances.

• Outdoor Discharge Mandate: Under IFGC Section 406.7.1 and Table 406.7.1, whether a system must be purged directly to the outdoors depends on both pipe size and length together. The table starts at NPS 2-1/2, and the length that triggers outdoor purging gets shorter as the diameter grows — the largest sizes in the table require outdoor purging at essentially any length, while piping smaller than the table's smallest listed size is never pushed outdoors by this rule. Read the actual table in the code book rather than memorizing a single diameter.
• Discharge Point Safety: Purge discharge lines must terminate outdoors at a safe point away from building air intakes, doors, windows, and active sources of ignition.
• Indoor Purging Limits: Indoor purging is permitted ONLY for small-diameter pipes ($\le$ 2 inches) of short length where discharge occurs into a well-ventilated space with no active open flames, pilot lights, or electrical arc sources present.

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Summary of IFGC Gas Pipe Materials, Testing Pressures, and Restrictions