Sediment Trap Requirements per IFGC 408.4

Why this matters

A sediment trap (informally "dirt leg" or "drip leg") is the short tee-and-nipple assembly that catches rust scale, mill slag, pipe-dope crumbs, and any other particulate before it reaches the appliance's gas valve. Modern modulating gas valves on tankless water heaters, condensing furnaces, and condensing boilers have small orifices and fine inlet screens; a single chunk of mill scale jams the valve and the appliance locks out on a vague code. The trap costs three feet of black iron and ten minutes to install, and it is required by IFGC at almost every appliance. Skipping it is one of the most common rough-in failures and a routine cause of warranty calls on otherwise-correct installs.

What the code actually says

IFGC 2021 Section 408.4 (and NFPA 54 Section 8.3.7):

Where a sediment trap is not incorporated as part of the gas utilization equipment, a sediment trap shall be installed downstream of the appliance shutoff valve as close to the inlet of the equipment as practical. The sediment trap shall be either a tee fitting with a capped nipple in the bottom opening of the run of the tee, or other device approved as effective.

Exceptions (per IFGC 408.4, with minor regional variation):

• Illuminating appliances (gas lights)
• Ranges
• Clothes dryers
• Decorative vented appliances for installation in vented fireplaces
• Gas fireplaces
• Outdoor grills
• Outdoor outlets (post lights, BBQ stub-outs)
• Other listed equipment with internal sediment-trap means

Everything else (water heaters, furnaces, boilers, unit heaters, generators, pool heaters, garage heaters) requires a sediment trap.

Construction

Standard tee-style sediment trap:

1. Install the appliance shutoff valve (usually a yellow-handled 1/4 turn ball valve listed for gas) upstream
2. Downstream of the valve, fit a black iron tee with the branch oriented downward (vertical drop)
3. The run of the tee continues to the appliance gas connection
4. The downward-facing branch gets a 3 in minimum nipple and a capped end
5. Total drip leg depth: 3 in minimum below the run of the tee. Note that the IFGC 408.4 text itself calls for a capped nipple in the bottom opening without stating a length; the 3 in figure is the common AHJ and manufacturer requirement. Check the local amendment and the appliance IO manual, and build to whichever is deeper.

Materials: black iron only (no galvanized, no copper). Approved pipe-thread sealant: gas-rated pipe dope (Rectorseal No. 5, Hercules Megaloc, Oatey Great Blue) or PTFE tape listed for fuel gas (yellow-flagged PTFE such as Blue Monster Compression Seal Tape). Standard PTFE tape (white) is not approved for gas; the FDA-grade PTFE is too thin and shreds at threads.

Common installation errors

• Branch facing the appliance, not down. Common shortcut when space is tight. Defeats the trap entirely; particulate carries straight through.
• Drip leg less than 3 in. Inspector failure. Inadequate capture volume.
• Galvanized nipple as the drip leg. Zinc flakes from galvanized in dry gas service shed into the line. Black iron only.
• Coupling cap instead of a cap and nipple. No volume to capture anything; the cap sits at the tee mouth.
• Located upstream of the appliance shutoff. Required to be downstream. Inspector failure.
• Located inside the appliance cabinet on a roof-top unit. Some RTU installs assume the OEM regulator handles it; verify. Almost all OEM literature still requires an external trap regardless.

When the appliance has an internal trap

Some modern appliances (Navien NPE tankless, Lochinvar Knight wall-hung boilers, Mitsubishi Diamond Cast pool heaters) include an internal sediment trap or fine inlet screen. The IFGC exception language says "Where a sediment trap is not incorporated as part of the gas utilization equipment." The relief from the external trap requirement only applies if the manufacturer documents that an integral trap is provided.

Verify in the IO manual. Generic statements like "the inlet contains a screen" do not satisfy the exception in most AHJs. Practical advice: install the external trap regardless. The marginal cost is trivial and it removes any inspector argument.

Inspection and pressure test

The sediment trap is tested as part of the gas-line pressure test (see the related procedure). It is included in the visual rough-in inspection before drywall. A trap installed after drywall (concealed behind a finished wall) is unacceptable; the inspector must see the trap orientation.

After the appliance is connected and gas is restored:

• Leak-check every joint added during install with leak-detection solution (Slime Gas Leak Detector, RectorSeal Bubble-Quick) at normal operating pressure, with the appliance connected and gas restored. Never with open flame.
• Do not confuse this with the rough-in pressure test. The 1.5x-working-pressure test figure belongs to the piping test BEFORE the appliance is connected, and the code requires appliances to be isolated or disconnected from the piping for it. An appliance gas valve is built for inches of water column and gets wrecked by test pressure. Once the appliance is on the line, the check is a soap-solution check at operating pressure, full stop.
• Confirm no leak at the cap of the drip leg. The cap is a common leak point because it gets minimal torque (it does not have a downstream pipe pulling on it).
• Annual maintenance: remove the cap, drain any captured liquid or particulate into a rag, replace the cap with fresh pipe dope. Document the find. A trap that captures pebble-sized scale on annual service indicates active corrosion upstream and a longer investigation.

Sizing the trap

Two different things get sized here, and only one of them carries gas.

The RUN of the tee is the flow path, and it is sized like any other segment of the gas line, never bushed down below what the appliance connection and the load require. The downward BRANCH is a dead leg; no gas flows through it. It gets sized to match so it has capture volume and so the cap and nipple are stock parts, not because it passes flow.

Common sizes:

• 1/2 in for water heaters and small appliances
• 3/4 in for typical furnaces and boilers
• 1 in or 1-1/4 in for high-fire-rate tankless water heaters (Navien 240A, Rinnai RUR199 require 3/4 in minimum and many installs spec 1 in)

A run bushed down below the required size chokes flow during peak appliance demand and can pull inlet pressure below the appliance minimum, causing lockouts on cold mornings when several appliances fire together. That is a run-sizing failure, not a branch failure. A short branch, by contrast, does not restrict anything; it just gives the trap nowhere to put what it catches.

References

• IFGC (International Fuel Gas Code) 2021, Section 408.4
• NFPA 54 (National Fuel Gas Code) 2024, Section 8.3.7
• IAPMO Uniform Plumbing Code 2021, Section 1213.6 (regional equivalent)
• ASME B16.3: Malleable Iron Threaded Fittings
• ASTM A53: Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless
• Rectorseal No. 5 Pipe Thread Sealant Technical Data Sheet
• Navien NPE-A Tankless Water Heater Installation Manual