Gas Line Basics & Leak Testing

Gas work is high-consequence plumbing. A sloppy threaded joint on a drain line is a callback; a sloppy threaded joint on a gas line is a news story. Most jurisdictions require a licensed plumber or a licensed gasfitter to run gas piping, and many require a separate gas endorsement beyond the plumbing license. This guide walks you through the materials, sizing logic, joint practices, bonding requirements, and pressure-test protocols every apprentice needs to know before touching a gas line.

Approved Materials - What You Can and Cannot Use

Approved for interior residential gas:

• Black iron / Schedule 40 steel pipe - The backbone of traditional gas distribution. Threaded joints with appropriate sealant.
• Corrugated Stainless Steel Tubing (CSST) - Flexible stainless tubing with manufacturer-specific fittings. Two common brands are TracPipe and Gastite. Must be bonded (see below).
• Copper tubing - Type L or K, approved in SOME jurisdictions for natural gas, more commonly approved for LP (propane). Never for sulfur-content fuel gas.
• Polyethylene (PE) pipe - Yellow outdoor underground only. Must transition to approved material at the point it enters the building.

Never use for gas:

• Galvanized steel pipe - The zinc coating flakes off into the gas stream and clogs regulators and orifices. Old installations still exist, but no new work is allowed.
• PVC, CPVC, PEX, or any other plastic inside the building - All are strictly prohibited for fuel gas. PVC shatters under thermal stress; PEX fails at any open-flame exposure.
• Cast iron or lead - No.

Pipe Sizing - Know the BTU Load

Sizing a gas line is a supply-versus-demand calculation. Every appliance has a BTU/hour input listed on its data plate. You total the load, find the longest run from the meter to the farthest appliance, and use the code sizing table for your pipe material and supply pressure.

Simplified residential BTU loads (verify on the actual appliance):

| Appliance                       | Typical Input     |
|---------------------------------|-------------------|
| 40 or 50 gal water heater       | 40,000 BTU/hr     |
| Condensing tankless water heater| 180-200,000 BTU/hr|
| 80,000 BTU furnace              | 80,000 BTU/hr     |
| 100,000 BTU furnace             | 100,000 BTU/hr    |
| Gas dryer                       | 35,000 BTU/hr     |
| Gas range                       | 65,000 BTU/hr     |
| Gas log set or fireplace        | 30-80,000 BTU/hr  |
| Outdoor grill                   | 60-100,000 BTU/hr |
| Pool heater                     | 200-400,000 BTU/hr|

Longest Length Method - IFGC Standard

International Fuel Gas Code provides tables for sizing based on the distance from the meter to the most remote appliance. Every branch run - regardless of its actual length - is then sized off that same longest-length column. This is conservative but eliminates hydraulic mismatch.

Example simplified table - 2-psi supply, schedule 40 black iron, 0.5 psi drop (for reference, always use code tables for real work):

| Nominal Pipe | 10 ft     | 30 ft     | 60 ft     | 100 ft    |
|--------------|-----------|-----------|-----------|-----------|
| 1/2 in       | 175 cfh   | 110 cfh   | 80 cfh    | 60 cfh    |
| 3/4 in       | 360 cfh   | 230 cfh   | 170 cfh   | 130 cfh   |
| 1 in         | 680 cfh   | 440 cfh   | 320 cfh   | 245 cfh   |
| 1-1/4 in     | 1,400 cfh | 910 cfh   | 660 cfh   | 510 cfh   |

Natural gas has roughly 1,050 BTU per cubic foot, so 1 cfh is about 1,000 BTU/hr. Divide your BTU load by 1,000 to get the required cfh.

A 200 kBTU tankless on a 60-foot run needs roughly 200 cfh. Looking at the table above, 3/4 inch supplies 170 cfh at 60 feet - not enough. You need 1 inch on that branch. This is why a half-baked "just use 3/4" answer gets tankless customers a starved appliance.

Spitzglass Formula - The Old School

Before computerized sizing tables, gasfitters used the Spitzglass formula. You will still hear old-timers reference it. The simplified concept: gas flow is proportional to the square root of pressure drop divided by length, multiplied by pipe diameter to the 2.5 power. In practice, trust the code table, but understand Spitzglass is the physics behind it.

Fittings and Joints

Black Iron Threaded Joints

• Cut pipe clean with a pipe cutter or band saw.
• Ream the inside burr with a reamer before threading.
• Cut threads with a threader to the correct length (six to seven threads engaged on a 1/2 or 3/4 pipe).
• Wipe threads clean of oil and metal chips.
• Apply pipe joint compound rated for gas, or yellow PTFE tape labeled for gas (not white plumber's tape, not standard white PTFE). Two to three wraps in the direction of thread rotation.
• Some gasfitters double up: pipe dope AND tape. Acceptable where the compound manufacturer approves it.
• Tighten with two pipe wrenches - one back-up, one driving. Engage 5-7 turns past hand tight.
• Never use flared or compression fittings on black iron.

CSST - Manufacturer-Specific Only

CSST uses proprietary fittings from the pipe manufacturer. You cannot mix Gastite fittings on TracPipe tubing. Every installer must be trained and certified by the manufacturer. The typical workflow:

1. Measure and cut CSST with the manufacturer's cutter.
2. Dress the end - strip the yellow jacket per the spec.
3. Install the fitting - usually a compression-style or flared-style unique to the brand.
4. Torque per manufacturer spec (these are torque-critical, not feel-based).

Copper - Flared, Not Soldered

For approved LP or gas applications, use flared fittings. Never solder copper for gas. The flare creates a metal-to-metal seal that is gas-tight and serviceable. Cut the tube, de-burr, slide on the flare nut, flare the end with a 45-degree flaring tool, assemble with a drop of gas-rated compound on the flare face if manufacturer allows.

Required Accessories at Every Appliance

Every gas appliance connection in most codes must include:

• Shutoff valve within 6 feet of the appliance, in the same room, accessible without tools.
• Sediment trap ("drip leg") downstream of the shutoff, upstream of the appliance regulator. This is typically a 3 inch minimum nipple, capped, hanging off the bottom of a tee so condensate, rust flakes, and pipe dope shavings fall into it instead of the gas valve. Dryers are often exempt from the drip leg in many jurisdictions.
• Union downstream of the shutoff so the appliance can be removed without unthreading live pipe.
• Appliance connector (flexible yellow-coated stainless or CSA-approved brass connector) if the appliance will be moved for service. Never use a connector through a wall or as a substitute for rigid piping.

No Unions in Concealed Spaces

A union is a serviceable joint. Code prohibits unions inside walls, ceilings, or inaccessible chases because if the union leaks, nobody can see or reach it. Pipe through the wall must be continuous with only welded or threaded non-union joints (some codes allow only welded in concealed runs).

CSST Bonding - The Lightning Story

Corrugated stainless steel tubing has a thin wall. A nearby lightning strike can induce enough current through the surrounding building electrical to arc between a metal object and the CSST, perforating the tubing and igniting a fire. Multiple fatal house fires in the early 2000s led to updated requirements.

Every CSST installation must be bonded to the grounding electrode system with a #6 AWG solid copper bond wire (some brands allow #4), attached to the manifold or first fitting at the meter side and run without splice to the grounding electrode (the bond wire should not be used as a service ground - it is a separate bond). Some newer CSST products with a black anti-arc jacket have relaxed bonding requirements, but default to #6 to the electrode system unless the manufacturer documentation explicitly says otherwise.

Failing to bond CSST is a life-safety issue and is an automatic inspection failure.

Pressure Testing - The IFGC Standard

Before a gas line is turned on, it must pass a pressure test witnessed or documented per code. Typical IFGC-based protocol for residential:

• Test pressure: The greater of 3 psi or 1.5 times the working pressure. For most residential natural gas (working pressure 7 inches WC, about 0.25 psi), the test pressure is 3 psi minimum, though many jurisdictions require 10 psi.
• Duration: Not less than 10 minutes, many local amendments require 15 minutes or longer. Some require 30 minutes, 2 hours, or 24 hours for new construction. Always check the local amendment.
• Gauge: A calibrated pressure gauge with an increment not greater than 0.1 psi, or 1/10 of the test pressure - whichever is smaller.
• Capped or valved off: All appliances and regulators must be isolated (valved off or physically disconnected and capped). Regulators cannot see test pressure or they will be destroyed.

The Hold Test

1. Install a test gauge on a tee at the meter or the inlet of the system.
2. Close all appliance shutoffs and isolate regulators.
3. Pressurize to the test pressure with a hand pump or shop air.
4. Close the air inlet, start a timer.
5. Read the gauge. Record at test start and test end.
6. Any loss is a failure. The gauge must not move during the test period.

Finding the Leak

If the test fails, two methods are standard:

• Soap solution - A squirt bottle of soapy water (standard commercial leak detector fluid, or just dish soap and water) painted on each joint. Bubbles indicate the leak. Cheap, effective, slow on long runs.
• Electronic leak detector (sniffer) - Handheld sensor that picks up methane or propane in ppm. Fast for locating a leak on a long run. Only works if the leak is venting to atmosphere.

Never use a flame to check for a gas leak. Yes, this has been tried, and yes, there are photos.

Purging Procedures - Getting the Air Out

A newly installed line is full of air. Before lighting any appliance:

• Purge through an appliance OR through an approved purge point (outdoor). Never purge into a closed room.
• Open the shutoff, crack the union fitting or bleed valve, and let the gas flow until the smell of mercaptan (the sulfur additive) is strong and consistent.
• Close the bleed and check for leaks at the joint you just disturbed.
• Light and verify the appliance cycles normally, checking for soot, flame color (blue with a soft inner cone), and cycle behavior.

Large systems or long runs may require a calculated purge volume - three pipe volumes of gas to fully displace air, released through a vented manifold outside the building. Purging a 100-foot 1-inch main into a small mechanical room can create an explosive atmosphere in minutes.

"I Smell Gas" - Stop What You Are Doing

If you or a customer smells gas:

• Do not flip a light switch, start a car, or light anything. Any spark can ignite an explosive atmosphere.
• Evacuate everyone from the building.
• From outside, shut off the gas at the meter if you can do so safely.
• Call the utility's emergency line (not 911 first in most regions - call the gas company). They come out free and fast.
• Do not return to the building until the utility clears it.

A suspected gas leak is never something a plumber handles alone. Utilities staff 24/7 dispatch for exactly this reason.

Permits, Inspections, and Paper

Almost every jurisdiction requires a permit for any gas work beyond replacing a flexible connector on an existing appliance. Typical inspection milestones:

• Rough / gas pipe inspection - Lines are installed but not yet connected to appliances. Inspector witnesses the pressure test or reviews the logged test.
• Final inspection - All appliances connected, combustion verified, labels in place (manifold labels, CSST bond labels, emergency shutoff labels where required).

Always leave the test gauge on the system until the inspector has seen it or, if not witnessed, photograph the gauge at the start and end of the test with a timestamp.

Day 1 Checklist

• Permit pulled, appliance data plates photographed for BTU values
• Sizing calculation done in writing (longest length, BTU load, cfh required)
• Black iron, CSST, or copper materials staged per design
• Pipe dope rated for gas OR yellow PTFE tape (not white)
• Shutoffs, drip legs, unions, flex connectors staged per appliance
• CSST bond wire (#6 solid copper) and clamps, if CSST in scope
• Test gauge (0-15 psi or 0-30 psi, with 0.1 or 0.5 psi increments)
• Hand pump or air connection for pressurization
• Soap solution bottle and electronic sniffer for leak detection

Expert Tips

• "Drip leg every appliance, cap down." Even where code does not require it, a drip leg saves the gas valve on every future callback.
• "Photograph the gauge at start and end of the test." Timestamped proof closes inspector disputes in seconds.
• "Bond the CSST manifold with #6 to the grounding electrode." Nothing destroys a career like a house fire from a lightning-induced CSST arc.
• "Never mix CSST manufacturers' fittings." A Gastite fitting on TracPipe tubing is a violation and a leak waiting to happen.
• "Purge outside, or through an appliance with a pilot flame." Never purge a new line into a closed mechanical room.
• "If you smell gas, leave and call the utility." Not 911, not the shop first - the utility's emergency line is free and they respond fast.