Fuel Gas Piping
Fuel gas questions are almost entirely arithmetic plus a short list of installation rules. You convert an appliance load in Btu per hour into cubic feet per hour using the heating value of the fuel, find the longest developed length from the meter to the farthest outlet, and use that one length to size every section of the system. Around that core sit the material rules, the shutoff valve and sediment trap requirements, connector limits, and the pressure test procedure. Get the two heating values and the longest-length method right and most of this section takes care of itself.
Load, Heating Value, and Converting Btu to cfh
Gas pipe sizing tables are written in cubic feet per hour, but appliances are rated in Btu per hour, so the first step is always a conversion. Natural gas carries about 1000 Btu per cubic foot, so cubic feet per hour equals the Btu per hour load divided by 1000. Propane carries about 2500 Btu per cubic foot, so the same load in propane divides by 2500 and produces a much smaller cfh number, which is why propane systems use smaller pipe for the same appliances. Propane is also rated at about 91,500 Btu per gallon of liquid, which is how tank capacity gets translated into run time. Start by listing every appliance and its input rating: a typical 40 gallon storage water heater is around 40,000 Btu/h, a range 65,000, a clothes dryer 35,000, and a forced air furnace 100,000. Total those inputs for the whole system, then total only the inputs downstream of a given section when you size that section. A 175,000 Btu/h total on natural gas is 175 cfh; the same total on propane is 70 cfh. Never mix the two divisors, and never use an output rating where the table calls for input.
Sizing by the Longest Developed Length
The longest length method is the one most exams test. Measure the developed length of pipe from the point of delivery, normally the meter or the second-stage regulator, to the most remote outlet on the system. That single distance sets the column you read in the sizing table, and you use that same column for every section of the system, no matter how short an individual run is. A branch that is only 8 ft long is still sized in the 90 ft column if 90 ft is the longest run in the building. The other input to the table is the allowable pressure drop, most commonly 0.5 in water column for a residential system operating at about 7 in w.c. of natural gas delivery pressure, with 0.3 in w.c. used where the design is tighter and 2 psi systems using their own tables with a line pressure regulator at each appliance drop. LP systems typically deliver about 11 in w.c. after the second stage regulator. Fittings add resistance, and the code allows the fitting allowance to be handled either by adding equivalent lengths or, in the simple longest-length method, by relying on the table's built-in allowance. Where a branch length method or the hybrid pressure system is used, follow that method's rules exactly rather than mixing methods.
Materials, Bonding, and Installation
Schedule 40 black steel pipe with malleable iron fittings is the traditional and universally accepted material for fuel gas inside a building. Galvanized steel is permitted in some jurisdictions but is disfavored because the zinc coating can flake and plug controls. Corrugated stainless steel tubing, or CSST, is listed and widely used, but it has one requirement that is a favorite exam item: the gas piping system containing CSST must be bonded to the electrical service grounding electrode system with a bonding conductor not smaller than 6 AWG copper, attached to a metallic component within 5 ft of where the gas piping enters the building. This bonding reduces the chance of arcing from a nearby lightning strike puncturing the thin tubing wall. Copper tube is permitted for natural gas only where the gas is not corrosive, which in practice means where the hydrogen sulfide content is below the code threshold, and many jurisdictions simply prohibit it. Polyethylene is for underground exterior use only and must transition to an approved metallic riser before it comes above grade, with a tracer wire or other means of locating the buried line. Piping through a foundation wall needs a sleeve, piping in concrete needs protection from corrosion, and gas piping may not run inside a duct, chimney, elevator shaft, or air plenum.
Shutoffs, Sediment Traps, Connectors, and Regulators
Every appliance needs an accessible manual shutoff valve located in the same room as the appliance and within 6 ft of it, ahead of the union or connector, so the appliance can be isolated for service without shutting down the building. A main shutoff at the meter controls the entire system. A sediment trap, sometimes called a drip leg, is installed downstream of the appliance shutoff valve and as close to the appliance inlet as practical, to catch rust, scale, and moisture before they reach the gas valve. The IFGC does not require sediment traps on illuminating appliances, ranges, clothes dryers, outdoor grills, and decorative appliances, but does require them on furnaces, boilers, and water heaters. Appliance connectors have their own limits: they are generally restricted to 3 ft in length, with 6 ft allowed for ranges and clothes dryers, they may not be concealed, they may not pass through a wall, floor, ceiling, partition, or appliance housing, and they may never be reused from a previous installation. Line pressure regulators reduce a higher distribution pressure at the appliance, must be accessible, and must have their vent piped to the outdoors where required by their listing so a diaphragm failure does not discharge gas indoors.
Pressure Testing and Leak Checking
Before a gas system is put into service it must hold a pressure test. The test pressure is not less than 1.5 times the maximum working pressure and never less than 3 psi, held for at least 10 minutes with no perceptible drop, using a gauge with a range and increment appropriate to the test pressure. Longer durations and higher pressures are common under local amendment, and many jurisdictions require 10 psi for 15 minutes or longer on new construction. The test medium is air, nitrogen, carbon dioxide, or another inert gas, never oxygen, which can ignite explosively with any oil residue in the piping. Appliances, regulators, and any device rated below the test pressure must be isolated by closing their shutoff valve and disconnecting them, or the test will damage them. After the system is charged with fuel gas, leak checking is done with a listed leak detection solution, an approved electronic detector, or by observing the meter dial with all appliances off. A flame is never used to check for leaks. Purging must be done to a safe outdoor location or in accordance with the code procedure, so that a flammable mixture is not released inside the building.
Last updated: July 2026