Sizing Gas Piping

Pick the Route Before You Pick a Size

Every gas system has one appliance that decides the pipe size for everything upstream of it, and locating that appliance is the first job on site. Measure developed length from the outlet of the meter, or from the second-stage regulator on a propane job, to the connector at each appliance, following the pipe the way it will actually be installed: up the wall, along the joist bays, back down into the utility room. A tape stretched across the floor plan lies. The route that totals the most developed feet is the one the entire system gets sized against under the longest-length method, which the National Fuel Gas Code (NFPA 54 / ANSI Z223.1) and the International Fuel Gas Code both recognise.

Farthest on paper and longest in pipe are different things. A furnace twelve feet from the meter but fed by a riser, two floor penetrations and a doubled-back leg through a crawl space will out-measure a range on the far side of the house served by one straight horizontal shot. Chalk the route on the deck, or mark it leg by leg on the plan, and total it. On a retrofit you inherit length you did not choose, so walk the ceiling with a torch and count what is already there before assuming the new work sets the number.

Two accepted methods split here and they are not interchangeable mid-job. Longest-length sizing takes the total developed length of that one worst route and sizes every section of the system from that single number, which is fast, conservative and forgiving of later changes. Branch-length sizing lets each branch be sized on its own developed length back to the source, which saves material on sprawling systems but leaves no margin when somebody adds a griddle to a short branch two years later. Pick one, write it on the drawing, and size the whole system with it.

Standing at the Source: the Pressure You Have to Spend

Sizing is not really about pipe; it is about spending a pressure allowance that cannot be topped up once the meter is set. A residential low-pressure natural gas system is typically delivered at around seven inches water column, and each appliance rating plate states the minimum inlet pressure it needs to fire at full input. The difference between those two numbers, less what the appliance regulator keeps for itself, is the whole budget for friction along the longest run, commonly allowed as half an inch water column and sometimes restricted to three tenths. Which figure applies is set by the utility delivery pressure and by the authority having jurisdiction, not by preference.

Capacity tables are printed per allowable drop, and reading the wrong one is the quietest way to undersize a system. A table built on half an inch hands you a smaller diameter than a table built on three tenths for the same length and load, and both look equally plausible on the page. Confirm delivered pressure at the meter outlet with a manometer before sizing rather than after, particularly on rural natural gas or on any system where a line regulator, an elevated two-pound service or a long service lateral sits between the main and the building.

Zero on the Route: the Vessel That Feeds It

On propane the route starts at a vessel, and that vessel does two jobs the pipe cannot help with: hold enough liquid to cover the coldest fill interval, and vaporise fast enough to meet peak simultaneous demand. Vaporisation rate depends on wetted surface area and outside temperature, so a tank that is adequate in October starves in January at twenty per cent full. The regulator sags, outlet pressure falls, and the appliance at the end of the longest run is the first one to drop out.

Two-stage regulation is standard on anything beyond a short cylinder installation. First stage drops container pressure to an intermediate pressure carried along the yard line; second stage, close to the building, drops it to the roughly eleven inches water column that propane appliances are set for. That intermediate leg is sized on pressure in pounds rather than inches of water and uses a different capacity table entirely, so the yard line and the house line are two separate sizing exercises that happen to share a trench. NFPA 58, Liquefied Petroleum Gas Code, governs the container, its separation distances and its relief arrangement.

The First Leg Carries Everything

From the source to the first tee, the pipe carries total connected input: every appliance rated input in BTU per hour added up, with no diversity taken unless the code in force explicitly permits it. Read inputs off rating plates rather than off memory or the homeowner description. A tankless water heater described as standard can be anything from 120,000 to nearly 200,000 BTU per hour, and one wrong plate on the trunk leg mis-sizes everything behind it.

Size that first section for the total load at the full developed length of the longest run, then hold the result. Where the answer lands between two nominal sizes, go up; the extra material is trivial against opening a finished ceiling later. Watch the table change when the material changes, too. Black iron, corrugated stainless steel tubing and polyethylene yard pipe each carry their own capacity tables, and CSST is rated by the manufacturer designation rather than by nominal iron pipe size, so its published capacity is not interchangeable with steel pipe of similar bore.

Past the First Tee: the Load Sheds, the Length Does Not

At every tee along the route, subtract the load that leaves and keep walking. The section downstream of the first branch carries what remains, and under longest-length sizing you still look that section up at the full developed length of the worst-case run, not at the distance from the tee. That is the step people get wrong after being taught to size each piece independently: length is a property of the system, load is a property of the section.

Branches taken off the trunk get sized on their own load, again at the governing length. A short spur to a 40,000 BTU per hour dryer read at 120 feet may come out a size larger than instinct suggests, and that is the method behaving correctly rather than being wasteful. Where a branch feeds a future appliance, size for the appliance you expect and cap it. Where a manifold serves several small loads, the section feeding it carries all of them at once unless the code in force allows otherwise.

Fittings, Risers and Length You Cannot Measure with a Tape

Elbows, tees taken on the branch, and full-port versus reduced-port valves all add equivalent length. Code capacity tables come with a stated assumption about how much fitting loss an ordinary installation carries, and that assumption belongs to the code rather than to the sizer. A run with a normal number of direction changes is usually covered; a manifold-heavy commercial rack, or a route with a dozen elbows inside fifteen feet, is not. When the fitting count is abnormal, add equivalent lengths from the fitting tables in the code before looking anything up.

Vertical legs behave differently for the two gases. Natural gas is lighter than air, so a tall riser gains a little pressure going up and gives it back coming down; propane vapour is heavier and does the reverse. On a single-storey dwelling the effect is negligible and no code asks you to chase it. On a multi-storey riser, or a hillside propane yard line with real elevation change between the tank and the building, it becomes a term you account for, and both the National Fuel Gas Code and CSA B149.1, Natural Gas and Propane Installation Code, publish the correction.

The Last Section: Connectors, Traps and the Appliance Shutoff

The final few feet undo more sizing work than any other part of the route. An appliance connector is not pipe: it carries its own capacity rating, its permitted length is shorter than most people assume, and it must not pass through a wall, floor or cabinet partition. A short connector on a 199,000 BTU per hour heater can throttle a system that was sized perfectly all the way to the shutoff valve, and the symptom on the burner looks exactly like undersized pipe.

Each appliance needs an accessible shutoff within the distance the code in force specifies, and both that distance and whether the valve must sit in the same room vary by jurisdiction. Sediment traps, where required, go close to the appliance inlet and downstream of the shutoff; they are not the same thing as the drip leg some codes require at the low point of a run. Ball valves fitted in the wrong orientation, unions placed downstream of the trap, and traps piped as a bullhead tee are the three details that fail inspection most often.

Proving It at the Far Appliance

A manometer settles what sizing only predicts. Pressure-test the pipework first, to the value and duration the code in force requires, with appliances isolated and their connectors capped, and log the result. Then bring the system into service, put a gauge on the inlet tap of the appliance at the end of the longest run, and fire everything at once: furnace, water heater, dryer, every range burner and the oven.

Watch the reading as the last load comes on. Static pressure with nothing running tells you almost nothing; the number that matters is inlet pressure at full fire, measured against the minimum on the rating plate, with the drop across the piping inside the allowance you sized to. A system that holds seven inches static and falls to four and a half under full load is undersized somewhere upstream, and the burners confirm it: lazy yellow flame, delayed ignition, a furnace that drops out on flame sense only while the water heater is calling.

Adding Load to a Run Already at Its Limit

Most gas sizing done in the field is not new construction. It is a generator, a pool heater or a second furnace added to a system laid out for the loads of the day it was built. Re-run the whole longest-length calculation with the new total, because added load does not simply require a branch; it can push the existing trunk out of compliance, and the appliance that suffers is the far one nobody touched.

Three outcomes cover almost every retrofit. The existing trunk still passes and you take a branch off it. It fails by one size, and a new dedicated run from the source usually beats replacing trunk that is buried or buried in a wall. Or the load has outgrown the source entirely, at which point the conversation moves to the utility about a larger meter or an elevated-pressure service with a line regulator, or to the propane supplier about a larger container. No amount of correct pipe sizing rescues a source that cannot deliver.

Leave the sizing sheet with the job. Route, method used, developed length, load at each section, allowable drop, table and material referenced, and the measured inlet pressure at the far appliance under full fire. The next person to add an appliance inherits your longest run, and a sheet taped inside the furnace cabinet saves them from re-measuring a system they cannot see.