Plumbing
Natural gas piping sizing and install field guide for gas fitters
Convert each appliance to CFH, total the connected load, run the longest length down to the farthest unit, size every section off the code table, then prove it with a pressure test.
Direct answer
Gas pipe sizing sets each section large enough to deliver every appliance its full BTU demand at the right pressure without too much pressure drop. You convert each appliance input from BTU per hour to cubic feet per hour, total the load, then size each section off the NFPA 54 or IFGC tables. The adopted code and gas utility control.
Key takeaways
- Convert each appliance input to CFH by dividing BTU/hr by about 1,000, total the connected load, then size each section off the NFPA 54 or IFGC table.
- Natural gas carries roughly 1,000 BTU per cubic foot (real pipeline gas 950 to 1,100); propane is near 2,500 and reads a different table.
- Most homes run low pressure near 7 inches water column, sized to lose no more than about 0.5 inch w.c. to the farthest appliance.
- Standard yellow CSST must be bonded, commonly with a 6 AWG copper conductor to the grounding electrode system, or lightning can arc through the thin tube.
- Pressure test with air or inert gas, commonly at least 1.5 times working pressure and not less than 3 psi, with appliances and regulators isolated; AHJ sets pressure and hold time.
Gas pipe sizing and what it has to deliver
Gas pipe sizing is the calculation that sets each pipe section big enough to carry the gas every downstream appliance needs, at a pressure high enough that the burner fires the way it was built to. Every appliance has an input rating in BTU per hour stamped on the nameplate. The piping has to hand that input to the appliance at the inlet pressure the appliance expects, after the gas has lost a little pressure pushing through the pipe and fittings on the way.
Undersize a line and the appliance starves. A starved burner does not just run small. It short cycles, the flame lifts or rolls, the heat exchanger sees incomplete combustion, and you start making carbon monoxide instead of just heat. A furnace that fires fine alone but drops out when the water heater and the range come on at the same time is the classic undersized-main symptom. The pipe was sized for one load, not the connected load.
The other half of the job is the install: the right material, joints that hold gas, a shutoff and a sediment trap at each appliance, a system that passes a pressure test, and a bond on the CSST if you used it. Sizing keeps the appliances fed. The install keeps the gas inside the pipe and the inspector signing off.
How do you size gas pipe?
Size gas pipe in five steps, and they go in this order every time. First, list every appliance and its input in BTU per hour from the nameplate, not from memory. Second, convert each input to cubic feet per hour using the heating value of the gas, and total the connected load. Third, find the longest run, the path from the meter to the farthest appliance. Fourth, decide the system pressure and the allowable pressure drop, because the capacity table you read is built for a specific pressure and drop. Fifth, look up each section in the NFPA 54 or IFGC table at the right length and carry the downstream load.
The piece beginners skip is that each section carries only the gas going through it. The section right off the meter carries the whole connected load. A branch feeding one appliance carries only that appliance. Size each section for the load it actually passes, at the length the method tells you to use, and you get pipe that is big where it has to be and no bigger.
The adopted fuel gas code, NFPA 54 (also published as ANSI Z223.1, the National Fuel Gas Code) or the IFGC as your jurisdiction adopts it, governs the tables and the rules. The gas utility sets the delivery pressure and the meter. Confirm both before you size anything.
How many BTU is in a cubic foot of natural gas?
One cubic foot of natural gas carries roughly 1,000 BTU of heat, so you convert an appliance input to cubic feet per hour by dividing its BTU per hour rating by about 1,000. A 100,000 BTU/hr furnace draws about 100 CFH. A 40,000 BTU/hr water heater draws about 40 CFH. The arithmetic is that simple, and it is the foundation the whole sizing job sits on.
The 1,000 figure is a round number, not a law. The real heating value of pipeline gas runs from about 950 to 1,100 BTU per cubic foot depending on the gas composition in your area, and many utilities sit near 1,020 to 1,030. The capacity tables in NFPA 54 and the IFGC are built on a heating value near 1,000, so using 1,000 to convert keeps you consistent with the tables. If you want the exact number for a large or marginal job, call the utility and ask for their current heating value. Propane is a different animal entirely, near 2,500 BTU per cubic foot, so a propane appliance pulls far fewer cubic feet for the same BTU and reads off a different table.
Convert every appliance, then add the CFH up. That total connected load is what the section off the meter has to carry, and it is the number people get wrong by sizing the main for a single appliance instead of the whole house firing at once.
CFH = (appliance input in BTU/hr) / (heating value in BTU/ft3)CFH ≈ (BTU/hr) / 1000CFHtotal = CFH1 + CFH2 + … + CFHn- BTU/hr
- British thermal units per hour, the appliance input rating on the nameplate
- CFH
- Cubic feet of gas per hour, the volume flow the pipe has to deliver
- Heating value
- Heat per cubic foot of gas, about 1,000 BTU/ft3 for natural gas, near 2,500 for propane
Totaling the connected load
The connected load is the sum of every appliance input the system can serve, converted to CFH. For residential and most light-commercial work, the trade sizes for the full connected load with no diversity reduction, because the code tables already assume you size for everything that can run. The assumption that the furnace and the water heater and the range will never all call at once is exactly the assumption that leaves a burner starving on a cold morning when they do.
Read the nameplate input, not the output. A furnace rated 96 percent efficient with an 80,000 BTU output has an input closer to 83,000 BTU, and the input is the gas it actually burns. Tankless water heaters are the load that surprises people. A single high-output tankless can demand 150,000 to 199,000 BTU on its own, which is more gas than the rest of a small house combined, and it is the appliance that most often forces the main and the meter up a size.
Large commercial and institutional systems with many like appliances can take a demand or diversity factor, but that is an engineered call under the code and the design documents, not a field guess. When in doubt, size for the full connected load. The cost of one size larger in pipe is small next to a callback for an appliance that will not run when its neighbors do.
What gas pressure and pressure drop do you size for?
Most homes run a low-pressure system, where the gas leaves the meter at about 7 inches of water column and you size the piping to lose no more than about 0.5 inch of water column from the meter to the farthest appliance. That 0.5 inch is the pressure-drop budget, and it is the column you read the capacity table under. Spend more than the budget and the appliance at the end sees too little pressure to fire right.
Inches of water column is a small unit on purpose. The whole supply is only about 7 inches, and 0.5 inch of that is only about 0.018 psi. The NFPA 54 and IFGC low-pressure capacity tables come in several drop columns, commonly 0.3, 0.5, 3.0, and 6.0 inches of water column, and which column you read changes the pipe size. A tighter drop budget means bigger pipe. Pick the drop your design or the AHJ specifies and read the whole system off that one column.
The other common arrangement is a 2 psi system, also called an elevated or hybrid system. The house line runs at 2 psi, which is about 55 inches of water column, and a line-pressure regulator at each appliance or zone steps it back down to the 7-inch range the appliance needs. The higher pressure lets you push the same gas through smaller pipe, which is why long runs and high-rises lean on it. Sizing a 2 psi system reads off a different table column built for that pressure and a larger allowable drop. Do not mix a 2 psi table size with a 7-inch system, or you will undersize the low-pressure runs.
| System | Typical supply pressure | Common drop budget | What it needs |
|---|---|---|---|
| Low pressure | ~7 in w.c. (~0.25 psi) | ~0.5 in w.c. | No appliance regulator beyond the meter |
| Elevated / 2 psi | 2 psi (~55 in w.c.) | Larger, per the 2 psi table | Line-pressure regulator at each appliance or zone |
| Propane vapor | Often 11 in w.c. at the appliance | Per LP table | Two-stage regulation off the tank |
Longest-length method vs branch-length method
NFPA 54 and the IFGC give two ways to read the capacity tables, and they trade simplicity for material. The longest-length method, sometimes called the maximum-length method, sizes the entire system off one number: the length of the longest run, meter to the farthest appliance. You find that single longest length, then size every section in the system at that length carrying its own downstream load. It is conservative on purpose, it is the most common method in the field, and it is the one that is hardest to get wrong on a small job.
The branch-length method is less conservative and can save pipe on a spread-out system. You size each section of the longest run using that longest length and the section load, the same as before. Then for branch piping that was not already sized, you use the length of piping from the meter to the most remote outlet on that branch, not the system longest length. Shorter branches get credited for being shorter, so some of them size down.
The longest-length method gives the same or larger pipe than the branch-length method, never smaller, so it is always safe to use. Reach for the branch-length method when the longest run is much longer than the branches and the savings are worth the extra bookkeeping. On a typical house, the longest-length method is the right tool and the branch savings are not worth the chance of a mistake.
Walking the longest-length method
Start at the appliance farthest from the meter, measured along the actual pipe route, not straight-line across the floor plan. Gas pipe goes up, over, and around the structure like any other pipe, and the routed length beats the plan distance on a real building. That farthest run sets the length you read every section at.
Now work back toward the meter, section by section. A section is a length of pipe between two fittings where the load changes, usually at a tee where a branch splits off. Each section carries the total CFH of everything downstream of it. The last leg to the farthest appliance carries only that appliance. The next section back carries that appliance plus whatever tees in there. The section at the meter carries the full connected load. For each section, go into the table for your material at the longest length, find the row, and read across to the smallest pipe size whose capacity meets or beats the section load.
Fittings cost you length. Every elbow, tee, and valve adds resistance, and the code and the manufacturers handle this with an equivalent length: an elbow might count as a few extra feet of straight pipe. For most rod-straight residential runs the table length already carries enough margin and you size off measured length. On a run with a lot of fittings, or a long commercial run, add the equivalent lengths of the fittings to the measured pipe and size off the total. When in doubt, round the length up to the next table row. Rounding length up never hurts you. Rounding it down starves the far end.
Black steel, CSST, copper, and the right table
The material decides which capacity table you read, because a smooth pipe and a corrugated tube of the same nominal size do not flow the same gas. Use the table for the material you are actually installing. This is where mixing tables quietly undersizes a job.
Black steel, schedule 40 threaded pipe, is the workhorse and the default many inspectors expect to see. It is rigid, takes abuse, and reads off the metallic-pipe capacity tables by nominal size. Corrugated stainless steel tubing, CSST, is flexible, fast to run through framing, and sized not by nominal diameter but by EHD, the equivalent hydraulic diameter the manufacturer assigns each tube size. CSST flows less than steel of a similar look, so you read the manufacturer's listed tables, and the size that fits the framing is often not the size that carries the load. Copper is allowed in many jurisdictions for natural gas, but not all, and only where the gas is not corrosive to it. Copper gas tables are built on Type K tube, the thickest wall with the smallest bore, so it carries less than its outside diameter suggests.
Polyethylene is for underground service only, never run inside a building. Confirm what your jurisdiction allows before you commit. Some areas restrict copper for natural gas over sulfur content in the gas stream, and some require steel for the first section off the meter. The AHJ and the gas utility have the final say on material.
| Material | Sized by | Notes |
|---|---|---|
| Black steel, schedule 40 | Nominal pipe size | Default, threaded, reads the metallic-pipe table |
| CSST | EHD per the manufacturer | Flexible, flows less than steel, listed tables only, must be bonded |
| Copper (Type K) | Tube size, Type K bore | Allowed in many but not all jurisdictions, check gas corrosivity |
| Polyethylene (PE) | Per PE table | Underground service only, never inside the building |
Does CSST need to be bonded?
Yellow-jacket CSST must be electrically bonded, and it is the install step that gets missed and fails inspection more than any other on a gas job. The bond is not the same as the equipment grounding that comes with the appliance circuit. It is a dedicated bonding conductor, commonly a minimum 6 AWG copper, run from the CSST system to the building's grounding electrode system. NFPA 54, commonly in the section on electrical bonding, and the NEC both drive this requirement, along with the CSST manufacturer's instructions, which govern the details.
The reason is lightning, and it is not theoretical. A nearby strike can put a large voltage on the gas piping, and the thin corrugated wall of standard CSST can arc and burn a pinhole through, which then leaks gas into a building that just took a lightning hit. Bonding drains that energy to ground and lowers the voltage the tube sees, so it does not perforate. This is why the bond exists and why an inspector looks for it specifically.
Two field details matter. Attach the bonding clamp to a rigid component, a steel pipe section, a malleable fitting, a manifold, or the brass CSST fitting, never to the corrugated tube itself, because the clamp cannot grip the thin wall safely. And know your product: newer arc-resistant CSST with a thick black jacket is listed to handle the arc energy without the separate bond, but the listing and the manufacturer's instructions control whether the standard bond is still required. Read the listing. Do not assume the black jacket exempts you.
Field example: a four-appliance house
Take a house with a furnace, a tankless water heater, a range, and a dryer on a low-pressure 7-inch system sized to a 0.5 inch w.c. drop in black steel. Convert each input to CFH at 1,000 BTU per cubic foot, then total the connected load. The longest run, meter to the dryer at the back of the house, measures 70 ft along the route, so the longest-length method reads every section at 70 ft.
The connected load comes to 355 CFH, so the section off the meter and the main trunk carry 355 CFH at 70 ft. Branches carry only their appliance. The point of the table is that each section drops in size as the load it carries drops. The numbers below show the demand, the longest length, and a representative result. The exact pipe size for each section comes out of the NFPA 54 or IFGC capacity table your jurisdiction adopts, at the length and drop you are sizing to, so treat the sizes here as the shape of the answer and read your own table for the job.
Notice the tankless. At 160,000 BTU it pulls 160 CFH on its own, nearly half the connected load, and its branch sizes up accordingly. That single appliance is what pushes the meter and the main on a small house, and it is the load to confirm with the utility before you commit the service size.
| Section / appliance | Input (BTU/hr) | Demand (CFH) | Length used | Representative size (steel) |
|---|---|---|---|---|
| Meter to main trunk (all) | 355,000 | 355 | 70 ft | 1-1/4 in |
| Tankless water heater branch | 160,000 | 160 | 70 ft | 1 in |
| Furnace branch | 100,000 | 100 | 70 ft | 3/4 in |
| Range branch | 65,000 | 65 | 70 ft | 3/4 in |
| Dryer branch (farthest) | 30,000 | 30 | 70 ft | 1/2 in |
| Connected load (total) | 355,000 | 355 | Confirm meter capacity |
The meter, the service, and the house regulator
The meter and the service regulator are the utility's hardware, and they cap how much gas the house can draw no matter how big you make the pipe downstream. Before you size anything, find out the meter's rated capacity in CFH and the pressure it delivers. A meter sized for an old house with a furnace and a water heater will not feed a new tankless and a pool heater, and the fix is a utility upsize, not bigger pipe past an undersized meter.
The service regulator at the meter steps the utility's distribution pressure down to the house delivery pressure, usually about 7 inches of water column for a low-pressure system or 2 psi for an elevated one. That regulator and the meter are matched to the connected load you tell the utility, which is why getting the connected-load total right matters before the gas company sets the equipment. Under-report the load and the meter throttles the house. Over-report it and you may pay for capacity you never use.
On the building side of the meter, the system either runs straight at 7 inches w.c. to the appliances or runs at 2 psi to line-pressure regulators near each appliance. Which one you have changes the table you size from and the hardware you install. Confirm the delivery pressure with the utility in writing for a job of any size. It is the input the whole sizing job depends on.
Shutoffs, sediment traps, and appliance connectors
Every appliance gets an accessible shutoff valve within reach of the appliance, so it can be isolated for service or in an emergency without shutting the whole house. The trade puts it where a person standing at the appliance can reach it, not buried behind the unit where you would have to pull the appliance to kill the gas.
Downstream of the shutoff, most appliances need a sediment trap, and it is worth knowing what it actually does. A sediment trap, the capped vertical leg of pipe below a tee at the appliance, catches rust, scale, and debris in the gas stream before it reaches the burner orifice and plugs it. The IFGC, commonly around the sediment-trap section, calls for it on most appliances, with a dirt leg generally at least 3 inches long, built of steel pipe and fittings. People confuse it with a drip leg, which is there to catch condensation at a low point. They are not the same fitting, even when one piece of pipe does both jobs. A range and an outdoor appliance are common exceptions, but the code and the appliance instructions control which appliances need a trap.
The flexible appliance connector is the corrugated line from the shutoff to a movable appliance like a range or a dryer. It is not gas pipe, it does not get sized off the capacity table the same way, and it never runs through a wall, floor, or cabinet partition. A connector concealed in or passing through a wall is a defect an inspector will write up, because it cannot be inspected and it chafes where it passes through framing. Keep connectors short, exposed, and within the length the manufacturer lists.
Joints, support, and the install that holds gas
Threaded steel joints get a pipe-joint compound or tape rated for fuel gas, applied to the male threads only, not the female fitting. Ordinary white plumber's tape is not the same as the yellow gas-rated tape, and the difference matters because gas finds a leak path that water would not. Cut clean threads, ream the burr, and make the joint up so a couple of threads still show. Buried thread count and a joint dripping with compound both read as a hide job to an inspector.
Unions belong where you need to break the line for service, at the appliance side of the shutoff, and they are never allowed concealed inside a wall or a floor. A concealed union is a hidden potential leak with no way to inspect it, and it is a flat violation. Run pipe so that any joint that could be a leak is accessible.
Support the pipe so it cannot sag, vibrate, or pull on a fitting. The code gives spacing by pipe size and material, tighter for small pipe and for CSST, wider for large steel, and the manufacturer sets the spacing for CSST and copper. Run pipe with a slight pitch back toward a drip point where the gas can carry moisture, protect it from physical damage where it is exposed, and sleeve and protect it where it passes through masonry or a slab. The install is half the job. A perfectly sized system that leaks at a lazy joint fails the same test as one that is undersized.
How do you pressure test gas piping?
You pressure test gas piping with air or an inert gas, never with the fuel gas, holding a pressure higher than the system will ever see in service and watching a gauge for any loss. The test proves the joints hold before any gas goes in the pipe. NFPA 54 and the IFGC set the framework, commonly calling for a test pressure of at least 1.5 times the maximum working pressure and not less than 3 psi, but the AHJ sets the pressure and the hold time it will accept, and many inspectors call for more.
Hold time scales with how much pipe you are testing. For a small system, a single-family dwelling or a piping volume under about 10 cubic feet, the common minimum is 10 minutes. Larger systems hold longer, often on the order of a half hour per 500 cubic feet of pipe volume. Disconnect or isolate the appliances and their regulators first, because appliance gas valves and regulators are not built for the test pressure and you will blow a diaphragm. Use a gauge or manometer with a range suited to the test, fine enough to show a small loss, commonly not more than five times the test pressure.
When the gauge drops, find the leak before you chase your tail. Brush a soap solution on every joint and watch for bubbles, the oldest and still the most reliable field test, or use an electronic gas detector once the system is on gas. After the test passes and the system is approved, purge the air out and bring gas in carefully, venting the purge to a safe place outdoors and away from ignition. Never purge into the room you are standing in.
Elevated 2 psi systems and appliance regulators
A 2 psi system runs the house piping at 2 psi instead of 7 inches of water column, and puts a line-pressure regulator at each appliance or zone to drop it back to the inlet pressure the appliance needs. The higher line pressure carries more gas through smaller pipe, which is the whole reason to use it: long runs and tall buildings size down, and the material savings can be real on a big job.
The trade-off is hardware and discipline. Every appliance fed at 2 psi needs its own line-pressure regulator, and that regulator has to be the right one, set right, and often vented or fitted with a vent limiter per its listing. Size the 2 psi piping off the table column built for that pressure and its larger allowable drop, then size the short low-pressure section between the appliance regulator and the appliance off the low-pressure table. The two pressure zones read different tables, and the boundary is the regulator.
The mistake that bites on these systems is mixing the zones. Someone sizes a low-pressure run off the 2 psi table because the project is a 2 psi job, and that run starves because the gas downstream of the regulator is back at 7 inches with a different capacity. Mark the drawing clearly where 2 psi ends and 7 inches begins, and size each side to its own table.
High-rise risers, commercial kitchens, and standby generators
Big loads and tall buildings stress the parts of gas sizing that a house never touches. A high-rise gas riser climbs through many floors, so the run is long and the elevation comes into play. Gas is lighter than air, so a tall riser actually gains a little pressure going up, a small credit the code lets you account for, but the dominant problem is still the length and the connected load of every floor tapped off the stack. These systems usually run elevated pressure with a regulator at each floor or tenant for exactly that reason.
Commercial kitchens stack large appliances. A bank of ranges, fryers, a char broiler, and a combi oven can total far more CFH than a house, and the appliances often have published inputs that are higher than they look. Size the kitchen for the full connected load, confirm the meter and service can feed it, and remember the makeup air and the hood interlock are part of the same combustion picture. A kitchen sealed up tight will starve its own burners for air no matter how well the gas is sized.
Standby and prime gas generators are the load that estimators miss, and they are showing up on more sites, including data centers running gas-fired backup. A generator's gas demand at full load can dwarf the building's normal connected load, and it draws that gas in a hurry when it picks up the load on a power loss. Size the generator's gas supply for its full input with the pressure it needs at the inlet under that surge, coordinate with the utility on whether the service and meter can deliver it, and treat it as a peak load that runs concurrently with everything else, because on a power failure it does.
Combustion air and the venting tie-in
Sizing the gas in is only half of feeding an appliance. The burner also needs combustion air, and the products of combustion need a way out, and a perfectly sized gas line means nothing if the appliance is choking. An appliance in a tight mechanical room or a closet with no dedicated combustion air will starve, soot up, and make carbon monoxide even with full gas pressure at the valve.
The fuel gas code sizes combustion air openings by the appliance input and the volume and tightness of the space, and the appliance instructions govern the venting category and the flue. This is its own design and its own set of tables, beyond gas sizing, but it lives on the same job and the same inspection. When you commission an appliance, confirm the combustion air and the venting are right before you call the gas sized correctly, because the symptoms of starved air and starved gas look alike at the burner.
The same building-air logic ties gas work to the rest of the trades. A kitchen exhaust hood or a big bath fan can depressurize a space enough to backdraft a natural-draft water heater. Coordinate the gas appliances with the building's airflow, not just the gas piping.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
What to document
The gas record is what answers the question a year out when someone adds an appliance and wonders whether the main can take it. Without it, the next person re-sizes the whole house from scratch or, worse, guesses. Capture the connected load and the sizing assumptions, not just the final pipe sizes.
Record each section, the appliances it serves, the CFH it carries, the length used to size it, and the resulting pipe size and material. Then record the system pressure and drop budget, the meter capacity the utility gave you, the CSST bond if you ran CSST, and the pressure test: the medium, the pressure, the hold time, and the result. Note the code edition and the AHJ, because the table you sized from is tied to a specific code cycle.
| Field to record | Why it matters |
|---|---|
| Section and appliances served | Shows what each leg feeds for the next change |
| CFH carried by the section | The load the section was sized for |
| Length used to size | Lets a reviewer reproduce the size off the table |
| Pipe size and material | Ties the size to the right capacity table |
| System pressure and drop budget | The table column the sizing came from |
| Meter capacity (utility) | Caps the whole system regardless of pipe |
| CSST bond, if used | Proves the bonding step was done |
| Test pressure, hold time, result | The leak test of record for the AHJ |
Common mistakes
- Sizing the main for one appliance instead of the full connected load, so the house starves when several fire at once.
- Reading the wrong table: steel sizes used for CSST, or a 2 psi table size used on a 7-inch low-pressure run.
- Using the plan distance instead of the routed longest length, which understates the run and undersizes the far end.
- Running CSST and never bonding it, the single most common gas inspection failure.
- Leaving out the sediment trap, or confusing it with a drip leg.
- Concealing a union or running a flexible connector through a wall where it cannot be inspected.
- Skipping or short-cutting the pressure test, or testing with appliance regulators still connected.
- Forgetting combustion air, so a correctly gassed appliance still sooths and makes CO.
Standards and references
The governing document is NFPA 54, the National Fuel Gas Code, also published as ANSI Z223.1, and the International Fuel Gas Code (IFGC) as your jurisdiction adopts it. These hold the capacity tables, the sizing methods, the pressure-test rules, and the bonding requirement. The sizing tables are organized by material, pipe size, length, and pressure drop, and the section numbers and table numbers move between code cycles, so confirm them against the edition the AHJ has actually adopted and any local amendments before you cite one on a permit.
The local gas utility governs the meter, the service regulator, and the delivery pressure, and it sets the real heating value of the gas in your area if you need a number tighter than 1,000 BTU per cubic foot. The CSST and connector manufacturers govern the EHD sizing tables, the bonding details, and the connector limits through their listings and instructions, which the code defers to. Pipe material standards cover the steel, copper, CSST, and PE themselves.
When two sources disagree, the stricter one usually controls, and the AHJ has the final call on the job. Size from the table, install to the manufacturer, and verify the edition before you write a number on a submittal. For the water side of the same building, see the water supply sizing guide, and for the drainage and venting, the DWV guide, since the three systems share the same walls and the same inspection.
Units, terms, and conversions
Gas work mixes heat units, volume-flow units, and several pressure units on the same drawing set, so the same quantity reads differently across a nameplate, a code table, and a utility sheet.
Appliance input is in BTU per hour, sometimes shown as MBH (thousands of BTU per hour, so 100 MBH is 100,000 BTU/hr). Gas flow is in cubic feet per hour, CFH, sometimes CFM on large equipment. Pressure for low-pressure gas is in inches of water column (in w.c. or in wg), where about 27.7 inches of water column equals 1 psi, so 7 inches w.c. is roughly 0.25 psi and 2 psi is about 55 inches w.c. CSST size is given as EHD, equivalent hydraulic diameter, not nominal pipe size. Heating value is BTU per cubic foot, near 1,000 for natural gas and near 2,500 for propane, which is why the two read off different tables.
- BTU/hr and MBH
- Appliance input as heat per hour, MBH meaning thousands of BTU/hr
- CFH
- Cubic feet of gas per hour, the volume flow the pipe must deliver
- in w.c. (in wg)
- Inches of water column, the low-pressure gas unit, about 27.7 in w.c. per psi
- EHD
- Equivalent hydraulic diameter, how CSST is sized instead of nominal pipe size
- Connected load
- The total CFH of all appliances the system can serve, summed for sizing
- Sediment trap
- A capped vertical leg that catches debris before the burner orifice
FAQ
How do you size gas pipe?
Convert each appliance input in BTU per hour to cubic feet per hour at about 1,000 BTU per cubic foot, total the connected load, then size each section off the NFPA 54 or IFGC capacity table for your material, at the longest run length and the allowable pressure drop. The adopted code and the gas utility control.
How many BTU is a cubic foot of natural gas?
A cubic foot of natural gas carries roughly 1,000 BTU, though real pipeline gas runs about 950 to 1,100 depending on composition, and many utilities sit near 1,020. Divide an appliance's BTU per hour input by about 1,000 to get its demand in cubic feet per hour. Propane is different, near 2,500 BTU per cubic foot.
What is the difference between the longest-length and branch-length methods?
The longest-length method sizes every section using the single longest run, meter to the farthest appliance, so it is conservative and simple. The branch-length method sizes branches off their own shorter length, saving some pipe. The longest-length result is never smaller, so it is always safe. Both come from NFPA 54 and the IFGC.
Does CSST need to be bonded?
Yes, standard yellow CSST must be bonded, commonly with a 6 AWG copper conductor to the building's grounding electrode system, per NFPA 54, the NEC, and the manufacturer. Bonding drains lightning energy so the thin tube does not arc and burn through. Newer arc-resistant black-jacket CSST may be exempt, but the listing controls.
How do you pressure test gas piping?
Test with air or an inert gas, never fuel gas, at a pressure commonly at least 1.5 times working pressure and not less than 3 psi, with appliances and regulators isolated. Single-family systems often hold 10 minutes minimum, larger systems longer. The AHJ sets the actual pressure and time. Soap-test joints, then purge safely outdoors.
What pressure does residential gas piping run at?
Most homes run a low-pressure system near 7 inches of water column, about 0.25 psi, sized so the system loses no more than about 0.5 inch of water column to the farthest appliance. Newer and longer systems may run 2 psi, about 55 inches w.c., with a regulator at each appliance. The utility sets delivery pressure.
What size gas line do I need for a tankless water heater?
Size it for the unit's full input, often 150,000 to 199,000 BTU, which is about 150 to 199 CFH at 1,000 BTU per cubic foot. That demand and the run length, read off the NFPA 54 or IFGC table for your material, usually drive 3/4 inch or larger. Confirm the meter can feed it too.
Does a gas appliance need a sediment trap?
Most appliances need a sediment trap, a capped vertical leg below a tee that catches rust and debris before the burner orifice, commonly with a dirt leg at least 3 inches long. The IFGC requires it on most appliances, with exceptions like ranges and some outdoor units. The code and the appliance instructions control which need one.
Why does my gas appliance starve when others turn on?
The main or a shared section is undersized for the connected load, so when several appliances fire together the pressure drop exceeds the budget and the far appliance loses inlet pressure. Re-total the connected load in CFH, check the longest-length sizing on the shared sections, and confirm the meter capacity. Often the trunk or meter is the limit.
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Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.