GAS LINE LEARNING LIBRARY / SALT LAKE VALLEY
Gas pressure & pipe sizing, explained
Pressure is the push. Flow is the amount moving. An appliance’s fuel demand tells us how much it needs. Pipe sizing connects those ideas across the entire route.
Try the interactive gas-system lessons ↗Gas words, in plain English No trade vocabulary needed.
- Demand / load
- How much gas the equipment needs. An appliance label gives its fuel input in BTU per hour; add the relevant appliances when assessing a shared pipe.
- BTU/h / input rating
- British thermal units per hour: the rate of fuel energy going into an appliance. Heat output is what it delivers after losses, so use the input rating for gas-supply calculations.
- Flow / CFH
- How much gas passes a point over time. CFH means cubic feet per hour. A cubic foot is a volume, like a box one foot on each side.
- Pressure / pressure drop
- Pressure is the push on the gas. A pressure difference moves it along a pipe. The reduction in pressure along the route while gas flows is called pressure drop.
- Capacity
- How much gas a pipe, meter or other part can deliver at the specified pressure. The connected appliances determine how much capacity is needed.
- Branch / upstream / downstream
- A branch takes gas from a shared route to part of the system. Upstream means toward the supply; downstream means in the direction gas travels toward the appliances.
- Meter / regulator
- The meter measures gas use. A regulator controls pressure.
- Shutoff / sediment trap
- A shutoff is a valve that stops supply to a section. A sediment trap is a capped pocket arranged to catch debris before it reaches an appliance’s gas controls.
- Inlet / manifold
- The inlet is where gas enters an appliance. The manifold distributes it to the burners inside. Pressure requirements at these two places can differ.
- Combustion air / venting
- Combustion air is air needed to burn fuel. Venting carries the resulting exhaust away.
- Clearance / access
- Clearance is required space around equipment or piping. Access means being able to reach a part for inspection or service.
- Code / commissioning
- Code means installation rules adopted for the job’s location. Commissioning means checking and starting equipment according to its instructions, then confirming that it operates as required.
THREE DIFFERENT IDEAS
Demand. Flow. Pressure.
- Demand: what the appliance needs
- An input rating in BTU per hour describes how much fuel energy an appliance can use in an hour. BTU means British thermal unit. Input is the fuel energy going in; output is useful heat delivered after losses. Gas-supply calculations need the input rating.
- Flow: how much gas moves
- CFH means cubic feet per hour: a volume of gas passing in an hour. Turning heat demand into gas volume requires the gas heating value—the energy in each cubic foot. The utility or fuel supplier provides the heating value for its gas.
- Pressure: the push through the system
- Pressure differences move gas through piping. The route resists flow, so pressure can fall between the supply and the appliance. The appliance needs the pressure stated in its instructions at its inlet—the point where gas enters—while operating.
Worked conversion: 100,000 BTU/h ÷ an assumed heating value of 1,000 BTU per cubic foot = 100 cubic feet per hour. The same calculation uses your supplier’s heating value when planning an installation.
WORKED EXAMPLE / CHANGE THE HOME
Same house.
A different gas demand.
Follow the demand through a heater replacement and a new grill connection. Each change affects the shared pipe supplying the appliances.
Read the numbers: BTU/h means British thermal units per hour—the fuel energy an appliance uses per hour. A branch is its route off the shared pipe.
Example assumptions: these practice ratings assume all listed appliances run at full input together. The gas heating value is 1,000 BTU per cubic foot.
STAGE 1 / OF 3
Start with the existing home
Kitchen range
65,000 BTU/hFurnace
100,000 BTU/hTank water heater
40,000 BTU/hOutdoor grill
0 BTU/hNot installed yet
FOLLOW THE ARITHMETIC
65,000 + 100,000 + 40,000 = 205,000 BTU/h
The shared pipe carries the combined demand of the range, furnace and tank water heater. Each final branch carries only the demand of the appliance it serves.
Turn energy into gas volume: if this example’s gas contains 1,000 BTU in each cubic foot, 205,000 ÷ 1,000 = 205 cubic feet per hour (CFH).
STAGE 2 / OF 3
Replace the tank with tankless
Kitchen range
65,000 BTU/hFurnace
100,000 BTU/hTankless water heater
199,000 BTU/hOutdoor grill
0 BTU/hNot installed yet
FOLLOW THE ARITHMETIC
205,000 − 40,000 + 199,000 = 364,000 BTU/h
Remove the old heater’s 40,000, then add the replacement’s 199,000. The demand rises by 159,000 BTU/h. The shared pipe now has to carry gas for the larger heater along with the range and furnace.
Turn energy into gas volume: if this example’s gas contains 1,000 BTU in each cubic foot, 364,000 ÷ 1,000 = 364 cubic feet per hour (CFH).
STAGE 3 / OF 3
Then add an outdoor grill
Kitchen range
65,000 BTU/hFurnace
100,000 BTU/hTankless water heater
199,000 BTU/hOutdoor grill
60,000 BTU/hFOLLOW THE ARITHMETIC
364,000 + 60,000 = 424,000 BTU/h
Keep the tankless replacement and add the grill’s 60,000. Demand is now 219,000 BTU/h above the original home. The new grill branch serves the grill; the shared pipe also serves the equipment indoors.
Turn energy into gas volume: if this example’s gas contains 1,000 BTU in each cubic foot, 424,000 ÷ 1,000 = 424 cubic feet per hour (CFH).
How this total is used
The total gives the fuel demand on each shared section. The installer uses it with the supply pressure, piping product, route length and fittings to select a size that delivers enough gas at the required pressure.
Why the whole route matters: every shared section carries the demand of the branches beyond it. A restriction near the meter can affect several appliances at once.
Check your understanding: do we add 199,000 to the original 205,000?
Subtract the removed heater’s 40,000 BTU/h, then add the new 199,000. That gives 364,000 BTU/h. If both heaters remain in use, include both input ratings, for a total of 404,000 BTU/h.
For how pipe product, lengths, shared sections and pressure affect a design, see the Gastite design guide (2019 edition).
WORKED EXAMPLE / INTERPRET A READING
A good reading with equipment off
can miss a problem while it runs.
For this example, the appliance needs at least 6 inches of water column (inWC) at its inlet while running. The inlet is where gas enters the appliance.
With no gas flowing, pressure is 7 inWC. The next reading shows what happens under load.
The 5 inWC reading is below the example’s 6 inWC minimum, so the technician needs to find where the operating supply is falling short.
Under load: pressure drops from 7 to 5 inWC, below the example appliance’s 6 inWC minimum. The supply needs investigation while the equipment is running.
What does the technician investigate next?
The technician checks supply pressure, regulator operation and pressure loss through the piping while appliances are running. Measurements along the route help locate where the pressure falls, which directs the repair toward the supply, regulator or piping.
The pressure reading
needs a location and a condition.
At the supply
The regulator controls the pressure entering the house piping. The meter measures the amount of gas delivered.
At the appliance inlet
The inlet is where gas enters the appliance. Available pressure there depends on the supply, route, other appliances using gas and any additional regulators along the route.
Inside the appliance
The manifold distributes gas to the burners. The appliance’s internal controls regulate that supply, and its manual specifies the pressure at this test point.
Static means a reading with no gas flowing at the point being assessed. Operating means a reading while equipment is using gas. Compare static and operating readings to assess delivery as demand changes.
A technician measures pressure with a manometer. Recording the test point and which appliances were running makes the result useful for diagnosing a delivery problem.
What do “inches,”
“ounces” and “pounds” mean?
Inches of water column (in. w.c. or inWC) is a pressure unit based on the height of a water column. psi means pounds-force per square inch. When pressure is described as “ounces,” the full unit is ounces-force per square inch.
| Pounds per square inch | Ounces per square inch | Inches of water column |
|---|---|---|
| 0.25 psi | 4 oz/in² | 6.92 inWC |
| 0.5 psi | 8 oz/in² | 13.84 inWC |
| 1 psi | 16 oz/in² | 27.68 inWC |
| 2 psi | 32 oz/in² | 55.36 inWC |
A house using a higher-pressure distribution arrangement may have additional regulators before lower-pressure equipment. Those regulators reduce the pressure to the range required at each appliance.
Try the pressure converter and system explorer ↗Size the shared pipe
for the appliances it supplies.
Imagine two appliances supplied by one shared pipe. One needs 40,000 BTU/h and the other needs 100,000 BTU/h. Their combined example demand is 140,000 BTU/h. The shared section supplies both appliances; the final branches carry 40,000 and 100,000 BTU/h respectively.
Length, fittings, material and the allowed pressure drop also affect delivery. A pipe’s trade size is called its nominal size. Products with the same nominal size can have different inside diameters and flow resistance, so sizing tables are specific to the piping product. The calculation includes the full route back to the meter.
- Identify the supply. Confirm fuel, delivery arrangement and available pressure.
- List connected loads. Record model-specific input ratings and future additions.
- Map the route. Identify lengths, branches, fittings and the exact piping product.
- Use the applicable design method. Use the tables for that exact product and the installation rules in force at the address. Establish how much pressure can be lost along the route while still meeting the appliance requirements.
- Verify the installed system. Test the piping for leaks, then check pressure and appliance operation during startup.
Technical reference: Gastite design and installation guide (2019 edition). Utility supply questions: Enbridge construction services.
PUT THE INFORMATION TO WORK
Tell us about
your project.
Mountain Man Plumbing is based in Sandy and serves the Salt Lake Valley. Send the appliance model, where it will go and your timing. For a repair after a shutoff, include the utility or inspection notice.