9.2 Gas Pipe Sizing & Longest Length Method

9.2 Gas Pipe Sizing & Longest Length Method

Accurate gas pipe sizing is vital to ensure that every gas appliance receives its required volumetric fuel flow under peak concurrent operating conditions without suffering excessive pressure drops. An undersized gas line starves appliances of fuel, resulting in incomplete combustion, lethal carbon monoxide (CO) generation, pilot outage, and premature heat exchanger failure.

The Texas State Board of Plumbing Examiners (TSBPE) tests candidates extensively on the Longest Length Method (also known as the Longest Run Method) outlined in IFGC Chapter 4 and NFPA 54 Chapter 6.

1. Fuel Gas Pressure Systems & Units of Measure

Fuel gas distribution systems are classified by operating gauge pressure:

+---------------------------------------------------------------------------------------------------+
|                         GAS PRESSURE CLASSIFICATIONS & CONVERSIONS                                |
|                                                                                                   |
|  SYSTEM TYPE            OPERATING PRESSURE            TYPICAL APPLIANCE / APPLICATION             |
|  ====================   ===========================   ==========================================  |
|  - Low-Pressure NG:     1/2 psi (0.5 psig)            Standard residential / commercial fixtures. |
|                         Nominal 7.0" w.c. (5"-7" w.c.)Manifold operates at 3.5" w.c.              |
|  - Low-Pressure LP:     1/2 psi (0.5 psig)            Propane residential systems.                |
|                         Nominal 11.0" w.c. (10"-12")  Manifold operates at 10.0" w.c.             |
|  - Medium-Pressure:     2.0 psig (55.4" w.c.)         Hybrid CSST / Steel residential mains with  |
|                                                       appliance line pressure regulators (MPR).   |
|  - High-Pressure:       5.0 to 10.0+ psig             Commercial / Industrial distribution loops. |
+---------------------------------------------------------------------------------------------------+

The "Inches Water Column" (in. w.c.) Unit

Low gas pressures are too minute to measure accurately on standard Bourdon tube PSI gauges. Instead, the industry utilizes a U-tube water manometer to measure the hydrostatic displacement of a column of water in inches:

$\text{Pressure Equivalents:}$ $1.0\text{ psi} = 27.7\text{ inches water column (in. w.c.)} = 16.0\text{ ounces/sq in}$ $0.5\text{ psi (1/2 psig)} = 13.85\text{ in. w.c.}$ $1.0\text{ in. w.c.} = 0.0361\text{ psi} = 0.578\text{ oz/sq in}$

• Allowable Pressure Drop (Δ P): In standard low-pressure systems (< 0.5 psig), sizing tables are calibrated to limit total friction loss between the gas meter outlet and any appliance shutoff valve to 0.5 in. w.c. (0.125 kPa) (or 0.3 in. w.c. in select conservation codes).
• Operational Logic: If utility meter delivery pressure is 7.0 in. w.c. and line friction drop is 0.5 in. w.c., appliance inlet pressure is 6.5 in. w.c., easily satisfying the 3.5 in. w.c. manifold burner requirement.

2. Converting Appliance BTU/hr Ratings to Volume Demand (CFH)

Gas sizing tables express pipe capacity in Cubic Feet per Hour (CFH). Before sizing pipe, all connected appliance nameplate ratings (expressed in BTU/hr) must be converted to volumetric flow rate:

+-----------------------------------------------------------------------------------+
|                         VOLUMETRIC DEMAND (CFH) FORMULA                           |
|                                                                                   |
|                         Appliance Input Rating (BTU/hr)                           |
|                  CFH = ---------------------------------                          |
|                         Heating Value of Gas (BTU/cu ft)                          |
|                                                                                   |
|  - FOR NATURAL GAS:  Use 1,000 BTU/cu ft  -->  CFH = BTU/hr / 1,000              |
|  - FOR LP-GAS:       Use 2,500 BTU/cu ft  -->  CFH = BTU/hr / 2,500              |
+-----------------------------------------------------------------------------------+

Standard Residential Appliance Demands

[!IMPORTANT] No Diversity Factor for Gas Piping: Unlike electrical load calculations or water supply fixture units (Hunter's Curve), fuel gas piping codes prohibit applying any diversity factor to general residential systems. The piping must be engineered to deliver 100% full-load capacity assuming every appliance operates simultaneously at maximum firing rate.

3. The Longest Length Method: Step-by-Step Procedure

The Longest Length sizing procedure prevents cumulative friction loss from choking the most remote appliance during simultaneous peak firing.

+-----------------------------------------------------------------------------------+
|                    THE LONGEST LENGTH SIZING METHOD WORKFLOW                      |
|                                                                                   |
|   [STEP 1] MEASURE TOTAL LONGEST DEVELOPED LENGTH:                                |
|   Measure from the outlet of the gas meter (or regulator) to the MOST REMOTE     |
|   appliance in the entire piping layout (summing pipe run + fitting allowances).  |
|                                                                                   |
|   [STEP 2] SELECT CODE SIZING TABLE & IDENTIFY LENGTH COLUMN:                     |
|   Choose the table for gas type, pipe material, inlet pressure, and ΔP.          |
|   Locate the column equal to or greater than the total longest developed length.  |
|                                                                                   |
|   [STEP 3] MAINTAIN THAT SINGLE COLUMN FOR THE ENTIRE SYSTEM:                     |
|   LOCK IN THIS COLUMN. Do NOT change columns when sizing shorter branch lines!    |
|                                                                                   |
|   [STEP 4] CALCULATE CUMULATIVE DOWNSTREAM LOAD FOR EACH PIPE SECTION:            |
|   For every main run, sub-main, and branch, sum all downstream appliance loads.  |
|                                                                                   |
|   [STEP 5] SELECT PIPE DIAMETER SATISFYING LOAD:                                  |
|   Read down the longest length column; choose the smallest nominal pipe diameter  |
|   whose tabulated capacity equals or exceeds the section's cumulative load.       |
+-----------------------------------------------------------------------------------+

Simplified IFGC Sizing Table Extract

Table: Natural Gas Schedule 40 Metallic Pipe (Inlet < 0.5 psig, Pressure Drop 0.5 in. w.c., Specific Gravity 0.60) — Capacities in CFH:

4. Comprehensive Worked Sizing Example

Consider a single-family residential natural gas system (1,000 BTU/cu ft, 0.5 psig, Δ P = 0.5 in. w.c.):

                     [FURNACE: 100,000 BTU / 100 CFH]
                                    ^
                                    | (10 ft Branch C-F)
[GAS METER] ====(20 ft A-B)====[B]====(20 ft B-C)====[C]====(20 ft C-D)====[WATER HEATER: 40 CFH]
                                |
                                +====(10 ft Branch B-R)====[RANGE: 65 CFH]
                                |
                                +====(15 ft Branch B-Y)====[DRYER: 35 CFH]

Sizing Calculation Steps:

1. Appliance Loads:

   • Water Heater (at D): 40,000 BTU/hr = 40 CFH
   • Furnace (at F): 100,000 BTU/hr = 100 CFH
   • Cooking Range (at R): 65,000 BTU/hr = 65 CFH
   • Clothes Dryer (at Y): 35,000 BTU/hr = 35 CFH
   • Total System Demand: 40 + 100 + 65 + 35 = 240 CFH

2. Determine Longest Developed Length:

   • Path to Range: 20 ft (A-B) + 10 ft (B-R) = 30 ft
   • Path to Dryer: 20 ft (A-B) + 15 ft (B-Y) = 35 ft
   • Path to Furnace: 20 ft (A-B) + 20 ft (B-C) + 10 ft (C-F) = 50 ft
   • Path to Water Heater: 20 ft (A-B) + 20 ft (B-C) + 20 ft (C-D) = 60 ft
   • Total Longest Developed Length = 60 feet (Governs the entire layout!).

3. Select Table Column: Use the 60-Foot Column for ALL pipe segments.

4. Size Each Individual Section:

   • Main Run A-B: Carries total load = 240 CFH. In the 60-ft column, a 3/4" pipe carries 138 CFH (too small); a 1-inch pipe carries 260 CFH (260 ≥ 240). → Select 1-inch.
   • Sub-Main B-C: Carries Furnace (100 CFH) + Water Heater (40 CFH) = 140 CFH. In the 60-ft column, 3/4" carries 138 CFH (fails by 2 CFH); a 1-inch pipe carries 260 CFH. → Select 1-inch.
   • Branch C-D (Water Heater): Carries 40 CFH. In the 60-ft column, a 1/2-inch pipe carries 65 CFH (65 ≥ 40). → Select 1/2-inch.
   • Branch C-F (Furnace): Carries 100 CFH. In the 60-ft column, 1/2" carries 65 CFH (too small); a 3/4-inch pipe carries 138 CFH. → Select 3/4-inch.
   • Branch B-R (Range): Carries 65 CFH. In the 60-ft column, 1/2" carries exactly 65 CFH (65 ≥ 65). → Select 1/2-inch (or 3/4" for field safety margin).
   • Branch B-Y (Dryer): Carries 35 CFH. In the 60-ft column, a 1/2-inch pipe carries 65 CFH (65 ≥ 35). → Select 1/2-inch.

5. Branch Method vs. Longest Length Method & Common Traps

A critical mistake made on licensing exams is switching table columns when sizing branches:

• The Sizing Trap: For Branch B-R (Range, 10 ft long), looking at the 10-ft column where a 1/2" pipe delivers 172 CFH. While 172 CFH seems generous, the Range is fed through Section A-B which has already consumed part of the available pressure budget over its 20-foot run. If the branch is sized using a short length column, the system pressure drop will exceed 0.5 in. w.c., causing low-pressure starvation.
• The Rule: The longest developed length represents the overall hydraulic resistance of the piping network; therefore, all downstream sections must be evaluated against the resistance of that longest path.

Hybrid 2-PSI Medium Pressure Systems

Modern residential construction frequently incorporates 2-psig medium-pressure systems utilizing CSST manifolds:

• Operation: Utility gas is delivered at 2.0 psig (55.4 in. w.c.) directly to a central distribution manifold. Because higher pressure pushes significantly more mass through smaller tubing, 1/2-inch and 3/8-inch CSST lines can run extended distances.
• Maxitrol / Line Regulators: At each appliance, an Appliance Line Pressure Regulator (ANSI Z21.18) reduces the 2.0 psig line pressure down to 7.0 in. w.c. for appliance operation.