9.1 Fuel Gas Properties, Heating Values & IFGC Scope

9.1 Fuel Gas Properties, Heating Values & IFGC Scope

Quick Answer: Natural gas has a specific gravity of about 0.60 and delivers roughly 1,000 Btu per cubic foot; commercial propane has a specific gravity of about 1.52 and delivers roughly 2,500 Btu per cubic foot. Natural gas rises when it leaks, propane sinks, and the same appliance load draws about 2.5 times more cubic feet per hour on natural gas than on propane.

Gas Appliances and Piping is 40 of the 100 scored items on the Maryland Journey Plumber/Gas Fitter exam — more than drainage, venting, water supply and backflow combined. Everything in that domain rests on the properties below, and you will reuse these numbers in every sizing, venting and combustion-air calculation.

What the IFGC Governs — and Where It Stops

Maryland adopts the International Fuel Gas Code (IFGC), the International Plumbing Code (IPC), the International Residential Code (IRC), NFPA 54 (National Fuel Gas Code) and NFPA 58 (LP-Gas) by reference at COMAR 09.20.01. In the testing room the IFGC 2018 is your reference for anything that burns gas; the IPC 2018 covers water, waste and vent.

IFGC Section 401.1 draws the boundary: the code applies from the point of delivery to the connections with the appliances. The point of delivery is the outlet of the utility meter for natural gas, or the outlet of the second-stage regulator for LP-gas; upstream of it the utility or propane supplier owns the pipe and federal pipeline rules govern. The IFGC also does not reach systems operating above 125 psig for natural gas or 20 psig for LP-gas. Chapter 24 of the IRC is text extracted from the IFGC, so residential and commercial answers are the same rule under two numbers (IFGC 310.2 is IRC G2411.2).

Natural Gas vs. Propane: The Comparison Table to Memorize

Specific gravity is the weight of a volume of gas compared with the same volume of air. Because propane is half again as heavy as air, a leak flows downhill and collects in basements, crawl spaces, pits and floor trenches — which is why NFPA 58 restricts LP-gas containers and certain appliances in below-grade spaces, and why propane detectors mount low. Natural gas at 0.60 gravity gathers at the ceiling, so its detectors mount high. Same leak, opposite search pattern.

Turning Btu Into Cubic Feet

Appliances are rated in Btu per hour — a Btu (British thermal unit) is the heat that raises one pound of water 1°F. The sizing tables in IFGC Section 402.4 are entered in cubic feet per hour (cfh). The heating value bridges them:

cfh = total connected load in Btu/h ÷ heating value in Btu/ft³

Worked example. A Maryland row house has a 120,000 Btu/h furnace, a 40,000 Btu/h water heater, a 65,000 Btu/h range and a 35,000 Btu/h dryer. IFGC Section 402.2 requires the sum of the maximum input of every appliance served, assuming all could run at full capacity at once:

120,000 + 40,000 + 65,000 + 35,000 = 260,000 Btu/h

• On natural gas: 260,000 ÷ 1,000 = 260 cfh
• On propane: 260,000 ÷ 2,500 = 104 cfh
• Combustion air on natural gas: 260 ft³/h × 10 = 2,600 ft³ of air per hour
• Liquid propane burned: 260,000 ÷ 91,500 = 2.84 gallons per hour

Two checks fall out of the same arithmetic. One gallon of liquid propane yields 91,500 ÷ 2,500 ≈ 36.6 cubic feet of vapor. And one therm — the billing unit on a Maryland gas bill — is 100,000 Btu, which is 100 cubic feet (one CCF) of 1,000-Btu natural gas.

Flammable Limits, Ignition and Odorization

A fuel gas burns only inside its flammable range, bounded by the lower explosive limit (LEL) and the upper explosive limit (UEL). Below the LEL the mixture is too lean to carry a flame; above the UEL it is too rich for the available oxygen. Methane runs about 5% to 15% by volume in air; propane runs about 2.1% to 9.5%. Propane's LEL is less than half of methane's, so a much smaller propane leak reaches an ignitable concentration.

Ignition temperature is the temperature at which a gas-air mixture ignites with no spark present. Published values vary by test method — methane is commonly listed near 1,000°F and propane near 850–920°F — so learn the ordering (propane ignites at a lower temperature than natural gas) rather than a single figure.

Both fuels are naturally colorless and odorless, so odorization is a legal requirement. 49 CFR 192.625(a) requires gas in a distribution line to be odorized so it is readily detectable by a person with a normal sense of smell at one-fifth of the lower explosive limit, and NFPA 58 applies the same one-fifth rule to LP-gas. The odorant is a mercaptan — usually ethyl mercaptan for propane, at roughly one pound per 10,000 gallons of liquid — and it smells of rotten eggs. Run the numbers: one-fifth of methane's 5% LEL is 1% gas in air, and one-fifth of propane's 2.1% LEL is about 0.42%. Those are the concentrations at which a customer must be able to smell the leak, well below anything that can burn.

Combustion Chemistry and What the Flame Tells You

Complete combustion of methane is CH₄ + 2O₂ → CO₂ + 2H₂O + heat. Air is only about 21 percent oxygen, so those two volumes of oxygen arrive as 2 ÷ 0.21 = 9.53 cubic feet of air, rounded in the trade to 10 cubic feet of air per cubic foot of natural gas. Propane needs five volumes of oxygen (C₃H₈ + 5O₂ → 3CO₂ + 4H₂O), so 5 ÷ 0.21 = 23.8, or about 24 cubic feet of air per cubic foot of propane. Burners run with excess air beyond these stoichiometric amounts so every fuel molecule finds oxygen.

Complete combustion produces only carbon dioxide, water vapor and heat, plus the nitrogen that rode along in the air, and the flame is blue with a sharp inner cone. Incomplete combustion — too little air, a blocked flue, a fouled burner, a cracked heat exchanger — leaves carbon partly oxidized and produces carbon monoxide (CO), soot and aldehydes. CO is colorless, odorless, lethal, and unlike the fuel gas it is not odorized.

Pressure Units, Input Rating and Appliance Categories

Fuel gas pressures inside a building are read in inches of water column (in. w.c.) — the height of a water column the pressure will support. The conversion is fixed:

27.7 in. w.c. = 1 psi, so 1 in. w.c. = 0.036 psi and 1 ounce per square inch = 1.73 in. w.c.

That makes 7 in. w.c. of natural gas 7 ÷ 27.7 = 0.25 psi, and 11 in. w.c. of propane 11 ÷ 27.7 = 0.40 psi. IFGC Section 402.7 limits piping inside buildings to 5 psig unless one of seven listed conditions is met — welded or brazed joints, flanges welded or brazed to the pipe, a ventilated chase, exclusively industrial/research/warehouse/boiler-room use, temporary construction piping, agricultural service, or an NFPA 58-compliant LP-gas system above 20 psi. Section 402.6 then requires that whatever pressure drop the design accepts, the pressure at each appliance inlet still meets that appliance's stated minimum.

Three definitions the exam reuses constantly:

• Input rating — the maximum gas consumption of an appliance in Btu/h, shown on the manufacturer's label; IFGC 402.2 sizes piping from the sum of these ratings. Above 2,000 feet of elevation, NFPA 54 §11.1.2 requires the rating to be reduced 4 percent per 1,000 feet.
• Manifold pressure — the pressure downstream of the appliance's own regulator, at the burner manifold: typically 3.5 in. w.c. for natural gas and 10 in. w.c. for propane. It is set by the appliance regulator, not the utility, and with orifice size it fixes the firing rate.
• Vented appliance categories — Category I (nonpositive vent static pressure, no excessive condensate), II (nonpositive pressure, excessive condensate), III (positive pressure, no excessive condensate), IV (positive pressure, excessive condensate). Categories II and IV are the condensing appliances, and vent material and sizing follow directly from the category.

Exam Trap: A stem gives a 200,000 Btu/h propane load and asks for flow in cubic feet per hour. The planted distractor is 200 cfh — what you get by dividing by 1,000 out of habit. Propane is 2,500 Btu/ft³, so the answer is 200,000 ÷ 2,500 = 80 cfh. The same reflex ruins pressure items: candidates read "11 in. w.c." and pick 11 psi, which is roughly 300 in. w.c. Confirm the fuel and the pressure unit before you touch a sizing table.