Photograph of  this gas flame which gives a clue that there may be an operating problem and an unsafe gas furnace in this buildingLP & Natural Gas Combustion Products
in Flue Gas Exhaust

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This article lists the components in the flue gas or exhaust gas produced by the combustion of natural gas (and similarly propane gas or liquefied natural gas (LNG) such as when gas is burned in a home heating appliance like a water heater or a heating boiler.

What are the normal combustion products and what dangerous combustion products may be produced when a gas fired appliance is not working normally? How much carbon dioxide, carbon monoxide, sulphur dioxide, or nitrous oxide are produced when burning natural gas? Methane gas hazards and hot water tanks - can a hot water heater tank produce methane gas? Methane gas production by methanogens & methanogenesis in water heaters?

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A Guide to LP or Natural Gas Fired Appliance Combustion Products found in Flue Gas Exhaust

LP gas tank (C) Daniel Friedman

Natural gas, currently supplying about 22% of energy in the U.S., has been used as a fuel since its development by the Chinese more than 2500 years ago.

Today it is used in buildings for heating as well as for production of hot water and in some cases even for cooling. In industry gas is used as a heating fuel for many processes.

In the U.S. the first natural gas well was dug in Fredonia New York in 1821.

Perfect combustion of natural gas (Methane – CH4) produces only CO2 and water vapor The equation for the combustion of natural gas is

CH4[g] + 2 O2[g] -> CO2[g] + 2 H2O[l] + 891 kJ

But this is a simplification since natural gas is not pure. Natural gas, principally methane (CH4) as it is produced from a gas well, also contains ethane.

C2H6, propane C3H8, butaneC4H10, carbon dioxideCO2, nitrogen (N), helium (He), and hydrogen sulfide H2S.

Before it is distributed to consumers, ethane, propane, butane are removed from natural gas. Small quantities of other molecules may be produced during natural gas combustion than those in the “pure” case we listed above.

In the table shown here we list the relative quantities of combustion products produced when burning natural gas. the numbers are pounds produced per million Btus of NG burned:

Table of Combustion & Flue Gas Products in Natural Gas

Combustion & Flue Gas Products in Natural Gas CH4

Combustion Product Pounds per Billion BTUs Burned Ratio to CO2 Power Plant Emissions Burning CH4
Carbon dioxide CO2
1135 lbs/MWh
Carbon monoxide CO
Nitrous oxide NO
1.7 lbs/MWh
Sulphur dioxide SO2
0.1 lbs/MWh
Mercury Hg

-- from various information sources on natural gas. We anticipate that the combustion products from burning liquefied natural gas LNG and propane (C3H8) will be similar.

Note that

Normal natural gas combustion: In a practical sense in a home or office building if we are considering a small natural gas appliance such as a water heater, and provided that the equipment and its flue vent connector and chimney area all working correctly and that there is adequate combustion air, once the equipment has warmed up and draft is established, the system is producing CO2 and H2O (in the form of water vapor) and not much else that will be detected by the building occupants.

Imperfect (and unsafe) natural gas combustion, short on Oxygen from too little combustion air or from a chimney problem, will produce CO as well as nitrogen oxides (NOx), organic particulate material, and polynuclear aromatic hydrocarbons (PAH). Incomplete combustion of natural gas may also release un-burned methane CH4 itself.

The reason for the initial versus stabilized- burn CO level spec is that until the appliance heats up combustion is incomplete and higher levels of CO are produced.

The percentage makeup in flue gas from a gas fired water heater will probably not be given as a general overall standard in many references and by most onsite HVAC technicians or inspectors except in theoretical combustion instances because of the wide variability in equipment, vents, and chimneys.

How much water is produced when burning natural gas?

Burning natural gas produces 1.25 gallons of water per 100,000 BTUH in a typical construction heater (less than 100% efficiency). That is, a 100,000 BTUH natural gas fired heater produces about a gallon and a quarter of water per hour or about a gallon and a quarter of water per 100,000 BTUs of heat produced by burning natural gas fuel.

Combustion & Flue Gas Products in Propane or LP Gas

Combustion & Flue Gas Products in Propane Fuel 1

Combustion Product Pounds per Million BTUs Burned Comments
Carbon dioxide CO2 139  
Carbon monoxide CO - with adequate
combustion air, negligible
amounts of CO
Water 150.4 18 gallons
Particulates 2 - Particulate matter
Sulfur dioxide 2 - Depending on
fuel source, small
amounts of SO2
Nitrogen 2 - small amounts of NOx


1. Assuming complete combustion with adequate combustion air supply, no CO produced.


Complete and perfect combustion of propane (also referred to as stoichiometric combustion) (C3H82) and water vapor.

For propane gas to burn we need a fuel-air mixture that is between 1.8% to 8.6% propane combined with 98.2% to 91.4% air.

The formula is

C3H8 + 5 O2 → 3 CO2 + 4 H2O + heat

or in chemistry-speak, burning one mole of propane will produce 3 moles of CO2 and 4 moles of H2O

or in text:

Propane + Oxygen → Carbon Dioxide + Water

or in wordier English, one molecule of propane (C3H8) combines with 5 molecules of oxygen (O2 ) to produce 3 molecules of carbon dioxide (CO2) and four molecules of water [vapor] (H2O) and some heat.

Actually the flue gases produced by burning propane will contain as well nitrogen (from the input air) and much smaller amounts of other gases.

Greenhouse gas emissions factors for burning propane fuel: 62.7 kg CO2/ mBTU - or - 138.2298 pounds of CO2 / mBTU or 1.55 kg of CO2 per litre or 73.7 kg / GJ.

Keep in mind that combustion will rarely be perfect, so this is the simplest but also a theoretical answer.

Watch out: If there is insufficient combustion air (that is if there is insufficient oxygen) then combustion of LP gas will also produce carbon monoxide (CO) - a potentially fatal gas. See CARBON MONOXIDE - CO.

How much CO2 is produced when burning Propane?

Burning 1 million BTUs of propane produces about 139 pounds of CO2

How much water is produced when burning propane?

Burning propane produces 0.98 gallons of water per 100,000 BTUH in a typical construction heater (less than 100% efficiency).

That is, a 100,000 BTUH propane heater produces about a gallon of water per hour or about a gallon of water per 100,000 BTUs of heat produced by burning propane. (CHI, 2017)

or from a more-scholarly treatment that is pure theory and that doesn't consider heater efficiency,

Burning a gallon of propane (91,502 BTUS) will produce 1.64 pounds of water.

11 gallons of propane = 1,000,000 BTUs

1,000,000 BTUs of propane will produce (1.64 x 11) gallons of water or about 18 U.S. gallons.

More Propane Fuel Conversion Constants

References for Products of Combustion of Propane Gas

Question: On 2017-10-08 by Frank - cubic of gases (including water vapor) generated from burning 1000 btus of propane?

Can someone tell me the cubic of gases (including water vapor) generated from burning 1000 BTUs of propane?

This question was posted originally at COMBUSTION AIR REQUIREMENTS

Reply: On 2017-10-08 by (mod) - exhaust gases produced from burning propane fuel


Since at NATURAL GAS & PROPANE COMBUSTION PRODUCTS we note that burning a gallon of propane produces 91,000 BTUS of energy (heat) and 1.64 pounds of water, we can figure that burning 1000 BTUs of propane will produce 1.64/91 gallons of water or 0.18 gallons of water.

That water is of course in vapor form whose vapor volume will vary depending on temperature and perhaps other factors.

There we also note the other combustion products and volumes from burning propane.

We avoid giving an answer in cubic feet (you didn't give me a cubic "what" measurement) because the volume of a gas depends also on its pressure (we can assume atmospheric pressure) and temperature (that we don't know for your heater but a typical gas furnace will be operating around 110-140 degF depending on when and exactly where you make the measurement).

An related question that helps us answer your question in greater depth is What will be the product of burning one cubic foot of propane gas (C3H8) ? Or what is the product of burning one cubic foot of propane?

What you get when burning 1 cubic foot of propane depends of course on the pressure at which that gas is delivered (thus how much propane is present), but roughly it might be 2516 BTUs of energy in the form of heat. From that fact we can estimate an answer to your question.

We can easily convert between thousands of BTUs and cubic feet of propane as you will see in the constants I'll give here.

Now, simplifying since actually additional factors such as Ambient Air Temperature and temperatures in the burner make a difference in story, metric combustion, let's review the following:

There is not one single “correct” answer because burning propane gas requires combining the propane with oxygen (in the combustion air provided) within a range in which the gas will burn.

In order for propane gas to burn we need to mix it with combustion air in the right range. We need an air-propane mixture between 2.15% and 9.6% propane in typical air (at sea level). At other gas-air ratios the propane will not burn.

So let’s do the math for a very rough estimate:

Lower (leaner) range of 2.15% propane in air means that in 100 cubic feet of gas-air mix we will have 2.15 cu.ft. of propane and 97.85 cubic feet of air.

Higher (richer) range of 9.6% propane in air means that in 100 cubic feet of gas-air mix we will have 9.6 cu.ft. of propane and 90.4 cubic feet of air.

In perfect combustion burning propane produces CO2 and water vapor. Propane + oxygen being burned, or C3H8 (g) + 5O2 (g) will produce when burnned: 3CO2 (g)+ 4H2O (g) + energy or re-stated: propane + oxygen = carbon dioxide + water + energy

Burning 100 cu.ft. of the gas-air mix will then produce about 100 cu.ft. of CO2 & water in perfect combustion along with the remaining un-burned or un-modified constituents of that same air (principally nitrogen and oxygen). You can, of course, divide by 100 to go back to your single cubic foot. I used 100 cu.ft. to make the writing easier.

See COMPLETE COMBUSTION, STOICHIOMETRIC for details about complete combustion of fossil fuels.

At GAS BTUH, CUBIC FEET & ENERGY we give details about the energy in fuel gases like LP and natural gas.

At GAS PRESSURES LP vs NATURAL GAS we discuss the properties of propane gas

See CONCENTRATIONS of GASES in AIR for details about the makeup of air, including combustion air.


Continue reading at GAS DETECTION INSTRUMENTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see these Gas Fuel & Piping Articles

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