Propane is a fuel gas with the chemical compound C3H8, forming a simple linear organic polymer called an alkane, the same family as other well-known organic fossil fuels such as methane, butane and octane.
It is often sold commercially in a mixture with other trace compounds, such as methyl mercaptan for olfactory detection of fuel leaks. Compared to natural gas, which is composed primarily of methane, propane's higher boiling point allows it to be condensed into a liquid under pressure, while similar storage of methane requires much higher pressures. The liquid propane, however, readily returns to a gas at atmospheric pressures and ambient temperatures, and burns with similar energy and cleanliness as methane. This interoperability with natural gas, combined with ease of storage and transport in pressurized tanks, makes propane highly suitable for a number of applications from combustion engines to cooking fuel.
In home construction, propane serves as a direct replacement for natural gas in fuel gas systems, in areas not served by natural gas infrastructure (for instance, in rural areas such as farms, although many urban and suburban neighborhoods lack natural gas service). Most appliances that run on natural gas can also accept propane, often with slight adjustments to the appliances or the supply pressure, to provide the correct fuel-air mixture (methane burns best in about a 10% concentration in air, while propane, needing more oxygen to burn completely, burns best at about a 4% fuel-air mixture). A typical propane system replaces the pipeline service of natural gas with a free-standing or buried propane tank, with a nozzle for regular fills from a truck. Most other aspects of a home propane system are similar in theory and practice to natural gas and thus bound by similar building code requirements.
Like any fuel gas, propane requires proper external ventilation to safely exhaust the combustion by-products, which is normally CO2 and water vapor, but can also include products of incomplete combustion due to insufficient oxygen, such as carbon monoxide and carbon soot. These are slightly more problematic with propane than natural gas, as the higher oxygen demands of propane stoichiometry often lead to insufficient oxygen supply causing production of these pollutants. Larger ventilation duct diameters, both for fresh air supply and exhaust venting, are often recommended in homes being built or retrofit for propane compared to homes designed for natural gas. A carbon monoxide detector is also required in new construction, and generally recommended for any home with a fossil-fuel heating system (gas, oil or wood-burning).