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Gas Principles You Should Know When Building a Fire Pit

Fire pits make a beautiful centerpiece for an enjoyable evening at home with family and friends. And using gas as fuel can make it as easy as pushing a button. No mess. Stress-free. No constant tending of the fire. However, working with gas can be dangerous. It is important to understand the properties of gas in order to make safe decisions.

The purpose of the information in this article is to provide a fundamental understanding of gas as it applies to the use of outdoor fire pits.  It is not intended as a substitute for hiring a licensed gas installer for your project.  Working with gas can be dangerous and should always be performed by a licensed gas professional.  You can search for a licensed gas installer at https://www.nficertified.org/public/ – alternatively, you can contact a local HVAC or Plumbing company to locate a licensed installer.

We’ll start with a quick chart outlining the essentials. But keep reading if you’re the type of person who was never satisfied when their mom said, “Because I said so.”

Beyond the chart, we’ll take a deeper dive into the technical nature of propane and natural gas, and how the chemical make-up of these gasses affects their behavior.  

You’ll gain a basic understanding of gas principles that will give you the confidence to create the fire pit of your dreams and make it easier for you to troubleshoot problems down the road.

Quick Guide to Propane vs Natural Gas (as used in fire pits)

Natural GasLiquid Propane Gas
Clean BurningDirty BurningPropane is prone to leaving a soot deposit behind; whereas natural gas is much less likely to leave any soot deposits.
Lighter than AirHeavier than AirNatural gas dissipates quickly and easily when used outdoors.  Propane pools and puddles inside enclosed areas won’t dissipate easily, potentially causing a dangerous build-up of gas.
Limited AvailabilityAvailable EverywhereNatural gas isn’t available in all locations; whereas propane gas is available in portable tanks at most local supermarkets, gas stations, and home improvement stores.
Needs a Gas Supply Line InstalledNo (or minimal) Gas Supply NeededNatural gas requires the installation of an underground gas line.  Propane tanks can be stored on the underside of a fire pit or fire table, and it doesn’t require the installation of extensive underground gas lines.

With the basics out of the way, we can dig a bit deeper.

Properties of Natural Gas

Natural gas exists in nature as a varying composition of hydrocarbon (organic compound composed solely of hydrogen and carbon) gases.  The exact composition of the raw mixture differs depending on the source and location.

It may be found in oil fields, produced from shale, isolated in natural gas fields, as crystallized natural gas hydrates, or in coal beds.

Once natural gas has been refined, the fuel consists almost entirely of methane gas.

Natural Gas in a fire pit
Natural Gas (Methane CH4)   

Methane is a hydrocarbon with the molecular formula CH4, indicating that each methane molecule is composed of one carbon atom and four hydrogen atoms. The single lines connecting each H to the center C in the illustration to the right represent four equivalent chemical bonds.

Natural gas has a very low boiling point. At room temperature, it exists as a gas. Therefore, it must either be pressurized or stored at extremely low temperatures for transport in its liquid state. Pipelines are usually the most economical way to transport natural gas fuel. Underground lines are typically used in residential settings.

Properties of Propane

Liquified Petroleum Gas (LPG) is commonly referred to as propane in the United States. It is a product derived from the processing of natural gas and the refinement of crude oil.

Chemical composition of propane gas for outdoor fire pits.
Propane (C3H8)

The structure of a propane molecule has much in common with the structure of a methane molecule. However, propane molecules are larger than methane molecules: each propane molecule contains three carbon atoms and eight hydrogen atoms.

Like methane, propane has a low boiling point and exists as a gas at room temperature.

Once it has been transformed into a liquid state, it too can be moved through gas pipelines. It is stored and distributed to consumers via pressurized, steel storage containers.

Specific Gravity of Gas

Specific gravity can also be thought of as relative density—how dense is one substance compared to another. By using a standard to measure different substances against, we can determine their specific gravity.

The table below uses air as the reference gas.  

GasSpecific GravityComments
Air1.00 
Natural Gas (methane)0.64Almost half the weight of air – rises easily.
Propane Gas1.52About 50% heavier than air – pools in low areas.

Knowing the specific gravity of each gas is very important in designing a safe fire feature and using it correctly.

Natural gas is lighter than air and dissipates easily. When it’s released, the gas flows upward.  Therefore, vents should be placed near the top of your structure.

Propane is heavier than air while Natural Gas is lighter than air.

Because natural gas rises, you have the option to face the orifice-side of the burner up or down. (Some people like to have the burner facing down to avoid getting water in the holes should it rain on the device.)  

Propane versus Natural Gas for Outdoor Fire Pits

It is recommended that the amount of fire glass over the burner should not exceed 1” to 2”.

Propane’s structure adds four extra hydrogen atoms, but they can’t compete with two additional carbon atoms. Carbon carries a heavier atomic weight, making propane denser than air. Therefore, propane “sinks” when released and pools like water.

For this reason, you must always have proper ventilation of an appropriate size, located low within your fire pit to allow pooling gas to disperse.  Failing to do so could lead to fire or an explosion.

Also, you will need to make sure the holes (orifices) in your burner are facing upward so the gas starts its journey in the right direction.

A thin layer of fire glass should cover the burner – no deeper than 1″ – in order to allow the gas to reach the surface easily and burn steadily. It also reduces soot build-up on the fire glass.

One More Thing

The heaviness of propane requires that you use an air mixer in your fire feature (it’s not necessary for natural gas). This device is installed between the gas source and the burner. It draws in the fresh air and dilutes the propane just before it reaches the burner. This results in a cleaner, more efficient burn.

Check out our article on air mixers for all the details you need to know.

Ask Us

By now, you should have a pretty good working knowledge of the properties of natural gas vs. propane as they relate to fire pits. However, if you still have questions or you want to run any of your ideas past our technical staff to make sure you’re on the right track, please don’t hesitate to to contact us.

We want you to not only have a beautiful fire feature—we want you to have one that is safe and cost-efficient.

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