Fire Loss from Gas-Fired Appliances in Your Home

August 2021 - Gas-Fired Appliances

Unless you heat your home with a wood-burning stove, you likely rely upon natural gas from your local gas company or from on-site propane (often referred to as LPG, liquified petroleum gas) to get the job done.  Even if you do use a wood-burning stove, you may rely upon gas for your water heater, your cook stove, your clothes dryer, or even your furnace (as a backup heat source).  Gas-fired appliances are quite common in the United States.  The purpose of this newsletter is to offer some insight into what a fire investigator looks for when inspecting a fire loss where gas-fired appliances are suspect.  Much of this information can be found in greater detail in the document titled Guide for Fire and Explosion Investigations (commonly referred to as NFPA 921) and in the book Kirk’s Fire Investigation, 8th Ed., by Icove and Haynes[1]

First, it is important to know and understand that natural gas is not the same as commercial propane (LPG).  The constituents of natural gas are manifold and vary from locale to locale.  There are no standards that mandate what natural gas contains.  Generally, natural gas is a mixture of predominately methane, with lesser amounts of other gases like ethane, propane, butane, and even nitrogen and oxygen, to name a few.  Natural gas has a specific gravity typically between 0.59 and 0.72, which means it is lighter than air[2].  Natural gas, at sea level, has a heat content of 1,030 BTUs per cubic foot.  In elevated regions, like say, here in the Denver, Colorado area, natural gas is typically derated (mixed with air) by the gas supplier to provide around 800 BTUs per cubic foot to account for the thinner air (less oxygen).


On the other hand, LPG commercially sold in the United States is predominately a mixture of propane and propylene with lesser amounts of other gases.  LPG has a heat content of approximately 2,500 BTUs per cubic foot, more than twice that of natural gas.  This is important to understand, as I will discuss below.  Propane has a specific gravity of around 1.5, which means it will generally sink in air.

Before I go much further, it is also important to note that fuel gases are not odorized in their pure state.  We are quickly reminded of the sulfur or rotten egg odor when we encounter gases like natural gas and propane, but these gases are generally odorless in their pure state.  Gas suppliers are required to add an odorant mixture, typically t-butyl mercaptan, to natural gas to provide a degree of safety in the event of a leak.  For LPG, Ethyl mercaptan is added as an odorant.  If the gases were not odorized, a dangerous leak could go undetected and result in a catastrophic explosion.  These added mercaptans provide a modicum of safety.

There are a few additional points on this topic to understand, or at least be aware of.  There are situations where the mercaptan odorants can be “scrubbed” from the gas.  This odorant loss is termed odor fade and can be very dangerous.  New propane tanks and new piping have been known to result in odor fade, leaving the fuel gas under-odorized.  Further, underground leaks where the escaping migrant gas passes through clay soils, have also been known to scrub odorant leaving the fuel gas odorless or severely under-odorized.  This odor-free gas can then migrate through the soil into structures and create an undetected, explosive mix[3].  Occupants of a structure suffering from ingress of de-odorized gas won’t benefit from the noxious mercaptans as a warning.  In explosions, the investigator will be very interested in checking buried gas lines for leaks and means of ingress into the structure. 

Figure 2Image

Now, back to some of the differences between natural gas and LPG systems.  Homes supplied with natural gas will be equipped with a pressure regulator next to the meter that drops the gas line pressure somewhere between 4” w.c. to 10” w.c.[4] (0.14 psi to 0.36 psi).  Homes supplied with LPG will also have a pressure regulator, often near or attached to the tank, which drops the gas pressure somewhere between 11” w.c. and 14” w.c. (0.39 psi to 0.5 psi).  We are not dealing with tremendous gas pressures inside residential structures, which is a good thing.    

Typically, each gas-fired appliance in your home is designed to accept both natural gas and LPG with the addition of a conversion kit[5].  I’ve found that most appliances come from the factory equipped to accept natural gas; a conversion kit is either provided with the appliance or can be purchased.  Let’s take a gas-fired water heater, for example.  The burner on the water heater consists of a gas nozzle (often referred to as an orifice) that is properly sized for a specific gas (typically natural gas) at a specific pressure.  These orifices often resemble a threaded cap with a hole in the top.  Generally, for a specific appliance, the natural gas orifice is larger than the propane-sized orifice.           

When a gas-fired appliance is suspect of being the cause of the fire, the investigator will want to verify that the correct orifice is in place.  An appliance set up for natural gas but attached to a propane gas system will result in a greatly over-fired appliance.  A few tell-tale signs of such a situation will be evidence of flame roll-out and heavy soot deposits inside the burner cavity and in the flue system.  Additionally, there may be testimony of poor or inconsistent performance of the appliance before the fire.   

One last thing, the investigator will also be very interested in the fresh air supply for the appliance.  If the appliance is in a closet or otherwise confined enclosure, the investigator will want to confirm sufficient air supply.  The National Fuel Gas Code (NFPA 54) and the specific appliance installation manual should be consulted in all situations.  In general, appliances are supposed to be provided with combustion, ventilation, and dilution air as specified by the appliance manufacturer. A common method used to provide combustion air is to install two ventilation openings to the enclosure, one within 12 inches of the top of the enclosure and one within 12 inches of the bottom of the enclosure.  Again, this is not always the case.  Still, if a fire investigator finds an appliance enclosure in the area of origin without openings that communicate with outside air or other indoor spaces, it is likely the investigator is looking at a starved appliance, which can lead to poor combustion, sooty combustion, possible flame roll-out, and other dangerous combustion performance issues.  Again, this is not always the case but certainly the installation of the appliance should be examined closely to make sure it was within the parameters dictated by the manufacturer and the National Fuel Gas Code.

In this short article, I’ve only scratched the surface on what a fire investigator should be tuned into when investigating a fire loss where gas-fired appliances are present.  A fire investigator should be questioning the type of fuel provided, that the appliance was properly set up for that fuel, that there were no leaks in the system, and that appliance was provided proper and sufficient combustion air.  Despite fuel gas systems today (natural gas or LPG) being safe and efficient, the potential problem areas are myriad and often complicated.  It is imperative that a savvy and informed fire investigator be employed to sift through the evidence and determine if the system or appliance was the source of the fire loss.  Until next time, be safe, and happy heating everyone!               

Meet Cord

Cord is a licensed Private Investigator in the state of Arkansas, and New Mexico.  He is a member of the National Association of Fire Investigators (NAFI), a member of the National Fire Protection Association (NFPA), and the International Association of Arson Investigators (IAAI). Give Cord a call to discuss more about fire investigations where gas-fired appliances may have been involved!

Learn More


[1] These two sources are just a short list of many technical and reliable literatures on the topic of fire investigation.

[2] Air has a specific gravity of 1.0.  Gases with a value higher than 1.0 will tend to sink; gases with a value lower than 1.0 will tend to rise.

[3] The explosive range for natural gas is approximately 3.9% to 15%.  The explosive range for propane is approximately 2.15% to 9.6%.  These are not high values, which means it does not take much to create a dangerous situation.

[4] Water Column.  A common method for noting low gas pressures.  One pound per square inch (psi) = 27.67 inches of water column.   

[5] As always, there are some appliances that are only designed for one type of gas, and are not able to be converted. 

Learn More

To find out more about how Ponderosa Associates can assist in fire origin and cause, please visit our Fire & Explosions consulting service page or contact us.