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Gas is Gas Right?...Wrong!


Despite the wide spread use of gas appliances such as furnaces, water heaters and barbeques, many people don’t understand the important differences between the two main fuels typically used with these appliances, propane and natural gas (methane).  Propane and methane have very different properties and ignoring this can lead to hazardous situations.  This article highlights the key differences between these fuels and the hazards associated with assuming that all gases are created equal.

Visions of a summer day with steaks sizzling on the grill are what most city dwellers tend to think of when asked what propane is used for.  Those same people tend to view natural gas as being the standard fuel for home water heaters and furnaces (at least in Alberta).  In reality, appliances such as barbeques, furnaces and water heaters can be configured to function equally well with either propane or natural gas (as most rural people well know).  So what dictates what type of fuel is used you ask?  The answer is a standard realtor’s catchphrase....location, location, location.  Rural areas may have natural gas distribution systems, but it is not uncommon to have appliances that are designed to operate with propane, using the propane that is supplied in pressurized cylinders.  As natural gas is not easily compressed into liquid form, it is not practical to store in cylinders.  Natural gas is generally only distributed through underground distribution networks in more densely populated, urban areas.  Consequently, location is usually the primary factor in determining what type of fuel is used.

As mentioned, most gas appliances can be configured to work with either propane or natural gas; however, the type of fuel used must match the particular configuration of the appliance.  If you only remember one thing about gas it should be that fuel sources and appliances cannot simply be interchanged.  For example, you cannot take a water heater that was designed to operate with natural gas and simply connect it to a propane supply.  Correction - you can do this, but the ensuing fire will probably make you wish you hadn’t. 

The primary reason gas appliances cannot simply be interchanged between fuel sources is that propane and natural gas appliances do not operate under the same gas pressures.  This difference dictates the size of the burner orifices and how the gas controller on the appliance is configured.  Natural gas appliances function under pressures between 3” to 4” water column (w.c.), while propane appliances range between 10” to 11” w.c..  To put this into perspective, there’s approximately 28” w.c. in one psig and a typical tire pressure on a vehicle is about 30 psi.

What are the implications?  Suppose you find a fantastic deal on a natural gas water heater and decide to install it at your cottage, which is fueled by propane.  The problems start because the water heater is designed to operate at lower gas pressures than that present in the propane being supplied.  As a result the burner orifices will be too large and excess propane will be released into the combustion chamber when the water heater is operating.  As a consequence, the water heater will repeatedly overheat.  Any combustibles near the heater may then be ignited from the overheating water heater. 

Other key differences between propane and natural gas are the specific gravity of their vapours (i.e. the weight of the gas), as well as their boiling points (the transition point between liquid and vapour at atmospheric pressure).  Propane vapour is heavier than air and will settle, whereas natural gas is lighter than air and will rise. In the event of a gas leak, propane will tend to settle and accumulate within a confined space, whereas natural gas will tend to rise and is more likely to harmlessly vent to the atmosphere.  As it relates to boiling points, propane has a boiling point of -42.2°C, which is considerably higher than that of natural gas (-161.5°C). Due to propane’s relatively high boiling point, the risk of propane gas condensing in fuel lines can be a concern during colder weather.  This can result slugs of liquid being supplied to the burner assembly.

The importance of these differences to fire investigators is to understand how some people may, unexpectedly, incorrectly fuel some of their appliances.  The effects of incorrectly fueling some domestic appliances can result in some pretty unusual looking consequences.


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