Preventing Fugitive Vapor Explosion with Design


Expertise Includes:

    • Machine Design & Safeguarding
    • Machinery & Equipment Analysis
    • Products Liability
    • Risk Assessment

For years, many water heater-related fires, injuries, property damage claims, and deaths were attributed to “consumer misbehavior.” By 2002, however, the NFPA National Fuel Gas Code began to reflect several key realizations: the existence of fugitive vapors and how appliance design can limit the risks they pose.

The Risks of Fugitive Vapor
Fugitive vapors from flammables are heavier than air and can travel as far as 40 feet. Because they stay low to the ground, they are particularly dangerous when a floor-mounted water heater is nearby. If vapor comes in contact with the water heater’s pilot light or burners, a flash fire can ignite.

A view of a water heater elevated 18 inches above the floor.

Code Evolves from Installation to Performance
Since the late 1980s, the code called for elevation of gas water heaters so that the burner and pilot flame are at least 18 inches above the floor. The code also required a label on the water heater storage tank regarding avoiding storage and use of flammable liquids in the vicinity of gas water heaters.

In 2003, the national safety code changed to prohibit the installation of gas appliances (including water heaters) in areas where the open use, handling, or dispensing of flammable liquids occurred, unless the design, operation, or installation reduced the potential for ignition of flammable vapors. This new requirement, identified as the ANSI Flammable Vapor Ignition Resistance (FVIR) test, when successfully passed, demonstrates an appliance’s immunity to nearby flammable vapors.

Combustion air holes on the bottom of a conventional water heater, through which fugitive vapors are drawn in.

Safety Influences Design
Because the changes affect all 30-, 40-, and 50-gallon atmospherically vented residential gas water heater models, manufacturers have adopted vapor resistance approaches into current designs. As a result, National and International Fuel Gas Codes now allow an exception to the 18 inch elevation requirement for floor-mounted water heater installations in garages, provided the unit is listed as flammable vapor ignition resistant (FVIR).


Water heater bottom with a flame arrestor that lets vapors in but prevents ignited vapors from flashing back to the pool of flammable vapors.

When technology advances, one of the greatest benefits can be a jump in human safety. That’s certainly been the case with gas water heaters. The result of the evolution in safety codes, product design and industry standards has been a path to safer use of gas water heaters in homes and businesses across the country.

Jeffery H. Warren, PhD, PE, CSP, is the chief engineer and CEO at Warren specializing in mechanical, machine design and safety. His deep expertise in machine design and safety analysis makes him a frequent presenter, trainer and expert witness. In addition to investigating more than 2000 claims involving property damage and injuries related to machinery and equipment since 1987, Jeff has an undergraduate degree in Mechanical Engineering from the University of North Carolina as well as a Master of Science and a Doctorate in Mechanical Engineering from Virginia Polytechnic Institute and State University — both with machine design emphasis.

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