In 2003, a new standard for water heaters was developed and phased in. It says, “The water heater should not ignite flammable vapors outside the water heater created by the spilling of gasoline onto the floor.” The Consumer Products and Safety Commission found thousands of fires, injuries and deaths were related to water heaters. Most of these cases were because of improper storage or handling/spillage of gasoline.
If the tank is in compliance with the FVIR standards, it does not have to be raised 18 inches in garages or similar locations, unless required by the manufacturer or local code authorities.
A FVIR water heater has the following components: 1) a device to prevent ignited vapors from passing out of the combustion chamber, 2) a one-way intake system to control the movement of makeup air into the combustion chamber, 3) an inner door and burner assembly to create a sealed junction with the combustion chamber, preventing combustion air and flammable vapors from entering the chamber through the front of the water heater.
All FVIR water heater tanks have things in common. 1) A flame arrestor plate. Located under the burner, the metal plate is designed to allow combustion air into the combustion chamber but keep flames from escaping downward and igniting flammable vapors below. 2) Thermal cutoff switch. It is designed to shut down the heater if it senses excessive temperatures caused by inadequate combustion air inside the chamber. Inadequate combustion air can be caused by an explosion of flammable vapors, inadequate venting, inadequate makeup air or the accumulation of lint, dust, or oil on the screen. 3) A lint, dust, and oil screen. The screen is designed to protect the combustion process from lint, dust, or oil. The screen openings can become clogged, especially when the tank is located in a basement or utility room.
FVIR System on a Bradford White Defender Water Tank
During normal operation, air for combustion is drawn into the water heater through the openings in the jacket. This air travels down and around the combustion chamber and enters through holes in the very bottom of the corrosion-resistant combustion chamber. The air then travels up through the oriented flame arrestor plate louvers, where the velocity of the air is increased and its direction altered. The air then mixes in a normal manner with the supplied gas and is efficiently combusted, producing very low NOx emissions (nitrogen oxides).
In the case where trace amounts of flammable vapors are present in the air flowing into the chamber, the vapors are harmlessly ignited by the burner/pilot flame. If flammable vapors are in sufficient quantity to prevent normal combustion, the burner/pilot flame is shut down.
Should the flammable vapors continue to the burner, the flame arrestor plate prevents the flames from traveling backwards and igniting vapors outside of the combustion chamber. The calibrated, multipurpose thermal switch recognizes this and shuts down the pilot and main burner. This switch also deactivates the burner and pilot in the unlikely event of restricted airflow caused by severe lint, dust, or oil accumulation on the arrestor plate.
“Hot to Properly Inspect a FVIR Hot Water Tank” – Online training video for property inspectors. https://www.nachi.org/advancedcourses.htm