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Understanding Flame Rollout In Gas Appliances

Flame rollout is the term commonly used to describe a fuel gas combustion process occurring outside the normal combustion area of a gas appliance. Any gas combustion not properly confined to the appropriate area could obviously have serious consequences. Electrical wiring, gas valves, and combustible materials located near appliances may be compromised by flame rollout. Flame rollout can cause gas control valves, usually made of metal alloys with relatively low melting points, to fail structurally. Such a failure can be responsible for the release of a devastating volume of gas with a ready source of ignition.

Almost any gas appliance that is configured in a way that permits the combustion process to be easily observed through large openings may experience flame rollout. Gas appliance designs that feature enclosed combustion areas with offset combustion air entry openings are less likely to experience rollout problems. Most of the gas fueled furnaces, boilers, pool heaters, and hanging unit heaters in use in America today have the potential for flame rollout problems.

Frequently, flame rollout is the result of a restriction within a flue gas passage in an appliance. Furnaces, boilers, pool heaters, and hanging unit heaters that employ multiple burners positioned beneath sectional heat exchanger passages are particularly susceptible to this problem. The most commonly found causes of restricted flue gas passages are soot and rust. Rust from degrading heat exchanger surfaces can accumulate in the narrow portions of a passage and impede the flow of combustion byproducts out of the appliance. Any gas combustion that occurs without a proper air/fuel mix can produce soot. Yellow flames in a gas appliance indicate an impaired combustion process and warn soot may be accumulating. Dirty or misadjusted burners are the usual causes of heat exchanger passages restricted with soot.

A restriction in a passage through a heat exchanger slows the flow of combustion byproducts out of the appliance. As the area required for flow diminishes, combustion byproducts begin to collect in the combustion area. These accumulated byproducts deprive the gas flame of the air required to support combustion. The air starved combustion process causes the flame to lift and appear to float above the burner. The size of the restriction will control how long the burner can operate before flame movement occurs. If sufficient material has accumulated in the passage to severely restrict the flow of combustion byproducts, the flame will be forced to seek air at the opening of the burner area. This is the situation that causes fire to emerge from within the combustion area of the appliance, the condition called flame rollout.

Simple equipment maintenance, periodic cleaning and adjusting, could prevent most of the rollout problems caused by restrictions. Unfortunately, another common cause of flame rollout in furnaces requires more extensive effort to understand and identify. Many of the first gas furnaces were coal furnaces converted to burn gas. These early units used large pipes and the natural flow of hot and cold air to move air through the furnace, not electrically powered blowers. These systems were called “gravity” or “warm air” furnaces. The heat exchangers in these furnaces, the enclosure that separates the combustion process and combustion byproducts from the air being heated, looked like a cast iron, “pot belly” stove. When material failure caused a crack or hole in one of these cast heat exchangers, combustion byproducts could emerge and join the heated air flowing up the supply ducts. Obviously, this could allow carbon monoxide to enter the living space. Most of the heat exchangers in furnaces today are made of steel, not cast iron, but the problem of cracks and holes still exists. If they remain in service, all the heat exchangers used in modern, forced air heating equipment will eventually fail from stress cracks and/or rust-through. The use of blowers to promote air movement causes another type of rollout problem. The operation of the blower in forced air heating units creates air pressure outside of the heat exchanger that is greater than the pressure on the inside where the combustion process occurs. Even a small hole or crack can permit enough air to enter the heat exchanger to cause an eddy, a rotational movement of gases, that disrupts the flow of combustion byproducts. This will have the same effect as a restriction in a heat exchanger passage, an impaired flow of combustion byproducts resulting in a flame deprived of combustion air and flame rollout. Small holes may require an extended period of burner and blower operation to backup enough exhaust gas in the passage to cause flame movement or rollout. Cold weather, causing long intervals of burner operation, may allow rollout to occur in furnaces with very small cracks or holes. All too often service technicians fail to observe burner operation for the extended period required to identify furnaces with either subtle restrictions or small cracks.

Intermittent flue gas spillage almost always precedes flame rollout. This spillage stains surfaces above burner openings and will eventually compromise paint and promote metal corrosion. The distinctive pattern created by flue gas spillage will often survive an intense fire event, alerting investigators to possible rollout problems.

Many equipment designs have incorporated removable panels above burner openings. Properly positioned, these panels will cause pilot light extinguishment and a safety control shutdown, if rollout problems start to occur. Equipment operation without these panels is a frequent cause of rollout fires. Newer units have safety controls that react to rollout temperatures and terminate burner operation. Unfortunately many individuals, including technicians who should know better, choose to bypass these safety devices.

Any gas appliance located in a possible area of fire origin should be carefully examined to eliminate it as a fire cause. Soot in gas appliance passages will usually survive fire extinguishment, rough handling, and even transport on open trailers. Rust may fall from gas equipment, but the evidence of heavily rusted passages will remain. Finding cracks or holes can be difficult and may require invasive disassembly and procedures that could be destructive. Ignoring the appropriate investigative protocols could result in a charge of spoliation.

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