Flame rollout occurs when burner flames exude from the normal combustion area of a gas boiler or furnace. Like furnaces, gas boilers incorporate a heat exchanger that accepts hot combustion gases produced by the burner, then transfers heat to the water. In proper operation, all combustion should be limited to the combustion chamber.

Venting Issues

In most cases, the sequence of events that leads to flame rollout in gas boilers begins with gradual accumulation of soot or dislodged rust inside the narrow flue passages of the heat exchanger. These passageways convey combustion gases to the vent pipe where they’re exhausted out of the home. As flue passages incrementally narrow, unvented combustion gases accumulate to high levels inside the combustion chamber.

The Floating Flame

Because combustion byproducts are inflammable, normal ignition of burner gas is inhibited due to a lack of sufficient oxygen. Initially, the burner flame may appear to float several inches above the burner, reflecting delayed ignition. As venting is increasingly obstructed and the air supply further diminishes, unburned burner gas exudes from the combustion chamber, instead igniting at the entrance to the chamber when it contacts oxygen. This results in the burner flame “rolling out” of the chamber.

Damage And Danger

When flame rollout occurs, boiler components outside the combustion chamber that are not fire-resistant may be compromised by exposure to the flame. Principally, these include the boiler gas valve and electrical wiring leading to the valve, as well as any other components or parts with a low melting point. Since combustion gases include dangerous carbon monoxide, insufficient venting also poses serious safety hazards.

Inspect To Prevent

Most boilers and furnaces incorporate safety sensors to shut down the system if flame rollout occurs. To prevent fire hazards and potential boiler damage from flame rollout, the combustion system, which includes the burner and heat exchanger, should be inspected annually for build-up soot or rust that narrows flue passages and obstructs proper venting.

For more information about the causes and cures of flame rollout in gas boilers, contact Overland Park Heating & Cooling in Kansas City.

Our goal is to help educate our customers in Overland Park, Kansas about energy and home comfort issues (specific to HVAC systems). 

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