Soot buildup is a common issue in boilers that use fuel sources like gas or oil. This buildup occurs beneath the boiler tank itself, around the burners that are used to heat the water. Over time, layers of soot can create a fire hazard. They will also interfere with how easily gas or oil can heat the water, impairing efficiency. Soot problems are caused by several different issues.
Your boiler may operate just as expected over a long time period. However, trace amounts of soot are usually unavoidable in boilers that have a fuel source. While this soot may not be a problem for years, over time it can build up, layer on layer. Finally, the boiler begins to suffer because of the soot and requires either thorough cleaning or replacement.
All fuel sources contain some level of contaminants. Propane, for example, has very few contaminants and burns clean enough to create very little soot. Natural gas is typically unfiltered and has more contaminants. It can create soot much more easily. Oil is thick and has even more contaminants that move through the exhaust as it is burned. These sources have different levels of quality as well. Low-quality fuels will create soot more easily.
If your boiler is receiving too much fuel through its lines, this extra fuel (especially gas) can float beyond the burners and help contribute to the soot problem. It also forms a combustion risk, since it might suddenly encounter enough heat to ignite and burn away, adding to the soot and potentially causing damage to other parts of the boiler.
Gas or oil pressure may also be low. When this occurs, the burners produce low flames that do not heat very well. These low flames also produce more soot than do hot, active flames, which can quickly cause buildup. In this case, the problem may be a clogged fuel line or nozzle that is restricting the flow of fuel into the burner.