What is the most likely cause of soot accumulation in an LP gas furnace that has been converted from natural gas?

Soot accumulation in an LP gas furnace that has been converted from natural gas is most commonly attributed to the orifice or spud being too large. When a furnace is converted from using natural gas to LP gas, it is crucial that the orifice size aligns with the specifications for LP gas, which has different combustion characteristics compared to natural gas. If the orifice is too large, it allows too much gas to enter the combustion chamber, resulting in an overly rich fuel mixture. This rich mixture can lead to incomplete combustion, which in turn causes soot to form as a byproduct.

Ensuring the correct orifice size is essential for maintaining optimal combustion efficiency and preventing soot buildup. Proper adjustment can help achieve the ideal air-to-fuel ratio, promoting cleaner combustion and reducing the risk of soot accumulation within the furnace system.