If a technician finds soot in the heat exchanger of an LP gas furnace converted from natural gas, what likely caused this issue?

Soot accumulation in the heat exchanger of an LP gas furnace that was converted from natural gas is primarily indicative of improper combustion due to fuel flow issues. When the orifice is too large for the fuel being used, it allows an excessive volume of gas to flow into the combustion chamber. This excessive flow can lead to incomplete combustion, resulting in the formation of soot.

Natural gas and LP gas have different combustion characteristics and energy content. If a furnace that was originally designed for natural gas is fitted with an orifice that is too large for LP gas, the mixture may be too rich in fuel. A rich mixture, where there is more gas than the air can support for complete combustion, will not burn cleanly, leading to carbon build-up in the form of soot.

This improper adjustment can create several issues, including decreased efficiency and potential safety hazards. Therefore, identifying that the orifice is oversized is crucial in resolving combustion problems and ensuring safe, efficient operation of the converted LP gas furnace.