A technician servicing an LP gas furnace notices large amounts of soot in the heat exchanger after it was converted from natural gas. What could be the cause?

When a technician observes large amounts of soot in the heat exchanger of an LP gas furnace, it typically indicates an issue with the combustion process. In this case, having an orifice that is too large is the most likely cause of the problem.

When the orifice (or spud) is too large for the type of gas being used, it allows an excessive volume of gas to flow into the combustion chamber. This can lead to a situation where there is not enough air to mix properly with the gas, resulting in an incomplete combustion reaction. Incomplete combustion produces carbon particles, which are seen as soot. Therefore, the large amounts of soot found in the heat exchanger are direct evidence of this inefficient combustion process, confirming that the orifice size is critical for proper operation.

In a correctly sized system, the ratio of gas to air would be balanced, leading to a clean burn and minimal soot production. The technician should consider resizing the orifice to ensure optimal fuel and air mixture, correcting the issue of soot formation.