What could cause large amounts of soot in a heat exchanger of an LP gas furnace that was converted from natural gas?

The presence of large amounts of soot in a heat exchanger of an LP gas furnace that has been converted from natural gas can primarily be attributed to issues with gas pressure. When the gas pressure is low, the furnace may not burn the LP gas efficiently, leading to incomplete combustion. Incomplete combustion is a key factor that can result in the formation of soot, which is essentially unburned fuel.

This situation can occur because LP gas requires a different pressure and flow rate compared to natural gas. If the system is not properly adjusted for the correct pressure settings after the conversion, it can lead to a condition where not enough gas is being supplied for optimal combustion. Consequently, the burner may not operate at the intended efficiency, resulting in soot buildup within the heat exchanger.

Factors like air flow and orifice size are also part of the combustion process but relate more to the balance of air and fuel. If the gas pressure isn't set correctly, even having the right air flow or orifice size wouldn't correct the incomplete combustion issue, which is fundamentally driven by the correct fuel pressure.