Gas Heat ESCO Practice Exam

A technician servicing an LP gas furnace notices a tag stating it was converted from natural gas. He finds large amounts of soot in the heat exchanger. What could be the cause of this soot?

1. Incorrect gas valve

2. Orifice (spud) too large

3. Blocked flue pipe

4. Faulty thermostat

The correct answer is: Incorrect gas valve

The presence of soot in the heat exchanger of an LP gas furnace converted from natural gas suggests incomplete combustion of the LP gas. The incorrect gas valve could be preventing the proper flow or mixture of gas and air, leading to improper combustion and the production of soot as a byproduct. When a furnace operates within the correct parameters, combustion should be efficient, producing primarily carbon dioxide and water vapor. However, if the gas valve is not set correctly, the fuel may not burn completely, resulting in soot accumulation. While the orifice being too large could potentially lead to a similar situation, it is more directly related to gas flow rates rather than combustion efficiency. In contrast, a blocked flue pipe would typically lead to other safety hazards such as carbon monoxide build-up, rather than a soot issue specifically related to combustion quality. A faulty thermostat may influence the operational state of the furnace but would not directly cause soot formation.