Final answer:
The minimum clearance from combustibles for a single wall furnace vent is typically 18 inches. This safety measure prevents the risk of fire by keeping the vent away from flammable materials. Always check local building codes and manufacturer's specifications for precise requirements.
Step-by-step explanation:
The minimum clearance from combustibles for a single wall furnace vent is generally understood to be 18 inches (A). This clearance is necessary to prevent heat from the vent from igniting nearby combustible materials, which could lead to a fire hazard. It is important to always refer to local building codes and the manufacturer's specifications as these clearances can vary based on the specific type of furnace and venting system used.