Which scenario would likely lead to a lack of combustion efficiency?

A blocked venting system would likely lead to a lack of combustion efficiency because it prevents the proper expulsion of combustion gases from the fireplace or appliance. For combustion to be efficient, a steady flow of fresh air is needed to facilitate the reaction with the fuel and enable proper exhaust of byproducts like carbon monoxide and water vapor. When venting is obstructed, it can create a backdraft, reducing the amount of oxygen available for combustion and leading to incomplete burning of fuel. This not only decreases efficiency but can also lead to safety hazards due to the accumulation of harmful gases within the living space. Proper venting is crucial for optimal combustion efficiency, hence a blocked system directly impacts this process.