Final answer:
The throat of the fireplace indeed affects its efficiency by controlling air flow and combustion gas expulsion, and efficiency can be increased through external air supply for combustion and by recycling the room's air via convection.
Step-by-step explanation:
The statement that the throat of the fireplace controls the efficiency of the fireplace is true. The efficiency of a fireplace can indeed be influenced by the design and operation of its throat. The throat of a fireplace is the narrow part near the base of the chimney, and it can affect the amount of air that flows into the fireplace and how well the combustion gases are expelled up the chimney.
To increase efficiency, an external air supply can be used for combustion, which minimizes the amount of heated indoor air being used for combustion and therefore lost through the chimney. Additionally, circulating room air around the firebox and having it returned to the room involves convection as the primary method of heat transfer. The air is warmed by the firebox and then circulated back into the room, reducing the amount of heat lost through the chimney.
An efficient device can also prevent most of the heat from just escaping through the chimney. This can include features like a wood stove insert, which retains more heat in the room rather than allowing it to escape.