Final answer:
The term 'zero clearance' describes fireplaces designed to safely contact combustibles like wood framing. Fireplaces achieve efficiency through convection and radiation, with an external air supply for combustion or room air circulation. Safety and efficiency are also guided by building codes to protect inhabitants.
Step-by-step explanation:
The term you are looking for in the question about factory-built fireplaces is "zero clearance." This term implies that the fireplace has been designed so that its components can safely be installed in contact with combustible materials, such as the wooden framing of a house, without requiring additional space for safety reasons. This is possible due to the fireplace's insulation and design that restricts the transfer of heat to surrounding combustible materials.
Speaking of heat transfer, the efficiency of a fireplace can be improved by using an external air supply for combustion or by utilizing room air to circulate around the firebox and back into the room. The primary methods of heat transfer involved in these cases are convection and radiation.
The external air supply helps to ensure a more complete combustion, improving efficiency. In contrast, circulating room air takes the heat from the firebox through the process of convection and disperses it back into the living space, thereby increasing the heating efficiency of the fireplace.
Such safety and efficiency design considerations are part of building codes in many developed countries (MDCs), aimed to protect the safety of inhabitants while maintaining structural integrity and livability standards.