Final answer:
Natural-draft furnaces are no longer available for new installations because they are not energy efficient, leading to higher costs and environmental impact. Heat pumps represent a more energy-efficient alternative, as they transfer heat rather than generate it through combustion.
Step-by-step explanation:
Natural-draft furnaces are no longer available for new installations primarily because they are not energy efficient. This lack of efficiency is problematic as it can lead to higher costs over time, increased energy consumption, and greater environmental impact. Additionally, with advances in technology, there are now better options available, such as heat pumps, which can provide energy more efficiently by transferring heat from accessible sources rather than generating heat through combustion.
Factors like temperature being too cold for the output of certain machinery, such as steam engines, also highlight the issues with systems that are not efficient in energy use. Moreover, assumed high efficiencies that are not feasible in practical scenarios further show why less efficient appliances, like natural-draft furnaces, are phased out in favor of more advanced and efficient technologies.