What type of furnaces and boilers use fuel as a heat source and require combustion air?

The correct choice identifies a category that encompasses various types of heating systems utilizing fuel combustion as a means of generating heat. Fuel-fired furnaces and boilers utilize a range of combustible fuels, including natural gas, propane, oil, and even solid fuels like wood or coal, to produce heat. One key requirement for these systems is the need for combustion air, which is essential for the efficient burning of fuel.

By requiring combustion air, fuel-fired systems can safely and effectively convert fuel into heat energy. This air is typically drawn from the environment or provided through specific ventilation systems designed to ensure adequate airflow for safe operation. Understanding this requirement is crucial for maintaining the efficiency and safety of heating appliances, as a shortage of combustion air can lead to incomplete combustion, resulting in lower efficiency and the potential for harmful emissions.

In contrast, other types of options listed either utilize electrical energy or do not encompass the entire spectrum of fuel options that such systems may include. Hence, the term "fuel-fired" accurately describes any furnace or boiler that operates primarily through the combustion of fuel, necessitating proper combustion air supply for optimal performance.