Air heaters and hot-air distributors under this heading heat and circulate air directly without generating steam or hot water as an intermediate medium. Boilers and steam-generating equipment fall under separate headings, and the classification turns on the heat-transfer mechanism — direct air heating versus steam or hydronic systems.

Parts that are identifiable solely or principally as components of iron or steel air heaters or hot-air distributors are generally classified within this heading. Generic components that could serve multiple appliance types may instead be classified under a more general parts provision, so specificity of design is the determining factor.