What are the subcategories of boilers?

The subcategories of boilers, as defined in § 63.11237 are: (a) Coal. (b) Biomass. (c) Oil. (d) Seasonal boilers. (e) Oil-fired boilers with heat input capacity of equal to or less than 5 million British thermal units (Btu) per hour. (f) Boilers with an oxygen trim system that maintains an optimum air-to-fuel ratio that would otherwise be subject to a biennial tune-up. (g) Limited-use boilers. [78 FR 7506, Feb. 1, 2013]