Pursuant to 26 CFR 1.48-1 (e) (2) [Title 26 Internal Revenue; Chapter I Internal Revenue Service, Department of the Treasury; Subchapter A Income Tax; Part 1 Income Taxes; Normal Taxes and Surtaxes; Determination of Tax Liability; Rules for Computing Credit for Investment in Certain Depreciable Property], the term "structural components" includes "such parts of a building as walls, partitions, floors, and ceilings, as well as any permanent coverings therefor such as paneling or tiling; windows and doors; all components (whether in, on, or adjacent to the building) of a central air conditioning or heating system, including motors, compressors, pipes and ducts; plumbing and plumbing fixtures, such as sinks and bathtubs; electric wiring and lighting fixtures; chimneys; stairs, escalators, and elevators, including all components thereof; sprinkler systems; fire escapes; and other components relating to the operation or maintenance of a building. However, the term "structural components" does not include machinery the sole justification for the installation of which is the fact that such machinery is required to meet temperature or humidity requirements which are essential for the operation of other machinery or the processing of materials or foodstuffs. Machinery may meet the "sole justification" test provided by the preceding sentence even though it incidentally provides for the comfort of employees, or serves, to an insubstantial degree, areas where such temperature or humidity requirements are not essential. For example, an air conditioning and humidification system installed in a textile plant in order to maintain the temperature or humidity within a narrow optimum range which is critical in processing particular types of yarn or cloth is not included within the term "structural components". For special rules with respect to an elevator or escalator, the construction, reconstruction, or erection of which is completed by the taxpayer after June 30, 1963, or which is acquired after June 30, 1963, and the original use of which commences with the taxpayer and commences after such date, see section 48(a)(1)(C) [26 USCS § 48(a)(1)(C)] and paragraph (m) of this section."