Pipes and ducts.
(4) Pipes and ducts. Pipes and ducts that are used exclusively to carry energy derived from solar energy are solar energy property. Pipes and ducts that are used to carry both energy derived from solar energy and energy derived from other sources are solar energy property (i) only if their use of energy other than solar energy does not exceed 25 percent of their total energy input in an annual measuring period and (ii) only to the extent of their basis or cost allocable to their use of solar energy during an annual measuring period. (See paragraph (d)(6) of this section for the definition of annual measuring period and for rules relating to the method of allocation.)