From the way I understand IRS publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf on page 110, a central air/heat system is classified as residential rental real estate and depreciated over 27.5 years. So it's not eligible for expensing or the SDA if I understand things correctly. On page 110 it states:
Structural components: Parts that together form an entire structure, such as a building. The term includes those parts of a building such as walls, partitions, floors, and ceilings, as well as any permanent coverings such as paneling or tiling, windows and doors, and all components of a central air conditioning or heating system including motors, compressors, pipes, and ducts. It also includes plumbing fixtures such as sinks, bathtubs, electrical wiring and lighting fixtures, and other parts that form the structure.
