I started renting a home in 1994. I have expensed 19 years of the 27 years of depreciation required on the building. There are 8 years remaing of depreciation. In 2013 I replaced the  central air conditioining system for $6000. Do I derpeciate the replacement A/C over 27 years as required under MACRs, or can I depreciate it over the remainin life of the buidling to which it is part of, which would be 8 years?