If the furnace (or any other asset) is "placed in service" (which I would assume it was installed and is operational not just decorative) and it is "paid for" in some fashion ( paid by CC, loan, installments, etc) then it is deducted in that tax year. Even if you default on the loan or never pay off the CC or go bankrupt the unit is still considered paid for thus it is an asset to be depreciated or expensed.
