Your tax professional may be the best person to answer this question. Here is my non-professional opinion which may or may not be true. If you had replaced a certain defective part with the same replacement part, this may not count towards home improvement. If you had replaced a certain defective part with a better (more expensive) replacement part, this upgrade may be considered home improvement. If you had fabricated the replacement part yourself from scratch, the raw materials and the equipments that you had bought during the tax year may be counted towards home improvement. If modifications had been made to make the chimney more efficient than before, any materials used and labor (if you hire somebody to do it) may be counted towards home improvement. For obvious reasons, IRS would not allow you to deduct your own labor if you did the work yourself.
