Thank you for the detailed explanation. The IRS instructions are very poorly written in this area and do not cover the situations that you described. In my situation I only have one home. Due to its size, the home has two central air conditioning systems. Each system has its own air handler inside the house and a condenser outside. From your explanation, it seems that the IRS considers both systems installed together in a single taxable year to be one unit and therefore the credit is limited to $600. However, for anyone else reading this explanation and contemplating the purchase of central AC, it seems to me that there is a strategy that would produce a $1,200 credit. Simply have one central AC system installed in December of the taxable year and install the second system a month later in January of the following taxable year. Doing so would produce a $600 credit in one taxable year and another $600 credit in the following year. I believe the way that the law is written, or maybe the way the Treasury Department is interpreting the law, this suggested strategy is totally legal and makes economic sense to do so. Many times, the IRS will issue Treasury Regulations that interpret the law under various scenarios. I could not find any regulations related to this tax credit
