You can deduct the time-space percentage of the cost of a new heating/ac system. Since this is a home improvement, you must depreciate it over 39 years.
Assuming your heating/ac system was energy efficient, you may also be entitled to claim a federal energy tax credit. See my recent blog post on this:
http://tinyurl.com/4rn3hmw
The blog post says that you can't claim the maximum energy tax credit if your time-space percentage was over 20%.