Hello again. Thank you for your patience.
Q: Is Sears responsible for selling us an AC that is too big for our existing ductwork. We bought what we thought was an efficient unit only to find out that the unit is too big.
A: The answer depends on whether you reasonably relied upon Sears to assess what you needed. For example, if a Sears employee visited your home and conducted an inspection, and then informed you that you need X, then Sears would be liable for selling you the wrong unit. On the other hand, if you simply told a Sears salesperson what you wanted, and they sold you what you asked for, then Sears probably wouldn't be liable because you got what you asked for and did not rely upon Sears' expertise. So, the answer really just depends on the facts in your particular case.
Q: Sears says that they do not warranty the installation only the mechanical part of the AC.
A: That would be an issue. You'll need to review the warranty to see what exactly it covers. Even if the warranty doesn't cover the installation, then you may have recourse against the contractor himself (though you'll need to track him down). He may even have a bond that would cover the re-installation. However, the statute of limitations may be an issue here depending on all of the facts. It would be between 3 and 6 years of when you knew that you had a potential case against Sears and/or the contractor. Accordingly, my suggestion would be to retain a local attorney who can review all of the facts in detail, determine whether the statute of limitations is an issue, and then make a demand upon Sears and/or the contractor, and then sue if necessary.
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.