Based on what you write, you appear to have a claim for fraud. Fraud is the knowing or intentional misrepresentation of a material or important fact, made to induce the other party to enter into the transaction and upon which the other party reasonably relied. While it's plausible that the seller might not know or remember the *exact* age of an A/C unit (e.g 1 year vs. 1 1/2 years or even maybe 2 years) and so an error of minor magnitude might not be fraud, there is no reasonable or plausible way that they could think that a 17-year-old unit had been replaced a year ago--clearly, there had been no replacement. This would therefore seem to be a knowing misrepresentation, and the age of the A/C unit is clearly material, since it goes to the efficiency of the unit, utility bills, and costs (e.g  to repair or replace) you would face. Accordingly, this appears to have been fraud, and you could potentially sue the seller to recover the lesser of the difference in value of the home with new vs. 17-year-old A/C, or the cost to put in a new system.