Terms vary among manufacturers and may change with changes in home ownership. The owner have had to register the system within a certain period and may have only a brief window in which to report problems for a claim to be successful.
The following is an example from Carrier:
You should recommend that your client read carefully the warranty terms for any system in the home that may still be under warranty. They may need to notify the manufacturer upon purchase of the home for the warranty to remain in effect. Your client should understand that warranties are typically prorated and may not include full replacement cost in the event of a failure, even if the warranty is still in effect.