I know many brands offer 5-year, and occasionally 10-year warranties. Seems simple enough, is it?

  It’s true that all manufacturers offer at least a 5-year warranty, but that covers one component, maybe two. One large volume, very low cost brand is well known to offer a 5 year parts-only warranty and will supply the free parts (you pay the labor costs) as long as you don’t mind waiting while they order it! All of Trane’s residential replacement systems have a 10-year compressor warranty and at least a 5-year warranty on all components, not just the compressor. Their XL series systems carry a 10-year warranty throughout. Don’t forget about the labor coverage. This is generally provided by the installing contractor and is not paid for by the manufacturer. Something to ponder: The best warranty is a warranty you never need to use. See Q9 below.