How long does a furnace and air conditioner last and what is the warranty like on them? Most new furnaces come with a 10 year manufacturer limited parts warranty and a 20 year manufacturer warranty on the heat exchanger which is the heart of the furnace, and the most expensive to repair or replace.

Furnaces have become more energy-efficient over the years. A gas furnace made in the early 1970s typically has an AFUE rating of about 65 percent. The lowest efficiency allowed by law for new gas furnaces is 78 percent, and some new models achieve 97 percent, near-total efficiency.