Gas, oil furnaces and air conditioners can last from 15 to 20 years. Heat pumps can last 12 to 15 years. If properly maintained, if not maintained they wont last as long and will be inefficent. Cost more to operate. If your system is over five years old its not near as efficient as whats available these days. Gas furnaces can be 96% efficient. Oil furnaces 80% and heat pumps and air conditioners can be over 16 seer nowadays. Something to think about since utility costs are always rising,

Ken s