Generally speaking it’s more cost effective to repair a broken furnace if it’s 12 years old or younger.  But probably better to replace a broken furnace if it is 15 years old or older.  The average life span of a furnace is 15-18 years. If a major component like a heat exchanger or a control breaks you’re probably better off to replace it.  To make sure your furnace lasts and runs optimally, keep up on maintenance  and change the filter frequently.  Make sure to use a dependable furnace repair company for maintenance. If you find you do need to replace your furnace remember that furnaces today produce heat more steadily than older furnaces and heat more evenly which makes your home more comfortable.  They also are a lot more energy efficient!