There are lots of household owners that are unaware of when it is the right time to change their furnace or air conditioner. Also there are some customers that literally have no knowledge about furnaces and air conditioners at all, but it is alright though because technology has advanced so much that anything that you want to learn without having to go through a lot of trouble to research on it is already here. The internet is one of the greatest assets to mankind, if you are unaware of something the information is literally at the tip of your fingertips. IN my honest opinion i think the best time to changing your furnace would preferably be when it is at least 17-18 years old. That is the best time because furnaces are run very well up until 17-18 years and after that is when problems start to add up and you end up spending a lot of money just to make sure it runs a couple of years longer. As your furnace gets older the efficiency of your furnace decreases as well, by the time your furnace is at least 17-18 years old your furnace is probably running at 60-70% efficient, and that gets costly when your furnace is running all the time and using a lot more power than it did before because its much older now. Last but not least i would just like to restate that the best time to change a furnace is when they are 17-18 years old.