It's a good idea to get your furnace replaced when it hits 20 years old. This is because most furnaces' parts only last for about 20 years, and if you've reached that age, then there's a good chance some of the components need to be replaced. However, older systems can also fail earlier if they're not properly maintained, so it's important to schedule regular maintenance checks. You should also consider replacing your furnace if it's not heating your home as efficiently as it used to. Talk to a heating specialist to find out if it's time for a replacement.