You should consider replacing your existing heating system in several different instances. For example, a heating system that requires repeated and frequent repairs is probably worth replacing. It’s likely not very efficient and may not last much longer anyway. You can save yourself a lot of aggravation as well as money by replacing the heater sooner rather than later.
You should also consider investing in a new heating system if your current unit is more than 10 years old. Even if it seems to still be working well, the newer systems are so much more energy efficient that you’ll come out on top anyway. The monthly energy savings you’ll incur immediately will quickly offset the replacement and installation costs for your new heating system.