replaced. But, this isn’t always the most effective way for determining if the machine needs to be changed. A system that is 8 years old may not be a top-quality unit and requires replacement. In contrast, a 30 year old system is not required to be replaced if it has been taken care of properly. A HVAC system is able to last over 15 years, on average. It is a good idea that you replace your HVAC if you have invested significant sums of money into the system. hwkwpmtbtm.