A HVAC unit should last around 15 years, but efficiency degrades over time, and your energy bill goes up the older your unit is. We have seen units over 30 years old still running, but the energy bill was quite high. We always try to help  you keep your HVAC system lasting as long as possible, but sometimes replacing a unit is cheaper than keeping a unit that is using more energy as well as has more expensive parts to replace. Technology within the heating and air conditioning realm has only been increasing in efficiency. Therefore, we suggest sometimes that the best choice is to update your system.