You could replace your outside AC unit, but you would likely be better off replacing your entire HVAC system. An AC unit is just one component of a larger system, and its important that all the components work together in order to get the best performance and longest life from your system. If you are not sure whether its time to replace your entire HVAC system or just your AC unit, a qualified HVAC technician can come out and do a full evaluation of your system to give you an accurate estimate of what needs to be replaced and what would be best for your home.